Why are strong acids unable to undergo reversible reactions?
9. Alcohols, ethers and phenols
In this chapter we will focus on the following; 1) structure, reactions and representation and 2) alcohols and phenols as acids and bases.
According to the IUPAC nomenclature, alcohols are treated as alkane derivatives. The ending -ol is added to the name of the alkane. In complex branched systems, the name of the alcohol is based on the longest chain, which carries the OH substituent :
To determine the position of the functional group in the molecule, start counting so that the OH group has the lowest possible number. The names of the substituents are placed in front of the alkanol:
Like alkyl substituents and haloalkanes, we divide alcohols into primary, secondary and tertiary Alcohols a:
Compounds with hydroxyl groups on the benzene ring are called phenols. When naming the substituted benzenes, the carboxy and carbonyl groups have a higher priority than the hydroxyl group:
The IUPAC nomenclature treats ether as an alkane with an alkoxy substituent, i.e. as an alkoxyalkane. The smaller substituent is considered part of the alkoxy group, the larger substituent forms the stem of the molecule:
9.2 Properties of alcohols, phenols and ethers
The structure of alcohols is similar to that of water. The structures of methanol, water and dimethyl ether are compared below:
The electronegativity of the oxygen atom causes an uneven charge distribution in the molecule, so that a dipole moment similar to that of water arises:
Compared to alkanes and chloroalkanes, alcohols have different physical properties: the higher boiling points are particularly striking.Boiling points of various alkanes, chloroalkanes and alcohols
|Alkyl group, R||Alkanes, R-H||Chloroalkanes, R-Cl||Alcohols, R-OH|
Phenols and aromatic hydrocarbons also have different boiling points: e.g. toluene bp 110OC, phenol bp 182OC.
For its molecular mass, water has an unusually high melting and boiling point, which, compared to other intermolecular forces, is explained by the formation of strong hydrogen bonds. These are formed between the oxygen atom of one molecule and the hydroxy hydrogen atom of another molecule. A regular T is formed in the crystalline stated-Lattice, with a maximum number of stable H-bridges. When warming up, this grid is only partially broken open at first, which leads to an even more densely packed state (at 4OC), which is then loosened up more and more (> 4OC):
Alcohols can also form networks of hydrogen bonds in the liquid state:
This interaction leads to a wide network of such linked molecules. Although a H-bond is much weaker (21 KJ / mol) As a covalent O-H bond (435 KJ / mol, the large number of hydrogen bonds present make the boiling process more difficult. This leads to relatively high boiling points. This effect is even more pronounced in water.
9.3 Acids and Bases
Alcohols and phenols are weak acids. But they can also react in a basic way, in that the free electron pair of the oxygen atom binds a proton (see also Section 8.5.3). What do acid and base mean, and how do their strengths compare?
According to the concept of Brønsted acids are substances that can transfer a proton. A base is a substance that can accept a proton - it must have a lone pair of electrons in order to form a σ-bond with H.+ to be able to form, e.g .:
The strength of an acid or base (in water) is through their Acidity constant Ka or pKadefined, e.g .:
The constant pKa is the pH for the case [HA] = [A-]. The smaller the pKa-Value, the stronger the acid. A weaker acid has a higher pKa.
If you are looking at a weak acid (e.g. ethanol) then its conjugate base is a strong base. If you are looking at a strong acid (e.g. HCl) then its conjugate base is a weak base:Relative strength of different acids and their conjugate bases
According to the concept of Lewis acids are substances that can accept a pair of electrons (Electron pair acceptors). In contrast, a Lewis base is a substance that can transfer a pair of electrons (Electron pair donors).
9.4 Alcohols and phenols as acids
When you dissolve an alcohol or a phenol in water, an equilibrium arises:
If we want to compare their strengths as acids, we need to look at their acidity constant:
|Alcohol or phenol||pKa|
|HIGH||[ 15.74 ]*|
|CH3COOH||[ 4.72 ]*|
|HCl||[ -7.00 ]*|
* As comparison
But notice! With alkali metals one takes place completely irreversible Reaction instead of:
Why are phenols stronger acids than alcohols?
The reason for this must be that the phenoxide anion is more stable compared to a phenol than an alkoxide anion is compared to an alcohol. The phenoxide anion is stabilized by resonance (mesomeric effect):
If the benzene ring also bears a substituent, the phenol can either be an even stronger acid or a weaker acid than the phenol itself:
Already at pH 7 it is almost up to 50% in the phenoxide form in the water.
Ether do not have a hydrogen atom on the O and are therefore not acidic. You behave neutrally.
9.5 Representation of alcohols
Alcohols occupy a central position in organic chemistry. They can be made from various functional groups and can be converted into numerous other functional groups.
What methods have we seen before?
1) Hydration of alkenes (see chapter 4.3):
(Warning - the reaction is reversible, and conditions that favor the desired direction must be found).
2) Oxidation of alkenes with OsO4 (see chapter 4.5):
3) Nucleophilic substitution reactions (see Chapter 8)
An important new approach to alcohols that we have not yet encountered is the reduction of carbonyl compounds. (The mechanisms of reactions at carbonyl groups will only be discussed in Chapters 10-12.
i) Through the reduction of aldehydes and ketones
This process can be described as follows:
There are several reagents that are suitable for these reactions. We're going to look at two because they have quite a broad scope:Reduction with sodium borohydride (NaBH4):
Reduction through heterogeneous catalysts and hydrogen:
These conditions are also used for the reduction of alkenes and alkynes to alkanes (see Section 4.5). But alkenes and alkynes cannot work with NaBH4 be reduced.
ii reduction of esters and carboxylic acidsThe reduction of esters and acids can be described in the following way (will be discussed later):
iii Nucleophilic addition of Grignard reagents to carbonyl compoundsThis topic is carefully dealt with in the next few chapters. We should note here that Grignard reagents for the synthesis of 1O-, 2O- and 3O-Alcohols can be used,
9.6 Preparation of ethers from alcohols
The easiest way to produce ethers is through the reaction of an alkoxide with a primary haloalkane or a sulfonic acid ester (mesylate or tosylate - see Section 8.4.2) under typical SN2 conditions. This procedure is called Williamson ether synthesis known, e.g .:
The alkoxides can be generated from the corresponding alcohol and Na. Since alkoxides are strong bases, their use in ether synthesis is limited to primary, unhindered systems, as otherwise a considerable proportion of E2 product would be produced.
9.7 reactions of alcohols
We have already got to know elimination reactions for the synthesis of alkenes on (see Chapter 8.5). Here we will cover the mechanisms of dehydration, e.g .:
Oxonium ions are formed by protonation of alcohols. Secondary and tertiary oxonium ions can then split off water and form relatively stable secondary or tertiary carbenium ions:
9.7.2 Formation of haloalkanes
Primary oxonium ions, on the other hand, are quite stable with regard to this further reaction, since the resulting primary carbenium ion is too energetic (unstable). However, they can be attacked by a nucleophile (see chapter 8.4.2):
2O- and 3O-Alcohols can also be reduced by treatment with HCl or HBr at 0OC (now pN1) convert into haloalkanes. It is best to 1O- and 2O-Alcohols by treatment with SOCl2 or PBr3 converted to haloalkanes (see Section 8.2).
9.7.3 OXIDATION of alcohols
Section 9.5 describes the preparation of alcohols from aldehydes, ketones and carboxylic acids that have been reduced with hydrogen (only RCHO and RCOR) or hydrides. The reverse reaction, the OXIDATION of alcohols to aldehydes, ketones or carboxylic acids, is also possible. Such OXIDATION REACTIONS form one of the most important reaction classes that are possible for alcohols, e.g .:
Primary alcohols can be oxidized either to aldehydes or to carboxylic acids; which product is formed depends on the reagents and reaction conditions. Secondary alcohols only provide ketones.
A frequently used reagent for the oxidation of alcohols is chromium (VI), a transition metal in a high oxidation state. In this form, chrome is usually yellow to orange. When reacted with an alcohol, chromium (VI) is reduced to deep green chromium (III).
e.g. Alcohol to aldehyde:
Further oxidation to acid can be avoided if you work anhydrous, since the aldehyde is then stable:
If the oxidation is carried out in aqueous solution, starting from 1OAlcohols carboxylic acids, e.g. B .:
The aldehyde occurs as an intermediate, but under these reaction conditions it is usually further oxidized to the acid. 2O-Alcohols can be oxidized to ketones either under anhydrous conditions or in aqueous solution:
9.8 reactions of ether
Ethers are quite inert. They are inert under most conditions and are therefore often used as solvents. However, some ethers react slowly with oxygen to form hydroperoxides and peroxides by a radical mechanism. Peroxides are dangerous because they can decompose explosively.
Although ordinary ethers are relatively inert, the strained ring of epoxides (oxacyclopropane) can undergo a variety of ring-opening reactions with nucleophiles. Epoxides react very quickly under mildly acidic conditions:
Two million tons of ethylene glycol for automotive antifreeze is produced annually from ethylene oxide through acid-catalyzed hydration.
9.9 Phenols: preparation and reactions
The direct electrophilic addition of OH to benzenes is difficult because reagents that contain an electrophilic OH group such as OH+ are very rare. But you can prepare phenol from sodium benzene sulfonate or chlorobenzene by heating in molten NaOH (mechanism is not discussed):
Phenols cannot be acid dehydrated, and they cannot be converted to aryl halogens with HX. But phenols are included in the Williamson ether synthesis (see Section 9.6). As electron-rich aromatics, phenols quickly enter into electrophilic substitution reactions such as nitration, sulphonation and halogenation (see also Section 5.5).
o- and p-dihydroxybenzenes (Hydroquinones) can easily be oxidized and thus form Quinones (Quinones) e.g.
Such quinones are found in important natural substances such as ubiquinone (coenzyme Q) and vitamin K.
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