Mustard gas, $\mathbf{Cl}\mathbf{-}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{-}{\mathbf{CH}}_{\mathbf{2}}{\mathbf{CH}}_{\mathbf{2}}\mathbf{-}\mathbf{Cl}$, was used as a poisonous chemical agent in World War I. Mustard gas is much more toxic than a typical primary alkyl chloride. Its toxicity stems from its ability to alkylate amino groups on important metabolic enzymes, rendering the enzymes inactive.

1. Propose a mechanism to explain why mustard gas is an exceptionally potent alkylating agent.
2. Bleach (sodium hypochlorite,$\mathbf{NaOCl}$, a strong oxidizing agent) neutralizes and inactivates mustard gas. Bleach is also effective on organic stains because it oxidizes colored compounds to colorless compounds. Propose products that might be formed by the reaction of mustard gas with bleach.

Sulfur present at the center of mustard gas acts as excellent nucleophile and internally or intramolecularly attacks at the carbon center and chloride is a decent leaving group as well. Sulfur can perform internal nucleophilic substitution which leads to the formation of reactive sulfonium salt and sulfur equivalent of an epoxide. This sulfonium salt is very reactive alkylating agent due to positive charge on sulfur and also increased ring strain which further increases the reactivity of the system; thus ring opens up quickly to stabilize positive charge on sulfur and alkylation can be performed easily.

Mustard gas as potent alkylating agent

$\mathrm{NaOCl}$is a powerful oxidizing agent. It oxidizes sulfur to sulfoxide or sulfone, either of which do not act as a nucleophile which prevents the formation of cyclic sulfonium salt and inactivates the mustard gas.

Mustard gas Sulfoxide Sulfone