Question

In this reaction find out what is P.

Answer 1

Hint: The purpose of making alcohol react with tosyl chloride is to convert the poor leaving group –OH into good leaving group –OTs , so later on nucleophilic substitution can be performed easily that was initially difficult with –OH group. Also as this is a secondary alcohol and pyridine is polar aprotic solvent so reaction will take place according to ${{S}_{N}}2$ mechanism.


Complete step by step answer:
When tosyl chloride will react with given alcohol then oxygen will donate its lone pair to the vacant orbital of sulfur and create a bond, chlorine will leave the group and take the hydrogen attached to oxygen along with it as oxygen-hydrogen bond got weaker because of the electron density transfer to the sulfur atom. The HCl formed will be neutralized by pyridine.
Reaction is shown below:

Now this Tosylate is made to react with sodium ethane thiolate. Now here reaction will take place according to ${{S}_{N}}2$ mechanism and inversion of configuration is going to take place. As ethane thiolate ions will attack from the back side and displace the good leaving group –OTs.
Reaction is shown below:

- So product P formed will be

The correct answer is option “B” .
Additional Information : If reaction of alcohol was taking place with thionyl chloride in place of tosyl chloride, the corresponding chloride would have formed with inversion of configuration.

Note: In case of secondary alcohols and secondary alkyl halides while performing nucleophilic substitution keep an eye on solvent. As in this case, the solvent will decide the mechanism. If solvent is polar protic reaction will take place via ${{S}_{N}}1$ mechanism and in case of polar aprotic solvent reaction will take place via ${{S}_{N}}2$ mechanism.