Question

Question. (a) Show how you would synthesize the pure (R) enantiomer of 2-pentyl methyl sulfide, starting with pure (R)-pentan-2-ol and any reagents you need.

(b) Show how you would synthesize the pure (S) enantiomer of the product, still starting with (R)-pentan-2-ol and any reagents you need.

Short answer

(a)

(b)

Step-by-step solution

Step-1. Explanation of part (a):

(R)-pentan-2-ol on reaction with phosphorous tribromide undergoes substitution reaction in which bromide ion attacks as a nucleophile and displaces hydroxyl group, this reaction is SN2 in nature and thus, inversion takes place at the chiral center, that is, “R” changes to “S”, and then on treatment with sodium methanethiloate, again SN2 reaction takes place and inversion occurs at the chiral center and “S” configuration changes to “R” configuration.

Formation of (R)-enantiomer of 2-pentyl methyl sulfide

Step-2. Explanation of part (b):

(R)-pentan-2-ol on reaction with tosyl chloride and pyridine undergoes acid-base reaction first, as pyridine acts as a base and abstracts proton from hydroxyl group, then, oxygen anion attacks at tosyl group and tosylate is formed. Configuration at chiral center do not changes. Then, on treatment with sodium methanethiloate, SN2 reaction takes place and inversion occurs at the chiral center and “R” configuration changes to “S” configuration.

Formation of (S)-enantiomer of 2-methyl pentyl sulfide