Dehydration of Alcohol

When alcohol reacts with protic acids, it tends to lose a molecule of water in order to form alkenes. These reactions are generally known as dehydration of alcohols. It is a basic example of an elimination reaction. The rates differ for the primary, secondary, and tertiary alcohols. The carbonation is very much stable in the case of tertiary alcohols; hence, the rate of dehydration is highest for tertiary alcohols as compared to secondary and primary alcohols. 

Dehydration can be performed in a 3-step mechanism:

1.  Formation of protonated alcohol

2.  Formation of carbonation

3.  Formation of alkenes 

Mechanism of Dehydration of Alcohols

Dehydration of alcohols follows the E1 or E2 mechanism. The primary alcohols, elimination reactions follow the E2 mechanism whereas the secondary and tertiary alcohols elimination reaction follows the E1 mechanism. Basically, it follows a 3-step mechanism. The steps that are involved are explained below:

1. Formation of Protonated Alcohol:

In this particular step, the alcohol is reacted upon by a protic acid. Due to the single pair present on the oxygen atom, it acts as a Lewis base. This step is easily and quickly reversible.

2. Carbocation Formation:

In this step, the C–-O bond breaks which generates a carbocation. This is the slowest step in the mechanism of dehydration of alcohol. Therefore, the formation of the carbocation is said to be the rate-determining step.

                               Image: Reaction of Carbocation

3. Alkene Formation:

This is the final step in the dehydration of alcohol. In this step, the proton generated is eliminated with the help of a base. The carbon atom neighbouring to the carbocation breaks the present C–-H bond to form C=C. Therefore, an alkene is formed.

                  Image: Reaction of Alkene Formation 

Dehydration is generally a condition when a person loses a lot of water by sweating or less intake of water, but such type of dehydration is different from dehydration that we see in chemical reactions.

Alcohol Dehydration Reaction

A dehydration reaction is a type of chemical reaction wherein water is formed from the extraction of the components of water from a single reactant. An alkene is produced when dehydration of alcohol is performed.

A basic structural equation for alcohol dehydration is as follows:

C2H5OH → C2H4 + H2O

Alcohol dehydration is an example of an elimination reaction which is quite the opposite of substitution reaction and addition reaction.

An elimination reaction is a type of reaction wherein 2 groups or 2 atoms on neighbouring carbon atoms are eliminated or removed from a molecule which leaves multiple bonds between the carbon atoms.

Dehydration of secondary and tertiary alcohols in acidic conditions follows the E1 method. The protonation of the hydroxyl group successfully converts the leaving group from hydroxide ion to water. The hydronium H₃O+ is way stronger than H₂O, the conjugate surface of the former H₂O is a better leaving group than that of a latter OH.

When a relatively stable carbocation is produced by dehydration of a protonated alcohol, an E1 elimination can take place. 

Because an unstable primary carbocation would be structured in the E1 dehydration on primary alcohol, acid-catalyzed E1 elimination through such a carbocation is so slow that different pathways are followed. 

An E2 reaction takes place, in which a proton is lost from carbon at the same time as water is lost from the neighbouring carbon.

                          Image: Formation of Alkene through E2 Reaction

This allows for the formation of an alkene without any in-between formation of an unstable carbocation.

A Protonated Primary Alcohol (Alkene)  

Dehydration is mainly easy when a neighbouring double bond is formed. Alcohol that bears a carbonyl group two carbons away readily goes through dehydration and this finally yields α, β- unsaturated carbonyl compound.

The location of the carbonyl group to the hydroxyl group in β hydroxyl carbonyl compounds opens the way for elimination under general conditions by the E1cB mechanism.

                                          Image: Elimination reaction

The carbonyl group plays two vital roles, it helps in stabilizing the transitional carbanion and it gives additional driving force for elimination in giving improved stability to the neighbouring product.

The base-catalyzed loss of water from β hydroxy carbonyl compounds is one of the examples of an elimination reaction that involves an sp³ hybridized carbon atom that follows the E1cB pathway.

Alcohol Dehydration Mechanism with Example

The mechanism of dehydration may vary from alcohol to alcohol even when the same catalyst is being used.

The dehydration of alcohol series done by Thomke over BPO₄, Ca₃(PO₄)₂, and Sm₂O₃ determined the mechanism by two precise criteria, uptake of deuterium from deuterated catalysts into produced olefin and un-reacted alcohol. 

The alcohols presented E1 on BPO4, E2 on Ca3(PO4)2, and E1cB on Sm2O3₃.

