Question

What product (s) would you expect from dehydration of the following alcohols with ${\mathbf{POCl}}_3$in pyridine? Indicate the major product in each case.

(a)

(b)

(C)

(d)

(e)

Short answer

a.

b.

c.

d.

e.

Step-by-step solution

Step-by-Step SolutionStep 1: Dehydration of alcohol

Alcohol dehydration takes place by an E2 elimination mechanism and is carried out with ${\mathbf{POCl}}_3$in pyridine.In the first step, dichlorophosphate intermediate is formed when OH group reacts with ${\text{POCl}}_{\text{3}}$. In the second step, E2 elimination occurs when dichlorophosphate group is leaving and on the other side, pyridine base abstracts proton from a neighbouring carbon.

The product expected from dehydration of alcohol in (a)

(a) The product obtained from dehydration of secondary alcohol with ${\text{POCl}}_{\text{3}}$in pyridine at ${\text{0}}^{\text{o}}\text{C}$is alkene products. This reaction takes place by E2 elimination because OH is a poor leaving group and on reaction with ${\text{POCl}}_{\text{3}}$, the conversion of OH group to ${\text{-OPOCl}}_2$takes place. The ${\text{-OPOCl}}_2$group formed is a good leaving group and thus eliminated. The pyridine acts as both base and solvent and it abstracts proton. This leads to the formation of alkene product. The major product is one which has the more substituted double bond.

Product obtained from dehydration of alcohol

The product expected from dehydration of alcohol in (b)

(b) In E2 elimination, the process of dehydration occurs more readily when the two groups that are to be eliminated present at an anti periplanar relationship. In this given compound, the only hydrogen which have a proper stereochemical relationship with –OH group is present at carbon 6. Thus, the formation of non-Zaitsev product 3-methylcyclohexene is seen.

Product obtained from dehydration of alcohol

The product expected from dehydration of alcohol in (c)

(c) In E2 elimination, the process of dehydration occurs more readily when the two groups that are to be eliminated present at an anti periplanar relationship. In this given compound, the hydrogen at C2 is trans to the OH group and thus, dehydration occurs.

Thus, the Zaitsev product 1-methylcyclohexene is formed.

Product obtained from dehydration of alcohol

The product expected from dehydration of alcohol in (d)

(d). The dehydration of 2,3-dimethyl-2-pentanol gives four different alkene products including E and Z isomers. The major product according to Zaitsev’s rule is one which has the most substituted double bond. Thus, out of four isomers, 2,3-Dimethyl-2-pentene is a major product.

Product obtained from dehydration of alcohol

The product expected from dehydration of alcohol in (e)

(e). The dehydration of 2-methylpentan-2-ol gives two different alkene products. The major product according to Zaitsev’s rule is one which has the most substituted double bond. Thus, out of two products, 2-Methyl-2-pentene is a major product.

Product obtained from dehydration of alcohol