Dehydrogenation reactions of alcohols are necessary to obtain aldehydes and ketones. Ketones are derived from secondary alcohols, and aldehydes from primary alcohols. Copper, silver, copper chromites, zinc oxide, etc. are used as catalysts in the processes. It should be noted that, in comparison with copper catalysts, zinc oxide is more stable and does not lose activity during the process, but it can provoke a dehydration reaction. In general, the reaction of dehydrogenation of alcohols can be represented as follows:

In industry, the dehydrogenation of alcohols produces compounds such as acetaldehyde, acetone, methyl ethyl ketone and cyclohexanone. The processes take place in a stream of water vapor. The most common processes are:

1. carried out on a copper or silver catalyst at a temperature of 200 - 400 ° C and atmospheric pressure. The catalyst is any Al 2 O 3, SnO 2 support or carbon fiber on which silver or copper components are supported. This reaction is one of the components of the Wacker process, which is an industrial method for producing acetaldehyde from ethanol by its dehydrogenation or oxidation with oxygen.

2. can proceed in different ways, depending on the structural formula of its parent substance. 2-propanol, which is a secondary alcohol, is dehydrated to acetone, and 1-propanol, as a primary alcohol, is dehydrogenated to propanal at atmospheric pressure and a process temperature of 250 - 450 ° C.

3. also depends on the structure of the starting compound, which affects the final product (aldehyde or ketone).

4. Dehydrogenation of methanol... This process is not fully understood, but most researchers identify it as a promising process for the synthesis of formaldehyde that does not contain water. Various process parameters are offered: temperature 600 - 900 ° C, active catalyst component zinc or copper, support silicon oxide, the possibility of initiating the reaction with hydrogen peroxide, etc. At the moment, most of the formaldehyde in the world is produced by the oxidation of methanol.

The generally accepted mechanism for the dehydration of alcohols is as follows (for simplicity, ethyl alcohol is taken as an example):

The alcohol attaches a hydrogen ion to step (1) to form a protonated alcohol, which dissociates from step (2) to give a water molecule and a carbonium ion; then the carbonium ion step (3) loses the hydrogen ion and an alkene is formed.

Thus, the double bond is formed in two stages: the loss of the hydroxyl group in the form [step (2)] and the loss of hydrogen (step (3)). This is the difference between this reaction and the dehydrohalogenation reaction, where the elimination of hydrogen and halogen occurs simultaneously.

The first stage is the Bronsted-Lowry acid-base equilibrium (Section 1.19). When sulfuric acid is dissolved in water, for example, the following reaction occurs:

The hydrogen ion passed from a very weak base to a stronger base to form the oxonium ion. The basic properties of both compounds are, of course, due to the lone pair of electrons that can bind the hydrogen ion. Alcohol also contains an oxygen atom with a lone pair of electrons and its basicity is comparable to that of water. The first stage of the proposed mechanism can most likely be represented as follows:

The hydrogen ion passed from the bisulfate ion to a stronger base (ethyl alcohol) to form the substituted oxonium ion of the protonated alcohol.

Similarly, stage (3) is not the expulsion of the free hydrogen ion, but its transition to the strongest of the available bases, namely to

For convenience, this process is often depicted as the addition or elimination of a hydrogen ion, but it should be understood that in all cases, in fact, a proton is transferred from one base to another.

All three reactions are shown as equilibrium, since each stage is reversible; As will be shown below, the reverse reaction is the formation of alcohols from alkenes (Section 6.10). Equilibrium (1) is shifted very much to the right; it is known that sulfuric acid is almost completely ionized in an alcoholic solution. Since the concentration of carbonium ions present at each moment is very low, equilibrium (2) is shifted strongly to the left. At some point, one of these few carbonium ions reacts according to equation (3) to form an alkene. Upon dehydration, the volatile alkene is usually distilled off from the reaction mixture, and thus equilibrium (3) is shifted to the right. As a result, the whole reaction comes to an end.

The carbonium ion is formed as a result of the dissociation of the protonated alcohol; in this case, the charged particle is separated from

a neutral particle Obviously, this process requires much less energy than the formation of a carbonium ion from the alcohol itself, since in this case it is necessary to separate the positive particle from the negative one. In the first case, the weak base (water) is cleaved from the carbonium ion (Lewis acid) much easier than the very strong base, the hydroxyl ion, i.e., water is a better leaving group than the hydroxyl ion. It has been shown that the hydroxyl ion is almost never cleaved from alcohol; the reaction of cleavage of a bond in alcohol in almost all cases requires an acidic catalyst, the role of which, as in the present case, is to protonate the alcohol.

Finally, it should be understood that the dissociation of the protonated alcohol becomes possible only due to the solvation of the carbonium ion (cf. Section 5.14). The energy for breaking the carbon-oxygen bond is taken due to the formation of a large number of ion-dipole bonds between the carbonium ion and the polar solvent.

Carbonium ion can enter into various reactions; which one occurs depends on the experimental conditions. All reactions of carbonium ions are completed in the same way: they acquire a pair of electrons to fill an octet of a positively charged carbon atom. In this case, a hydrogen ion is split off from a carbon atom adjacent to a positively charged, electron-depleted carbon atom; a pair of electrons that previously made a bond with this hydrogen can now form a -bond

This mechanism explains acid catalysis during dehydration. Does this mechanism also explain the fact that the ease of dehydration of alcohols decreases in the series tertiary secondary primary? Before answering this question, it is necessary to find out how the stability of carbonium ions changes.