How do I convert paraxylene to benzene

9 aromatics. 9.1 Benzene is an aromatic. To the picture of the homepage


1 9 Aromatics To the picture on the homepage The regular hexagon is the basic structure of many aromatic compounds. The equality of bond lengths and bond angles can be explained with the mesomerism model (Sections 9.3 and 9.4) or with the rbital model (Section 9.5). The regular hexagon occurs in the benzene molecule (all --bond lengths are 139 pm, all bond angles are 120), also approximately in the molecules of the benzene derivatives (Section 9.6 ff.) And in the molecules of the polycyclic aromatics such as . B. naphthalene, anthracene and benzopyrene (Sections 9.4 and 9.13). In the case of benzene derivatives, the deviations are small: For example, ff fi in the phenol molecule the two - bond lengths between the three atoms close to the group are slightly smaller (139.1 pm) than the four other - bond lengths (139 , 4 pm). In the case of polycyclic aromatics, the deviations are somewhat greater: in the anthracene molecule there are four - bond lengths of 137 pm and seven - bond lengths of 142 pm. There are aromatic compounds with other regular polygons; however, these are not dealt with in the student's book: the ylopropyl cation (triangle), the yclopentadienyl anion (pentagon) and the ycloheptatrienyl cation (heptagon). 9.1 Benzene an aromatic To exercises A1 In comparison with an alkane of the empirical formula 6 14, eight atoms are missing in the molecule of the compound with the empirical formula 6 6, i. That is, an open-chain molecule would have to hold either two triple bonds, one triple bond and two double bonds, or four double bonds. Examples: A2 a) The structural formula of Dewar does not agree with the experimental finding that only one ombenzene isomer is formed when benzene is converted. The other two structural formulas are consistent with the findings. b) Monosubstitution products of the first two examples from A1: (E) -1-om-hexa-1,5-diene-3-in (Z) -1-om-hexa-1,5-diene-3-in 2- om-hexa-1,5-dien-3-yn 1-om-hexa-1,5-diyne 3-om-hexa-1,5-diyne A3 The Ladenburg benzene is in agreement with the experimental finding that with Only one ombenzene isomer results from the omation of benzene, since all atoms are equivalent. At first glance, it is consistent with the fact that there are three dibromobenzene isomers: On closer inspection, however, one recognizes that the molecule shown on the right is chiral, i.e. that is, there are four Ladenburg dibromobenzene isomers, two of which are mirror images of one another. Elements hemie upper level 901

Note: The hirality of the molecule shown on the right can be checked on a simple model: A rectangular piece of paper is divided into four quarters by folding and then two of the surfaces are glued together so that an equilateral triangular prism is created. Mark two of the corners with a felt-tip pen or similar, as in the molecule shown on the right. The corners are mirror-inverted on a second triangular prism. Turning tests reveals that the two models are actually different. Additional information on the figure B5 Some suggestions for the structure of the benzene molecule The Dewar benzene and Ladenburg benzene were synthesized later (see additional information. James Dewar published its structure as one of several possibilities for the empirical formula 6 6, but was based on his experiments believes that the structure of August Kekulé is correct. The Ladenburg benzene is now mostly referred to as the prismane. Discovery of benzene and the first benzene derivatives M. Faraday discovered this in 1825 while studying the undesirable liquid deposits in the oil gas of Gordon's portable gas lamps bicarburet of hydrogen. E. Mitscherlich and E.-M. Péligot obtained a liquid which was identical to Faraday's bicarburet of hydrogen when heating benzoic acid (from benzoin resin) with lime from 1833 to 1834. E. Mitscherlich called it gasoline, but J. Liebig's proposal for benzene prevailed, and Mitscherlich presented the first B enzene derivatives, inter alia. A. W. Ofmann isolated benzene from coal tar for the first time with chloro- and ombenzene, nitrobenzene, and benzenesulfonic acid. August Kekulé's description of his vision, which led to the setting up of the ring formula I turned the chair towards the fireplace and fell asleep. The atoms wobbled again before my eyes. This time, smaller groups stayed modestly in the background. My spiritual eye, sharpened by repeated visions of a similar kind, now distinguished larger structures of various forms. Long rows, often closer together; Everything in motion, twisting and turning like a snake. And see what was that? One of the snakes took hold of its own tail and the structure whirled scornfully in front of my eyes. I woke up as if by a bolt of lightning; this time, too, i spent the rest of the night working out the onsequences of the ypothesis. Hemehistorians, however, doubt whether this description is correct. yclic valence isomers of benzene About 100 years after the formulas of Dewar benzene and Ladenburg benzene had been established, the appropriate compounds were synthesized, which of course have completely different properties from benzene. These and a few other cyclic valence isomers of benzene are briefly presented below. 2 Dewar-Benzol Prisman Benzvalen Fulven Bicyclopropenyl-Isomere Dewar-Benzol (bicyclo [2.2.0] hexa-2,5-diene, see B5 in the student book) was first synthesized in 1963. It isomerizes to benzene with an idle time of 37 hours. The molecule is not flat, but angled (like a postcard folded in half). Prismane (tetracyclo [, 6.03,5] hexane, Ladenburg benzene, see B5 in the student book) was first synthesized in 1973. It is a colorless, volatile and explosive liquid. Prisman does not isomerize to benzene, since rearrangement is forbidden according to the Woodward-Offmann rules due to the rbital symmetry. Robert B. Woodward and Roald Offmann described Prisman as an angry tiger unable to break out of a paper cage. Benzvalene was first synthesized in 1971. The basic substance is explosive. Benzvalene isomerizes to benzene with an idle time of approx. 10 days. Fulvene (5-methylene-1,3-cyclopentadiene, pentafulvene) is a yellowish liquid that polymerizes easily. It belongs to the fulvene group of substances; these were discovered as early as 1900. Fulvene can be produced by a condensation reaction of yclopentadiene with formaldehyde, or by a photochemical isomerization of benzene. 902 elements high level

3 Bicyclopropenyl is a compound whose molecules consist of two linked yclopropenyl groups. Depending on the position of the double bonds, there are three isomers (from left to right in the figure): bicycloprop-2-enyl, bicycloprop-1,2-enyl and bicycloprop-1-enyl. All three isomers were first synthesized in 1989. The (relatively) most stable isomer, bicycloprop-2-enyl, polymerizes above 10. Literature E. Mitscherlich: About the benzene and the acids of the oil and tallow. Annalen der Pharmacie 9 (1834), readable online from (as of April 2020): J. Dewar: n the xidation of Phenyl Alcohol, and a Mechanical Arrangement adapted to illustrate Structure in the Nonsaturated ydrocarbons. Proceedings of the Royal Society of Edinburgh 6 (1869), readable online from (as of April 2020): Binding relationships in the benzene molecule For tasks A1 109.5 109, ethane ethene ethine benzene note: The more precise angles in ethene are 117.4 () and 121.3 (). A2 The particular stability of the benzene molecule is due to the delocalization of the six ring electrons. The delocalization means that the benzene molecule is significantly lower in energy than the hypothetical yclohexa-1,3-5-triene molecule. A3 a) Molecular formula of toluene: 7 8 Molecular formula of xylene: 8 10 b) Mesomeric limit formulas of the toluene molecule: Mesomeric limit formulas of the xylene isomers: Note: Toluene is also known as methylbenzene or phenylmethane. The three xylene isomers are called ortho-xylene (1,2-dimethylbenzene, top), meta-xylene (1,3-dimethylbenzene, middle) and para-xylene (1,4-dimethylbenzene, bottom). Elements hemie upper level 903

4 9.3 Mesomerism and Aromaticity Exercise A1 If you first look at the ethene molecule, each of the two carbon atoms is connected to the other carbon atom via a double bond and to two hydrogen atoms via single bonds. According to the electron pair repulsion model (EPA model), the double bond is treated like a single bond and thus the repulsion of only three electron clouds is considered. These have the greatest possible distance in a trigonal-planar arrangement with a bond angle of approx. In the case of the 1,3-butadiene molecule, one expects analogously that the two halves of the molecule are flat. However, the EPA model would allow free rotation around the single bond in the middle, so that the molecule as a whole would not be flat. However, the mesomeric boundary structures show that the middle - bond also has the character of a double bond: Delocalization is energetically more favorable than localization of the electrons in the terminal double bonds. If the EPA model is applied to the polar boundary structures and the central part of the molecule, a planar arrangement also results there. Hence the whole molecule is expected to be planar. Notes: Measurements have shown that in buta-1,3-diene the two outer bonds are slightly longer than would be expected with a normal double bond, while the middle bond is slightly shorter than with a normal single bond would be expected. According to the EPA model, the four atoms and the two middle atoms should lie in one plane. The four terminal atoms could also lie in planes perpendicular to the 4 plane. According to the rbital model, however, one expects that four terminal atoms are also in the 4 -plane. A2 Naphthalene: There is a continuous ring-shaped system of 5 conjugated double bonds. The number of delocalized electrons is therefore 10. This corresponds to the ückel rule, if you insert n = 2: 4 n 2 = 8 2 = 10 According to the ückel rule, naphthalene is an aromatic. Anthracene: There is a continuous ring-shaped system of 7 conjugated double bonds. The number of delocalized electrons is thus 14. This corresponds to the ückel rule if you insert n = 3: 4 n 2 = 12 2 = 14 Anthracene is an aromatic according to the ückel rule. 904 elements low level

5 9.4 Examples of aromatics For the exercises a yclohepta-1,3,5-triene 1-methyl-yclohexa-1,3,5-triene 1,2,3,4-tetramethyl-cyclobut-1-en no aromatic aromatic none Aromat A2 Pyrimidine: NN Ring-shaped molecule Continuous system of conjugated double bonds, i. That is, the electrons are delocalized The number of delocalized electron pairs is 3, consequently the number of delocalized electrons is 6. This corresponds to the ückel rule, if you insert n = 1: 4 n 2 = 4 2 = 6 pyrimidine is after the ückel-rule an aromatic. Since it contains nitrogen atoms, it is a heterocyclic aromatic. A3 a) Ring-shaped molecule Continuous system of conjugated double bonds, d. That is, the electrons are delocalized. The number of delocalized electron pairs is 9, consequently the number of delocalized electrons is 18. This corresponds to the ückel rule, if you insert n = 4: 4 n 2 = 16 2 = 18 The ring system is according to the ückel rule an aromatic. Note: The empirical formula is; the name of the compound is benzo [a] anthracene. b) or Both isomeric molecules () are circular, but the system of conjugated double bonds is not continuous. The ring systems as a whole are therefore not ückel aromatics. In the left molecule, however, the upper three six-membered rings can be viewed together as a ückel aromatic, i.e. that is, the molecule can be viewed as an anthracene derivative. In the right molecule, the two six-membered rings on the right can be viewed together as ückel-aromat, i.e. that is, the molecule can be viewed as a naphthalene derivative. Additional information Change in the term aromatic An aromatic can be recognized by its smell. (before 1825) characterized by high carbon content. (before 1865) a descendant of benzene. (Kekulé 1865) a compound whose reactivity is similar to that of benzene. (Erlenmeyer 1866) a compound whose ring-shaped molecules have an electron sextet. (Armit and Robinson 1925) found a compound that preferentially enters into substitution reactions. (from 1925) a compound whose planar and ring-shaped molecules have an electron system of (4 n 2) delocalized electrons with n = 0, 1, 2, 3. (ückel 1931) a compound in whose molecules a diamagnetic ring current can be induced . (from 1931) Elements hemie upper level 905

6 If the definitions given for the term aromaticity are reduced to the most important points, there are essentially three features that can serve as the basis for a meaningful differentiation: 1. the structure (a plane system), 2. the type of delocalized ring electron system and 3. the chemical behavior. Non-benzoic aromatics In addition to the classic aromatic compounds that are derived from benzene, there are other compounds with an aromatic character which, in contrast to benzene, have an odd number of atoms in the ring. These compounds are grouped together in one class, the non-benzoid aromatics. These are usually present as cations or anions and therefore have a different reaction behavior than benzene. If the aromatic state is to be defined beyond that of benzene, the chemical behavior cannot be used as a basis, but the physical properties must be considered. Aromatic compounds in the broadest sense are understood to mean cyclic, unsaturated ring systems in which all ring atoms are involved in a mesomeric system and which are characterized by a high mesomeric energy. Another essential criterion is the level or almost level construction of the ring system. Like the benzenoid aromatic compounds (4 n 2), all non-benzoid aromatic compounds have delocalized ring electrons (π-electrons). The simplest carbocyclic system of this class of substances is the yclopropenylium cation with two delocalized ring electrons (n ​​= 0). Both this and the more stable trisubstituted cations have been synthesized: A non-benzoid aromatic with six delocalized ring electrons is the yclopentadienide anion: A particularly stable compound, ferrocene, is formed when iron (II) chloride acts on the Grignard compound Magnesium bromide cyclopentadienide. It was first synthesized in 1951 (orange crystals, ϑ sm = 173) and belongs to the so-called sandwich compounds (metallocenes): Fe 2 [5 5] Mg Fel 2 Fe (5 5) 2 Mg 2 Mgl 2 The ycloheptatrienylium cation ( Tropylium cation) has six delocalized ring electrons. It can be described by seven limit formulas with the same energy: ückel rule Erich ückel (, professor for theoretical chemistry in Marburg) established a rule for conjugated monocyclic hydrocarbons (annulenes) in 1931, according to which those turn out to be particularly stable if have π-electrons (n ​​= 0, 1, 2). These are the aromatic systems. Systems with 4 n π-electrons are called antiaromatic. The particular stability of hydrocarbons with (4 n 2) π electrons (so-called ückel aromatics) is explained by the fact that in this case all binding π molecular orbitals are completely occupied and a closed-shell state is achieved, similar to that of the noble gas Atoms. This is shown in the following molecular orbital scheme (M scheme): 906 elements hemie berstufe

7 Number of electrons Energy E = 18 E = 14 E = 10 E 1 E = = 2 The M-scheme shown can be derived from the model of the electron in the one-dimensional box (see Section 1.8). In the present case, a ring-shaped lounge area is used as a basis. On this ring with the circumference L there should be a constant potential to which the value zero can be assigned. The total energy of an electron therefore only consists of kinetic energy: E = 1_ 2 mev 2 = mevmev 2 me = p 2 _ 2 me (with the momentum p = mev) If you consider the electron as a standing wave, the deflection of the wave must be adjusted one revolution around the ring have the same value again. This condition is fulfilled if the ring circumference L is an integer multiple of the wavelength λ n: L = n λ n (The quantum number n is an integer.) Since the ring circumference L is given by the geometry of the molecule, there are only certain wavelengths λ n possible. In the case of the ring-shaped box potential, n = 0 is also possible, where λ 0 must be viewed as infinitely large. Negative values ​​of n are also possible. With the De Oglie equation λ p = h or λ = h / p one can convert the wavelength of the electron into the momentum. The condition for a standing wave is then: L = n λ n = n h_ p n p n = n h_ L Because the wavelengths are quantized, only quantized pulses are possible. Inserted into the above equation for the kinetic energy results in: _ E n = pn 2 = 2 m n2 h 2 e 2 me L 2 Except for the lowest energy level E 0 (n = 0) there are two quantum numbers n for every further energy level with the same amount, but with a different sign. In other words: there is a lowest energy level that is not degenerate, and above it any number of energy levels, each of which is doubly degenerate. These energy levels correspond to the molecule orbitals in the M-scheme above. 2 electrons can be accommodated on the lowest energy level, 4 electrons each on all other. If the energy levels are either fully occupied or not at all, then the total number of electrons must be 4 n 2 with n = 0, 1, 2.(This n agrees with the quantum number used above, except for the fact that no negative values ​​are possible here.) With this, the ückel rule has found an elementary justification, which, strictly speaking, can only claim validity for systems from a ring. The prediction of the particular stability of a carbocyclic system with 2 π electrons (n ​​= 0) is an outstanding achievement of the ückel rule: the yclopropenylium cation was first produced in 1965 in the form of salt-like compounds. Elements hemie upper level 907

