Q12E - Organic Chemistry (Mcmurry)

Question

How might you carry out the following transformation? More than one step is needed.

Step-by-Step Solution

Answer

The below transformation is used to prepare 2-cyclopentyl-1-ethanol form cyclopentylmethanol.

Step1: Preparation of (bromomethyl) cyclopentane from cyclopentylmethanol.

In the first step, the given starting material cyclopentylmethanol is treated with $PB{r}_3$ to form a P-O bond by displacing the bromide ion.

In the complex cation, the oxygen atom in the alcohol group bears a positive on it and makes it as a good leaving group. This process is known as activation. 

The bromide ion attacks the carbon atom through $S_{N}2$ a mechanism to form a new carbon-bromine bond as shown below.

Step 2: Conversion of (bromomethyl) cyclopentaneinto 2-cyclopentyl acetic acid.

In the second step, (bromomethyl)cyclopentane reacts with $NaCN$ to give 2-cyclopentylacetonitrile through a $S_{N}2$ nucleophilic substitution pathway.

In the third step, the hydrolysis of 2-cyclopentylacetonitrile in the presence of aqueous acid $\left(H_3{O}^{+}\right)$ to produce 2-cyclopentylacetic acid as shown below.

 

Step 3: Conversion of 2-cyclopentylacetic acid into 2-cyclopentyl-1-ethanol.

In the fourth step, 2-cyclopentylacetic acid is successfully converted to 2-cyclopentyl-1-ethanol by using the strong reducing agent, $LiAl{H}_4$. This above-said reaction is drawn as follows: