When alcohol is allowed to react with protic acids, it is prone to lose a water molecule to form alkenes. This type of reaction is commonly known as dehydration of alcohols. This is a basic example of an elimination reaction. The mechanism rate is different for primary, secondary and tertiary alcohols. In the case of tertiary, the carbonation is much stable so the rate of hydration is greatest for tertiary alcohols as compared to primary and secondary alcohols. The dehydration process takes place in the three-step mechanism.
Mechanism of dehydration of alcohol
The dehydration of alcohol follows the E1 or E2 mechanism. The primary alcohols follow the E2 mechanism for elimination reaction while the E1 mechanism is followed by secondary and tertiary alcohols. Normally, it is a three-step mechanism. The steps are explained as follows
- The formation of protonated alcohol
- The formation of carbonation
- The formation of alkenes
- The formation of protonated alcohol
This step involves the reaction of alcohol by a protic acid. Due to the presence of a single lone pair on the oxygen atom, it acts as a Lewis base. The protonation of alcoholic oxygen makes it a good leaving group. This step is not complicated and hence quickly reversible.
- The formation of a carbocation
In this step, the breakdown of the C-O bond takes place which generates a carbocation. This step is considered as the slowest step in the mechanism of dehydration of an alcohol. Hence, the formation of the carbocation is considered to be the rate-determining step.
- The formation of alkenes
This is the ultimate step in the dehydration of alcohols. Here, in this step, the generated proton is eliminated with the help of a base. The carbon atom near to the carbocation breaks the present C-H bond to form C=C. therefore, an alkene is produced.
Alcohol dehydration reaction
A dehydration reaction is considered as that type of chemical reaction where water is extracted from a single reactant. A production of alkene takes place when dehydration of an alcohol is carried out. A basic equation for alcohol dehydration is
C2H5OH C2H4 + H2O
Alcohol dehydration is an example of an elimination reaction. An elimination reaction is the type of reaction in which two atoms adjacent to carbon atoms are eliminated from a molecule leaving multiple bonds between the carbon atoms.
Dehydration of Primary, Secondary and Tertiary Alcohols
Alcohols and ethers own leaving groups that are stronger Lewis bases than halide ion. this property makes alcohols and ethers less reactive than the alkyl halides (where halogen atoms replace one or more compounds of hydrogen atoms in an alkane). They required to be protonated before undergoing a substitution or elimination reaction.
The primary, secondary and tertiary alcohol go through a process called the nucleophilic substitution reactions with HI, HBr and HCl to form alkyl halides. With the help of chromic acid, the secondary alcohol gets oxidized to ketones and that of primary are oxidized to carboxylic acids.
These are categorized as SN2 reactions in primary alcohols and SN1 reactions in secondary as well as tertiary alcohols. Tertiary alcohols are easy to dehydrate but on the other hand, primary alcohol dehydration is very tough.
The dehydration of secondary and tertiary is known as an E1 reaction (it’s a two-step mechanism), whereas the dehydration of primary alcohol is known as E2 reaction (it’s a one-step mechanism) because it is difficult to form primary carbocations.
Dehydration of alcohols using Aluminum Oxide as the catalyst
The dehydration of ethanol to give Ethene
It is considered the simplest way to make gaseous alkenes like ethene. if ethanol vapor is allowed to pass overheated aluminum oxide powder, the ethanol is cracked to generate ethene and water vapor.
To prepare some test tubes of ethene, the following
This method is easy to perform as just by boiling some ethanol in a flask and pass these vapor over aluminum oxide heated in a long tube.
Dehydration of alcohols using an acid catalyst
Concentrated Sulphuric acid or concentrated phosphoric acid are normally used acid catalysts.
The results obtained from concentrated Sulphuric acid are messy. It is not only acid but also a strong oxidizing agent. Some of the alcohol is oxidized to carbon dioxide by concentrated Sulphuric acid and simultaneously it reduces itself to Sulphur dioxide. Both of these gases need to be removed from the alkene. It also reacts with alcohol to generate a carbon mass.
The dehydration of ethanol resulting ethene
In an excess presence of concentrated Sulphuric acid ethanol is heated at a temperature of 170 ᵒC. the gases which are produced during the reaction are allowed to pass through sodium hydroxide solution to remove carbon dioxide and Sulphur dioxide released from side reactions.
The resulted ethene is collected over water in the presence of conc. Sulphuric acid as given in a reaction below
The Dehydration of cyclohexanol resulting cyclohexene
This preparation is used to form and purify a liquid product. It is a fact that carbon atoms that are joined in a ring make no difference to the chemistry of a reaction.
Cyclohexanol is allowed to heat with concentrated phosphoric acid and the liquid cyclohexene is distills off which later can be collected and purified.
Here phosphoric acid is used instead of Sulphuric acid because it is much safe and produces a less messy reaction. Phosphoric acid is not a strong oxidizing agent.
The Dehydration of Complicated Alcohols
There is a possibility of forming more than one alkene. Butan-2-ol is its good example which forms three different alkenes being formed when it is dehydrated.
When alcohol is dehydrated, -OH group and a hydrogen atom from the next carbon atom in the chain is removed. There are two possibilities of happening with molecules like butan-2-ol.
This gives us these products:
The products are but-1-ene, CH2=CHCH2CH3 and but-2-ene, CH3CH=CHCH3.
The situation is more complicated than it looks because but-2-ene displays geometric isomerism. Mixtures of two isomers are formed –cis-but-2-ene and trans-but-2-ene.
The –cis-but-2-ene is also termed as (Z)-but-2-ene while –trans-but-2-ene is termed as (E)-but-2-ene.
Dehydration of butan-2-ol results in a mixture containing
- Cis-but-2-ene (also (Z)-but-2-ene)
- Trans-but-2-ene (also (E)-but-2-ene)