8 yclobuta-1,3-diene (see B1 in the student's book) is antiaromatic, i. That is, the number of its π-electrons satisfies the condition 4 n (with n = 1, 2, 3,). It is an extremely reactive molecule that can only be observed at all at extremely low temperatures. ycloocta-1,3,5,7-tetraene, a yellow liquid (ϑ sd = 152), is also very reactive and only stable at low temperatures. It is oxidized in the air, can easily be hydrogenated to yclooctane and undergoes electrophilic addition reactions. This type of reactivity shows that it is also not an aromatic compound. The yclo octa tetraene molecule is not referred to as an anti-aromatic because it is not flat, but rather a tub-shaped structure. The four conjugate double bonds are aligned orthogonally to one another. 117.6 148 pm 134 pm 126.1 Literature E. eilbronner ,. Bock: The M-Model and its application. V, Weinheim Excursion: The Benzene Molecule in the rbital Model Preliminary Remarks Concerning the Tasks A detailed introduction to the rbital model can be found in Chapter 1 of the student book. The representation in this excursus does not explain the red or blue coloring of the rbitals. It corresponds to a positive or negative sign of the wave function. This corresponds to the deflection of a classic wave upwards or downwards. For the sake of clarity, the areas of the hybrid orbitals with a negative sign are not shown in B2. A1 The six atoms of the molecule are assumed to be sp 2 hybridization. The three sp 2 hybrid orbitals lie in one plane and form an angle of 120. Two hybrid orbitals of one atom form a σ bond by overlapping with the hybrid orbitals of the two neighboring atoms. Another σ-bond is created by combining it with the s-rbital of an -atom. The six atoms and the six atoms thus form the σ-bond structure of the benzene molecule. The six p-rbitals that are not used for hybridization are perpendicular to the molecular plane. These six p-rbitals overlap to form a closed ring-shaped π-electron system. A2 In sp 3 hybridization, one s-rbital is computationally mixed with three p-rbitals. Four identical sp 3 hybrid orbitals are obtained. The axes of symmetry of the sp 3 hybrid orbitals are at tetrahedral angles (109.5) to one another. In sp 2 hybridization, one s-rbital is computationally mixed with two p-rbitals. Three identical sp 2 hybrid orbitals are obtained. The axes of symmetry of the sp 2 hybrid orbitals lie in one plane and form angles of 120. The axis of symmetry of the rest of the p-rbital is perpendicular to this plane. With the help of sp 3 hybridization, inter alia Constructing alkane molecules. With the help of sp 2 hybridization, inter alia Construct alkene molecules and aromatic systems. Note: The pupils can solve this task with the help of Kap. 908 elements high level

9 9.6 Alloyment of benzene To exercises A1 A2 The electrophilic substitution on aromatics and the electrophilic addition on alkenes have in common that an electrophilic attack takes place on the substrate. The reactive particles that attack the substrate are cations or regions of particles with a partial positive charge. Differences: With electrophilic substitution, the aromatic system is retained; with electrophilic addition, double bonds (or triple bonds) are broken. In electrophilic substitution, the main steps are an addition and an elimination step. In electrophilic addition, the first addition step is followed by a second addition step. A3 When adding omwasser, two liquid phases (layers) form in both cases, since the liquids do not dissolve in one another. Benzene does not react with the omwasser under the specified conditions, i. That is, the omwasser is not discolored and remains yellow. ycloxen reacts with the omwasser; the omwasser is discolored. An electrophilic addition takes place. Literature N. Vorwerk ,. Schmitt, M. Schween: Electrophilic substitution reactions on aromatics understand σ-complexes as (experimental) key structures. hemie specifically 22 (2/2015), 59 Peter Sykes: reaction mechanisms of organic hemie An introduction. V, Weinheim Types of reactions in aromatics To tasks A1 The electrophilic substitution on aromatics and the electrophilic addition on alkenes have in common that an electrophilic attack takes place on the substrate. The reactive particles that attack the substrate are cations or regions of particles with a partial positive charge. Differences: With electrophilic substitution, the aromatic system is retained; with electrophilic addition, double bonds (or triple bonds) are broken. In electrophilic substitution, the main steps are an addition and an elimination step. In electrophilic addition, the first addition step is followed by a second addition step. A2 When adding omwasser, two liquid phases (layers) form in both cases, as the liquids do not dissolve in one another. Benzene does not react with the omwasser under the specified conditions, i. that is, the water does not become discolored and remains yellow. ycloxen reacts with the omwasser; the omwasser is discolored. An electrophilic addition takes place (see B1 in the student book). Elements hemie upper level 909

10 A3 Reaction of toluene with om under the influence of light: 1. Start reaction 2. Reaction chain etc. 3. Termination reactions Under the influence of light (or at a higher temperature) one or more hydrogen atoms of the methyl group of toluene are replaced by om atoms. 1. Start reaction: Due to the energy supplied, a small part of the om molecules is split homolytically. A homolytic cleavage separates electron pair bonds in such a way that radicals (particles with unpaired electrons) are created. 2. Reaction chain: In the reaction chain following the start reaction, toluene and om-radicals (om-atoms) are formed alternately. When the reaction mixture is exposed to light or heated, many initial reactions are triggered, which is why many reaction chains take place next to one another in the mixture. 3. Termination reactions: In addition to the reactions that lead to the formation of omtoluene and hydrogen molecules, termination reactions also take place in which the radicals necessary for the reaction chain react with one another. The entire reaction is called radical substitution because of the radicals involved. Depending on the reaction conditions (duration, concentrations of the different particles, temperature), different numbers of hydrogen atoms are replaced. 910 elements low level

