Dehydrogenation (Dehydration) of Alcohols

What is Dehydration of Alcohols?

Alcohol upon reaction with protic acids tends to lose a molecule of water to form alkenes. These reactions are known as dehydrogenation or dehydration of alcohols.

It is an example of an elimination reaction. Its rate varies for primary, secondary and tertiary alcohols. This variation of rate can be attributed to the stability of carbocation generated. Since the carbocation is most stable in the case of tertiary alcohols, the rate of dehydration is highest for tertiary alcohols in comparison to secondary and primary alcohols.

Dehydrogenation is the removal of hydrogen from the feedstock, such as the treatment of paraffin for the production of olefin. The degree of dehydrogenation during thermal cracking of petroleum varies with the starting material and operating conditions, but due to its practical significance, methods have been found to increase the level of dehydrogenation and, in some cases, to make it almost the only reaction.

Dehydrogenation

Dehydrogenation is one of the most important processes in the chemistry of petroleum because it turns the starting inert alkanes into olefins and aromatic compounds, starting points towards other functional groups.

Catalytic dehydrogenation plays an significant role in the development of olefin light (C3–C4 carbon range), detergent range (C10–C13 carbon range), and dehydrogenation to styrene by ethylbenzene. During the Second World War, catalytic dehydrogenation of butane over a catalyst for chromium – alumina was practiced to generate butenes that were dimerized to octenes and hydrogenated to octans to yield high octane aviation fuels.

Dehydrogenation is a highly endothermic process, and as such, a restricted reaction to the equilibrium. While important aspects of dehydrogenation include reaching equilibrium or conversion to near-equilibrium while reducing side reactions and coke formation.

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Dehydrogenation Reaction

Dehydrogenation reactions in the presence of oxygen are conducted on silver catalysis to transform alcohols into the corresponding aldehydes. The reaction types can be extended to prepare fine chemicals thus dec-9-en-1-ol was on silver catalysis with good yields. In contrast, dehydrogenation reactions can be conducted in the absence of oxygen on platinum or palladium catalysts to aromatize substituted cyclohexyl or cyclohexenyl compounds. Thus, in the field of fine chemistry p-cymene was obtained by dehydrogenation of limestone with a 67% yield on active carbon supported Pd catalysts.
The dehydrogenation reactions are endothermic and require appropriate heat input. Dehydrocyclization reaction is slower than dehydrogenation per sec, which requires higher temperature being the most difficult reaction in catalytic reforming. Skeletal isomerization reactions are very mildly exothermic. Hydrocracking is exothermic being favoured at high temperatures and high hydrogen pressures. In dehydrogenation pressure increases and decreases conversion. Dehydrogenation is also preferred at high temperatures.
Cyclohexane dehydrogenation, similar to other dehydrogenation reactions, is an endothermic and equilibrated reaction, which means that its conversion is limited to thermodynamics and increases with temperature. Temperature increases mean higher energy consumption and an increase in side reactions and coke formation. Considering that the removal of hydrogen from the reaction side causes an increase in conversion, the membrane reactor is a potential candidate for this reaction.

Dehydration Mechanism Steps

If either Raney-Ni, Al(i-ORr)3, or alumina are absent from the catalytic mixture, secondary alcohol dehydrogenation reaction to ketones does not occur. When the Raney-Ni is substituted with other Ni(II) salts I e. NiCl2) or complexes I e. Ni(PPh3)2Cl2), no reaction is noted.

Dehydration of alcohols follows a three-step mechanism.

  1. Formation of protonated alcohol
  2. Formation of carbocation
  3. Formation of alkenes

General dehydration reaction of alcohols can be seen as,

Dehydration Reaction

Mechanism of Dehydration of Alcohols:

Dehydration of alcohols can follow E1 or E2 mechanism. For primary alcohols, the elimination reaction follows E2 mechanism while for secondary and tertiary alcohol elimination reaction follows E1 mechanism.

Generally, it follows a three-step mechanism. The steps involved are explained below.

1. Formation of protonated alcohol:

In this step, the alcohol is acted upon by a protic acid. Due to the lone pairs present on the oxygen atom it acts as a Lewis base. Protonation of alcoholic oxygen takes place which makes it a better leaving group. It is a reversible step which takes place very quickly.

Carbocation Formation

2. Carbocation formation:

In this step, the C-O bond breaks generating a carbocation. This step is the slowest step in the mechanism of dehydration of an alcohol. Hence, the formation of the carbocation is considered as the rate-determining step.

Formation of Protonated Alcohol

3. Alkene formation:

This is the last step in the dehydration of alcohols. Here the proton generated is eliminated with the help of a base. The carbon atom adjacent to the carbocation breaks the existing C-H bond to form C=C. Thus, an alkene is formed.

Alkene Formation

Dehydrogenation and rehydrogenation reactions are reversible and the reagents and components are recyclable as a result of which the device can be used more efficiently as a hydrogen supply network technology compared to other hydrogen storage materials.

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