What is meant by the acidic dehydration of alcohols?

A dehydration reaction as the name suggests is a reaction where compounds(organic or inorganic) undergo a loss of water molecules during thisparticular reaction. In most organic reactions the organic compounds when they lose water molecules they usually form an alkene as a product which is still an organic compound containing a C=C (Carbon-Carbon double bond).There can be many products using such type of reaction depending on the conditions, catalysts used, and so on. For alkene preparation, usually occurs under an acidic medium. This is known as the acidic dehydration reaction or acid-catalyzed dehydration reaction of alcohol. An example of the conversion of alcohol into alkene iswhen cyclopentanol which is an organic compound under acidic conditions loses a water molecule leading to the formation of cyclohexene. The reaction is shown below for a clearer understanding:

Under acidic conditions dehydration of cyclopentanol leads to the formation of cyclopentene

The dehydration reaction is also called the dehydrogenation reaction and is considered to be one of the most important processes that play a major role in chemistry applications like e.g., the Chemistry of petroleum. Here, the use of this reaction helps in the conversion of inert alkanes into required olefins for specific purposes as well as other aromatic compounds that are needed for application requiring functional groups.This reaction also helps in thedehydrogenation of alcohol-producing required products like for example of alkene preparation. Another usefulness of the dehydrogenation process is in the times of the 2^nd world war where high octane fuels are needed. For this, the process uses a catalyst for the dehydrogenation reaction of butane which is expected to produce the required aviation fuels of very high octane.

The dehydration reaction of alcohols or the dehydrogenation reaction of alcoholsusually takes place in the oxygen’s presencewhere silver is used as a catalyst to give aldehydes from alcohols. However, under acidic conditions, the dehydration reaction of alcohol gives out alkene. The reaction can also take place in the absence of oxygen and such cases,  Palladium is used as a catalyst for aromatizing substituted cyclohexyl or cyclohexenyl compounds. An example where Pd is used as a catalyst with carbon support is in fine chemistry for the formation of p-cymene by the process of dehydrogenation of limestone and this results in 67% yield.

Acidic dehydration of alcohols

In the case of alcohols as being said earlier, the acidic dehydration process leads to the formation of alkenes. The reaction takes place when strong acids used as catalysts such as Sulphuric acid (H₂SO₄), Phosphoric acid (H3PO4), etc. are heated up with alcohols. In this 1,2-elimination reaction is undergone by alcohol resulting in the generation to alkene and water as the by-product. This is specifically known as the acidic dehydration of alcohol reaction as in this reaction there is a removal of water molecule taking place.

The relative reactivity order of alcohol is given in this : 3°> 2°> 1°. According to Zaitsev’s rule the major product of such a reaction is mostly the more highly alkene substituted or in other words, the most stable alkene. The term explaining this is known as Regioselectivity which in chemistry is a term used when there is a formation preference of one product over the other in a reaction. Hence, this reaction is selective. In the case of Stereoselectivity the trans- is preferable over cis- again due to the reason of stability.

Mechanism Stepsfor Acidic Dehydration Reaction of Alcohol

Thedifferent steps involved are:

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

The reaction for the dehydration of alcohol is given below:

The mechanism involved in the Dehydration of Alcohols:

The acidic dehydration reactions of alcohols take place either by the E1 or E2 mechanism. For primary alcohols, the E2 mechanism is being followed whereas secondary as well as tertiary alcohols follow the E1 mechanism.

The three-step mechanism behind the process is explained as under:

1. Protonated alcohol formation:

Firstly, the alcohol is allowed to react with a protic acid which acted upon it. There is the formation of lone pair that actsas a Lewis base present on the oxygen atom. With this,  protonation of alcoholic oxygen takes place making an easy way for the group that is leaving. This is a reversible stepthat takes place very fast.

1. Formation of Carbocation:

In this second step, there is a breakage of the C – O( bond between carbon and oxygen)which forms a carbocation. This step is very slow and is said to be the rate-determining step.

• Formation of Alkene:

Thisfinal step in the acidic dehydration of alcohol leads to alkene formation. The formed proton gets eliminated with the help of a base. The carbon atom adjacent to the carbocation is needed for breaking the C-H bond resulting in the C=C (double bond).