11 A4 In nucleophilic substitution, the substrate is attacked by a nucleophilic particle (that is, a negatively charged particle or a particle with a negative partial charge). In radical substitution, the attacker is a particle with one or more unpaired electrons, a radical. In nucleophilic substitution, an atom or a group of atoms is split off heterolytically from the substrate. This heterolytic cleavage can be the first step in a nucleophilic substitution before the nucleophilic particle is bound (S N 1). The binding of the nucleophile and the heterolytic cleavage of an atom or an atom group can, however, also take place simultaneously (S N 2). The radical substitution takes place as a chain reaction that is triggered under the influence of light or at a higher temperature. A5 a) Partial reactions from benzene to polystyrene: sulfuric acid catalyst 2 n dibenzoyl peroxide nb) The following reaction steps can be formulated for the reaction of ethene with benzene under the catalytic effect of sulfuric acid: Formation of an arbocation: 2 S 4 S 4 Electrophilic attack of the arbocation on the benzene molecule: Deprotonation and rearomatization: S 4 2 S 4 The sulfuric acid molecule reacts with the ethene molecule to form an arbo-cation and a hydrogen sulfate ion. The arbo-cation is a strongly electropile particle; it reacts with a benzene molecule. By reacting with the arbo-cation, the benzene molecule loses its aromaticity; a new arbo-cation is formed. The arbo-cation gives off a proton to a hydrogen sulfate ion. This creates an ethylbenzene molecule and the aromatic state is restored. The electrophilic substitution is now complete. Since the released proton has reacted with a hydrogen sulfate ion to form a sulfuric acid molecule, the catalyst is also restored. Elements hemie higher level 911

12 About the experiments V1 Two liquid phases (layers) are formed, the lower phase (omwasser) is initially yellow. When shaken, discoloration occurs immediately. 1,2-Dibromocyclohexane is formed. Instructions for implementation: If you want to demonstrate the reaction of pure om with yclo hexen, you have to note that it is very violent. Therefore, only very small drops are given to the cyclohexene from a pipette. (Caution: Due to its low surface tension and high density, om often drips from the pipette by itself.) After adding one drop, wait until the next addition. If the omtropfen comes into contact with the cyclohexene, a violent reaction ensues. There is always a little steam rising. In total, no more than three to four small drops should be added. The reaction must be carried out under the hood. V2 B3 in the student book shows the two possible reaction paths. a) The substitution takes place catalytically in the dark. Preferably 2-omtoluene and 4-omtoluene and a little 3-omtoluene are formed. Instructions for carrying out: The two partial experiments can each be carried out well in an Erlenmeyer flask (50 ml), which is closed with a suitable stopper. The three products have similar properties: 2-omtoluene: ϑ sm = 28 ϑ sd = omtoluene: ϑ sm = 40 ϑ sd = omtoluene: ϑ sm = 27 ϑ sd = 185 Warning: The hydrogen product irritates the eyes! b) In the light z. B. a slide projector or on a Verhead projector takes place in a radical reaction preferentially the omation of the methyl radical; benzyl bromide is formed. Instructions for implementation: Caution: The products are irritating to the eyes! This applies to hydrogen, but especially to benzyl bromide. Properties of the product benzyl bromide: ϑ sm = 4; ϑ sd = 201 Note on the reaction mechanism: Under the conditions of the radical reaction, the reaction with the methyl radical, i. H. the substitution, clearly preferred over the radical addition on the benzene nucleus. The reason for this is the stability and easy formation of the benzyl radical (). In contrast, in the yclohexadienyl radical, which would have to be formed on the nucleus in a radical reaction, the aromatic stabilization of the starting material would be abolished: Benzyl radical 2 3 yclohexadienyl radical 2 3 c) After both reactions, hydrogen can be detected in the flask, e.g. B. with damp universal indicator paper (turns red) or with a glass rod with a drop of ammonia hanging from it (ammonium bromide smoke). Literature N. Vorwerk ,. Schmitt, M. Schween: Electrophilic substitution reactions on aromatics understand σ-complexes as (experimental) key structures. hemie specifically 22 (2/2015), 59 Peter Sykes: reaction mechanisms of organic hemie An introduction. V, Weinheim Elementary level

13 9.8 Benzene derivatives For exercises A1 Phenol molecules can give off protons to water molecules in an acid-base reaction: This reaction can be explained by the stability of the phenolate ion, the corresponding base. The negative charge is not localized on the oxygen atom, but delocalized over the entire anion. Several limit formulas of the anion can be drawn up in which, in addition to the phenyl radical (6 5), a non-binding electron pair of the oxygen atom is also involved in the mesomerism. The phenolate ion is therefore mesomeric stabilized: Such a stabilization of the anion cannot occur with non-aromatic hydroxyl compounds. In the acid-base reaction of ethanol with water, the ethanolate ion would be formed as the corresponding base. With this ion, the entire negative charge would be concentrated on one atom: 2 3 Ethanol is therefore a very weak acid and practically does not react with water. In addition, one can also argue from the starting molecules: The phenyl group has an I-effect, which facilitates the heterolytic cleavage of the proton of the hydroxyl group. The ethyl group has an I-effect, which makes it difficult for the proton to split off from the hydroxyl group of the ethanol molecule. A2 N N addition: aniline (pk B = 9.4) is a weak base. An aniline molecule has a non-binding pair of electrons on the nitrogen atom and can thus bind a proton. Aniline is a much weaker base than other amines because the aniline molecule is mesomeric stabilized. The non-binding pair of electrons on the nitrogen atom is included in the mesomerism of the phenyl ring and is therefore poorly available for binding a proton. Diethylamine (pk B = 3.0) is a much stronger base than aniline, since the nitrogen atom is not bound to a phenyl ring, and consequently the non-binding electron pair is more available for binding a proton. The two ethyl groups also have an I effect, which increases the electron density on the nitrogen atom. Elements hemie upper level 913

