Sulfides neighbouring group participation

The mechanism by which they speed up the reactions is known as neighbouring group participation. Compare the reaction of this ether and this sulfide with an alcohol. 

You ve already met the most important ones—sulfides, esters, carboxylates. Ethers and amines (you will see some of these shortly) can also assist substitution reactions through neighbouring group participation. The important thing that they have in common is an electron-rich heteroatom with a lone pair that can be used to form the cyclic intermediate. Sulfides are rather better than ethers—this sulfide reacts with water much faster than rc-PrCl but the ether reacts with acetic acid four times more slowly than rc-PrOSC Ar. 

Participation of sulfides through three-membered rings was used to gruesome effect in the development of mustard gas during the Second World War. Mustard gas itseff owes its toxicity to the neighbouring group participation of sulfur, which accelerates its mustard gas... 

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