Qualitative organic analysis carboxylic acids

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... 

Chlorosulfonic acid is widely used in organic qualitative analysis to prepare solid derivatives from liquid or low melting aromatic compounds, e.g. hydrocarbons, halides and ethers." The procedure involves conversion of the aromatic compound into the sulfonyl chloride, which is subsequently reacted with ammonia to yield the solid sulfonamide derivative which is suitable for melting point determination, Chlorosulfonic acid reacts with carboxylic acid anhydrides to give excellent yields of the acid chlorides (Equation 11). 

---