14 A3 Benzoic acid (E 210) and its salts sodium benzoate (E 211), potassium benzoate (E 212) and alcium benzoate (E 213) inhibit the growth of fungi and ivy and are mainly found in pickled foods (mayonnaise and delicatessen products containing mayonnaise such as meat and sausage salads, marinades, canned vegetables (especially pickled vegetables), canned sour fruit and fruit juice concentrates). But they can also be found in ketchup, mustard, sausage and margarine as preservatives. Benzoic acid and sodium benzoate are also approved for preservation in tobacco products. Note: Benzoic acid and its salts are prohibited as preservatives in cat food and undefed. It should be strictly ensured that the catfish or the and are not fed food containing these preservatives. A4 Example: Catalytic oxidation of toluene with oxygen: III 0 Cat. III II II II (Only the oxidation numbers that change are written here. The -atom of the methyl group is oxidized: The oxidation number changes from III to III. The - Atoms are reduced: The oxidation number changes from 0 to II. As a result, it is a redox reaction. Note: To solve the problem, it is sufficient to consider the oxidation number of the atom without setting up a reaction equation. A5 Comparison of safety data sheets from April 2020: Hazard pictograms - sets Hazard classes and categories Benzene GS02 (flame) GS07 (exclamation mark) GS08 (health hazard) 340 May cause genetic defects. 350 May cause cancer. 225 Liquid and vapor highly flammable. 304 May be fatal if swallowed and enters airways. 315 Causes irritation. 319 Causes Do not cause serious eye irritation. 372 Causes damage to organs (blood) through prolonged or repeated exposure. 412 Harmful to aquatic life with long lasting effects. Flammable liquid, category 2 irritant effect on the aut, category 2 eye irritation, category 2 germ cell mutagenicity, category 1B carcinogenicity, category 1A specific target organ toxicity repeated exposure, category 1, blood aspiration hazard, category 1 long-term (chronic) aquatic hazard, category 3 aniline GS05 (caustic effect) GS06 (skull) GS08 (health hazard) GS09 (environment) Toxic if swallowed, in contact with or if inhaled. 317 May cause an allergic reaction. 318 Causes serious eye damage. 341 Suspected of causing genetic defects. 351 Suspected of causing cancer. 372 Causes damage to organs (blood) through prolonged or repeated exposure. 400 Very toxic to aquatic organisms. Carcinogenicity, Category 2 Germ cell mutagenicity, Category 2 Acute toxicity, Category 3, inhalation Acute toxicity, Category 3, aut Acute toxicity, Category 3, ral Specific target organ toxicity, repeated exposure, Category 1, blood Serious eye damage, Category 1 Sensitization through car contact , Category 1 Short-term (acute) hazardous to the aquatic environment, Category Elements hermetic level

15 Toxicity Assessment: Benzene can cause genetic defects and cancer. However, it is not classified as toxic; it only causes irritation. Aniline is poisonous and can presumably cause genetic defects and cancer. Overall, benzene seems to cause damage over the long term, while aniline is acutely more dangerous. Note: Obviously, long-term damage is viewed as more serious for admission to schools. According to D-GISS (as of March 2020), no experiments with benzene are allowed in schools. Aniline is even approved for student experiments, but with activity restrictions up to grade 4 and for pregnant or breastfeeding teachers and students. Concerning experiments V1 a) The solubility of phenol in water at 20 is 84 g / l. The phenol content in the first test tube is approx. 2 g / 0.005 l 400 g / l. By diluting to almost five times the volume, a clear solution is created. Their p-value is around 4.5.b) The addition of caustic soda creates phenolate ions which are readily soluble in water. By adding hydrochloric acid, the phenolate ions are converted back into less soluble phenol molecules. Notes: Phenol dissolves in water in all proportions above 68.8. Below this temperature, however, segregation occurs and the liquid becomes cloudy (emulsion). Two phases arise because it is a two-substance mixture with a miscibility gap. So z. B. at 25 an aqueous phase containing dissolved phenol with a mass fraction of 7 to 8%, and a phenolic phase in which about 28% water is dissolved in the phenol. At 40 there are two phases side by side, of which the aqueous phase has a mass fraction w (phenol) of at most 10%, the other a mass fraction w (water) of at most 33%. Phenol crystallizes out of the solution below the melting temperature of phenol (ϑ sm = 40) and a mass fraction w (phenol) of more than approx. 74%. The melting temperature of hydrous phenol depends on the water content: With a mass fraction of 2% water, the melting temperature of phenol is 33, with 6% water it is already 20. Additional tasks Additional task 1 Research and draw the structural formulas of the benzene derivative phthalic acid and phenylalanine. Solution: N phthalic acid phenylalanine additional task 2 Research the structure of the benzene derivative DDT. Explain the problem that arises from the use of DDT. Solution: The DDT molecule (dichlorodiphenyltrichloroethane) contains two aromatic rings and five chlorine atoms. DDT has high toxicity to insects, but low toxicity to mammals. Therefore, it has long been used as an insecticide on a large scale. The development of resistance and the recognition of the negative effects of DDT on the environment led to the fact that DDT is now only used to combat the malaria mosquito (Anopheles). DDT is lipophilic and chemically very stable. The stability means that DDT is difficult to biodegrade. If DDT gets into the food chain, it can accumulate in the tissues of animals and humans due to its fat solubility. DDT and its breakdown products have an endocrine effect, i. That is, they can act like ormons or inhibit the body's own ormons. This can have fatal consequences for organisms with a high DDT concentration, e.g. B. Too thin eggshells in birds. l l l l l dichloro-diphenyl-trichloroethane (DDT) element inhibitor level 915

