5- Alkylisothiuronium salts, reaction with

A problem with the first route is that even with excess of the thiolate, it is difficult to avoid some further reaction leading to formation of the sulfide (R2S). The secondary reaction can be avoided by starting with thiourea (la) and obtaining the thiol via the intermediate S-alkylisothiuronium salt (2) by subsequent hydrolysis (Scheme 2). The formation of the isothiuronium salt (2) depends on the relative instability of the thione ( C=S ) bond in thiourea (la), which therefore exists mainly as the enethiol form isothiourea (lb) which then reacts with the halide. S-Benzylisothiuronium salts are used in organic qualitative analysis for the characterisation of carboxylic acids with which they form nicely crystalline... 

Thioureas generally react with alkyl halides to give S-alkylisothiuronium salts. On the other hand, by reaction with acyl halides, either the N-or S-acylthioureas may be formed. The nature of the product appears especially dependent on the reaction temperature. S-Acylthioureas (108) on heating rearrange to the N-acyl derivatives (109) (Scheme 59). 

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