The dehydrogenation of alcohol accompanied the dehydration of alcohol over some basic oxides. In the E2 mechanism, there is a different kind of selectivity, anti and syn elimination. The syn elimination products are made when the groups are eliminated from the same side of the molecule and anti-elimination products are made when the groups are removed from the opposite side.

The dehydration of alcohol is catalyzed with the help of boron phosphorus oxide. 

The reactivity of alcohol in the dehydration decreases in the order of: 

Tert- amyl alcohol> 3 pentanol > 2 propanol> 1 pentanol> ethanol.

The catalytic activities of the oxides are made up of a different number of bBoron and phosphorus for propanol dehydration which shows a relation with the total amount of acid sites.

Butanol goes through dehydration on boron phosphorus oxide. The activity shows a relation with the total of Lewis and Bronsted acid sites and in all of these reactions, the carbonium ion mechanism is in service.

In the E1cB mechanism, the initial step of dehydration is the formation of carbanion, which means that a C–-H bond is broken in the first step. The starting step of dehydration is the formation of a carbonium ion by the abstraction of an OH group.

This mechanism takes place with strongly acidic catalysts like aluminosilicate.

E2 mechanism includes the elimination of a proton and hydroxyl group from alcohol which is converted without the formation of an ionic intermediate. Alumina is a basic E2 oxide. The three mechanisms can be differentiated in various ways but unlike the liquid phase reactions, the kinetic method cannot be used. With the E1 mechanism, the isomerization occurs in the carbonium ion stage.

                       Image: Starting step of E1cB mechanism

The formation of 2 butenes from 1 butanol depicts the E1 mechanism. High selectivity for 1 butene from butane-2-ol depicts the E1B mechanism.  The dehydration of isobutyl alcohol over SiO₂–Al₂O₃ yields a combination of butene in which the fractions of n-butane is around 33%. Since the rate of skeletal isomerization of isobutene to n-butene is comparatively lower than the rate of formation of n-butene in dehydration, n-butene is a primary product. This shows that the reaction goes through the E1 mechanism.

The formation of n-butene is related to the formation of the least stable isopropyl carbonium ions which are already rearranged by hydride or methyl transfer to form more stable tertiary or secondary carbonium ions.

Secondary Alcohol Dehydration

Dehydration of alcohol requires cleavage of a C–-O bond with loss of a proton from the beta position. The result of dehydration is either an alkene or a mixture of the alkenes and the order of dehydration is first tertiary, then secondary, and finally primary. 

Tertiary Alcohol Dehydration

Tertiary forms of alcohol are easiest to dehydrate as the carbocations are more stable and thus easier to form compared to primary and secondary carbocations.

                                Image: Reaction of Tertiary Alcohol Dehydration

For dehydration to take place, the alcohol must be heated to roughly 50℃ in 5% H₂SO₄. Secondary alcohol needs about 100℃ in 75% H₂SO₄ and primary can only be dehydrated at 170℃ in 95% H₂SO₄ which are under extreme conditions. The dehydration process of both secondary and tertiary alcohols involves the formation of a product called the carbocation intermediate. Here, the carbocation gets rearranged if the result is a more stable carbocation.

Dehydration of Primary Secondary and Tertiary Alcohols

Alcohol and ethers possess leaving groups that are stronger Lewis bases than halide ions (is a halogen atom that has a negative charge). This makes alcohols and ethers less reactive than the alkyl halides (compounds where one or more hydrogen atoms in an alkane get replaced by halogen atoms). They need to be protonated before undergoing an elimination or substitution reaction.

The primary, secondary, and tertiary alcohols undergo a process called the nucleophilic substitution reactions (in chemistry, a nucleophilic substitution is a class of reactions where the nucleophile bonds with or attacks the positive charge of an atom or a group of atoms to substitute a leaving group) with HI, HBr, and HCl to form alkyl halides.  Secondary alcohols get oxidized to ketones and primary are oxidized to carboxylic acids by chromic acid.

They are categorized as SN₂ reactions in primary alcohols and SN₁  reactions in secondary as well as tertiary alcohols. Tertiary alcohols tend to be easier to dehydrate and primary alcohols to be the hardest.

The dehydration of either a tertiary or secondary alcohol is known as an E1 reaction (two-step mechanism), the dehydration of primary alcohol is an E2 (one-step mechanism) reaction because of the difficulty encountered in forming primary carbocations.

Conclusion

Hence, the article explained the important concept of organic chemistry. The mechanism of dehydration of alcohol is explained in the article, which will help students to grab the concept of dehydration of alcohol.