16 Additional experiment literature Oxidation of toluene (hood! Safety glasses!) A few drops of toluene are shaken vigorously with 10 ml of dilute potassium permanganate solution and a few drops of conc. Sulfuric acid. The solution is then gently warmed. One carefully examines the smell. Observation: Due to the oxidation of toluene, the characteristic smell of benzaldehyde (bitter almond aroma) appears after a short time. K. üner: 75 years of DDT. hemie in our time 48, 3 (June 2014), second substitution on aromatics To the tasks A1 M-effect: N 2 ,,, R, S 3, N M-effect: ,, R, N 2, NR 2, 6 5 , 2 = 2, F, l ,, I Note: The solution can be found in the student book (right margin). 1st order substituents usually have an M effect (only the methyl group does not); Second order substituents have an M effect. The methyl group does not have an M effect, but an I effect. In part of the edition of the student book, the I-effect is not mentioned in the margin column. It can be used to explain why the methyl group is also a first-order substituent, see additional information. A2 The atom is a first-order substituent and thus ortho / para-directing. If one considers the mesomeric limit formulas of the arbo-cation with the three possibilities of the position of the second substituent, one recognizes that with the second substitution in the ortho or para position there is a limit formula in which the -atom carries the positive charge. In the case of a second substitution in the meta position, there is no limit formula with the positive charge on the atom: Only in the case of a second substitution in the ortho or para position is the positive charge delocalized beyond the ring. These arbo-cations are therefore more stable than the arbo-cation in the case of a second substitution in the meta position. This is why ombenzene is preferably substituted in the ortho and para positions. A3 Statistically, one would expect that a second substitution would produce twice as much ortho as para product, since there are two ortho positions and only one para position on the ring. However, the second substituent sometimes takes up the same space as the first substituent. Second substitution in the para position is therefore preferred. Note: This effect is known as steric change. 916 elements low level

17 A4 N 2 F N 2 The ring system is deactivated. Reason: The nitro groups have an M and I effect. The fluorine atom has an I effect. 3 The ring system is activated. Reason: The hydroxyl group has an M effect that outweighs the I effect. The methyl group has an I effect. N 2 The ring system is activated. Reason: The amino group has an M effect that outweighs the I effect. The hydroxyl group exerts an M-effect, which outweighs the I-effect. S 3 N 2 The ring system is deactivated. Reason: The sulfonate group has an M and I effect. The nitro group exerts an M and I effect. Additional task Explain (with the help of Section 9.7) how one could produce (a) 2- and 4-chlorotoluene or (b) chlorophenylmethane from toluene as the starting material. Also draw the structural formulas of the three compounds. Solution: a) Toluene reacts with chlorine in the presence of a suitable catalyst to form 2- and 4-chlorotoluene (KKK rule: cold, catalyst, nucleus; the methyl group is ortho / para-directing). b) Under the influence of light or at a higher temperature, one or more hydrogen atoms in the methyl group are replaced by chlorine atoms (SSS rule: sunlight, boiling point, side chain). It arises, inter alia. chlorophenylmethane. l l l 2-chlorotoluene (o-chlorotoluene) 4-chlorotoluene (p-chlorotoluene) chlorophenylmethane elements hemi-level 917

18 Additional information Influence of alkyl groups as first substituents Due to their I-effect, alkyl groups are activating and ortho / para-directing. In the student book it is only shown to what extent the conducting is achieved through the M-effect. Using the example of the methyl group, the following shows how the I-effect is also directed. For the three possible cases of second substitution, three mesomeric limit formulas of the arbo-cation can be drawn up: Second substitution in ortho position: Second substitution in meta position: Second substitution in para position: In the case of a second substitution in ortho and in para position, the positive Charge on the atom carrying the methyl group is reduced by the I effect of the methyl group. This stabilizes these arbo-cations. In the case of a second substitution in the meta position, there is no limit formula with the positive charge on the atom that carries the methyl group. Consequently, the second substitution in the ortho and in the para position is preferred. Influence of the analogous atoms as first substituents alogeneic atoms as first substituents have a deactivating effect. The electronegativity (and thus the I-effect) decreases in the fluorine-to-iodine series, but the M-effect also decreases due to the increasing size of the atoms, so that the I-effect predominates in all alogenic atoms. Nonetheless, alogenic atoms are first-order substituents (i.e. ortho / para-directing), since they also exert an M effect due to their non-bonding electron pairs (see also solution to A2). Product proportions in the second substitution In the second substitution, the orthometa- and para-product practically always occurs, but with very different proportions. The following table shows some examples: Educt Reaction ortho-product meta-product para-product acetanilide (6 5 N 3) nitration 21% traces 79% anisole (methoxybenzene,) nitration 40% 2% 58% chlorobenzene (6 5 l) nitration 29% 1% 70% ombenzene (6 5) conversion 13% 2% 85% benzoic acid (6 5) nitration 18.5% 80.0% 1.5% Literature N. Vorwerk ,. Schmitt, M. Schween: Electrophilic substitution reactions on aromatics understand σ-complexes as (experimental) key structures. hemie specifically 22 (2/2015), 59 Peter Sykes: reaction mechanisms of organic hemie An introduction. V, Weinheim Elementary level

