91Ó°ÊÓ

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.

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.

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: