Halides, alkyl reaction with NaSH

Reaction of alkyl halides with NaSH cleavage of isothiuronium salts... 

So the reaction of an alkyl halide with NaSH or Na2S cannot usually be made to stop after one alkylation as the anion of the first product is at least as nucleophilic as HS or S2. This is obvious in reactions with Na2S. Less obviously with NaSH the first reaction 18 gives the thiol 19 but this is in equilibrium with RS- and a second displacement 20 gives the sulfide 21. We shall see shortly how to get round this problem. A more important example—the failure of the alkylation of ammonia to give a useful amine synthesis—has chapter 8 to itself. 

Hydrogen sulfide also reacts by nucleophilic substitution with alkyl halides and alcohols under appropriate conditions to form thiols (Scheme 10) (see Chapter 4, p. 42). The reaction with an alkyl halide is generally facilitated by using sodium hydrogen sulfide (NaSH) rather than hydrogen sulfide since NaSH is a much better nucleophile. Hydrogen sulfide also adds to ketones to give gem-dithiols (13) (Scheme 11). 

Sodium sulfhydride (NaSH) is a much better reagent for the formation of thiols (mercaptans) from alkyl halides than H2S and is used much more often. It is easily prepared by bubbling H2S into an alkaline solution, but hydrosulfide on a supported polymer resin has also been used. " The reaction is most useful for primary halides. Secondary substrates give much lower yields, and the reaction fails completely for tertiary halides because elimination predominates. Sulfuric and sulfonic esters can be used instead of halides. Thioethers (RSR) are often side products. The conversion can also be accomplished under neutral conditions by treatment of a primary halide with F and a tin sulfide, such as PhsSnSSnPhs. An indirect method for the preparation of a thiol is the reaction of an alkyl halide with thiourea to give an isothiuronium salt (119), and subsequent treatment with alkali or a... 

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