19 9.10 ASA, a medicinal product of the century To tasks A1 By splitting the glycosidic bond of salicin and oxidation one obtains: Salicylaldehyde 2 b-d-glucose A2 Known side effects of acetylsalicylic acid are e.g. E.g .: heartburn, nausea and vomiting, diarrhea, bleeding, iron deficiency, gastrointestinal ulcers, allergic reactions, gout attacks (in gout patients). Literature. G .. Becker et al .: rganikum rganischchemisches Grundpraktikum. German publishing house of the sciences, Berlin. From the 20th edition: Wiley V, Weinheim M.A. Fox, J.K. Whitesell: rganische hemie (fundamentals, mechanisms, bio-organic applications). Spectrum Akad. Verl., Eidelberg, Berlin, xford L. Gattermann, Th. Wieland: Practice of the organic hemikers, part 1. Walter de Gruyter, Berlin G. Vollmer (rsg.): Aspirin and Bayer AG Elberfeld in hemie in Wuppertal and surroundings A. Düntsch: No headache from organic hemia with aspirin. Mathematical and scientific teaching 53 (7/2000), 414 A. Kleemann ,. ffermanns: Milestone salicylic acid synthesis. hemie in our time 46, 1 (February 2012), U. Gessner (rsg.): Medicines and hemie teaching materials for a contemporary hemie lesson. Bayer Vital Gmb, Available from (as of July 2020): Practical course: Acetylsalicylic acid To the experiments V1 Synthesis of ASA Task solution To (a): The actual synthesis of ASA takes place here. B3 in Chap. Shows the reaction equation. Regarding (b): Acetylsalicylic acid is very poorly soluble in cold water, so that it can be filtered off with good yield. To (c): The slow evaporation of the ethanol produces larger crystals. Notes: Acetic anhydride (acetic anhydride) was not allowed in student experiments at times, for this reason part of the experiment (a) is described in such a way that it is carried out by the teacher. According to D-GISS (as of January 2020), there is a ban on activities for pupils up to and including grade 9 and activity restrictions for pregnant and breastfeeding teachers. The decision as to whether the experimental part (a) can be carried out by the students rests with the teacher, who should find out about the current regulations. A double hour is sufficient for the experiment if hot water for the water baths and ice water are available. The reaction time given in the literature is two hours, but usable yields are obtained in just two hours. It is advisable to first add the sulfuric acid (2 to 3 drops) to the acetic anhydride (ρ = 1.087 g / ml at 15) and then add this mixture to the salicylic acid. If you give the conc. Sulfuric acid to salicylic acid, this is partially charred. Caution: Immediately after the components have been combined, a strongly exothermic reaction sets in and there is a risk of splashing. Only after this reaction has ended is the mixture heated on the water bath. It is therefore beneficial to add the acetic anhydride with the sulfuric acid in small portions to the round bottom flask with the salicylic acid and to put the reflux condenser on immediately. Elements hemie upper level 919

20 Using glacial acetic acid instead of acetic anhydride reduces the risk of splashing. However, the reaction time is then longer and problems with crystallization can occur (see below). When using salicylic acid that has not been dried beforehand or contaminated salicylic acid, the product does not precipitate in crystalline form in ice water, but a slightly yellowish, oily liquid is formed. If you put the beaker in the refrigerator overnight at about 6, i. d. As a rule, until the next day a crystal slurry that can be separated. Attention: Salicylic acid sublimes when drying in the drying cabinet. It is therefore recommended that the salicylic acid be concentrated for a few days. Sulfuric acid to dry in the desiccator. In a general working instruction for the preparation of acetic acid esters from acetic anhydride (cf. rganikum) there is the recommendation to carry out the reaction with freshly distilled acetic anhydride. However, the implementation also succeeds if the anhydride is not distilled beforehand. If the anhydride is to be cleaned first, it is distilled over anhydrous sodium acetate. This can be obtained by evaporating the water of crystallization from sodium acetate containing water of crystallization (3 mol of water). The anhydrous melt is poured onto an iron plate, allowed to solidify, the salt is pulverized and it is stored in the desiccator. ASA dissolves well in ethanol and can be recrystallized from a solution of ethanol in water (volume ratio 1: 4). However, this significantly reduces the yield. Under no circumstances should the ethanolic solution be heated more strongly, otherwise ASA will noticeably hydrolyze. A solution of 1,4-dioxane in water (volume ratio 1: 1) is more suitable. After recrystallization once, the yield here is between 70 and 50% (literature value: 85%). Warning: Dioxane is harmful to health. Schoolchildren experiments with dioxane are not prohibited, but testing for substitutes is of particular importance. Therefore, ethanol should be preferred. When acetic anhydride is used as the acetylating agent, the following reaction mechanism can be formulated for the esterification: Acetic anhydride Acetylsalicylic acid 920 elements hemie upper level V2 Experiments with the product from V1 Solution to (b): Observation: The p value of the product from V1 lies between Values ​​of salicylic acid and ASA. Values ​​determined with the p-meter: p (ASS) 3.1 p (salicylic acid) 2.4 p (product) 2.6 Interpretation: The product probably still contains salicylic acid. Note on implementation: A p-meter is recommended to compare the p-values. With universal indicator paper it is very difficult to tell a difference between the three solutions and the determination of the p-values ​​is imprecise. Alternatively, the p-values ​​can also be determined on the spot plate or in another vessel using a liquid universal indicator. Re (c): Observation: In the case of salicylic acid, the solution turns dark violet, in the case of pure acetylsalicylic acid, it turns slightly brownish red, in the case of the product produced, it turns brownish violet. Interpretation: The color reaction with iron (III) chloride solution shows that the product probably still contains salicylic acid.