Acid Halides (Acyl Halides)

Acid chlorides -- the most important and most reactive of the four carboxylic acid derivatives (Section 12.14) -- are covered here in their own right: how they are made, their physical character, and the wide …

Acid chlorides are prepared from carboxylic acids by treating them with any one of three chlorinating agents: SOCl2, PCl5, or PCl3 (the same three reagents already introduced for exactly this transformation in Section 12.12.B.1). The book highlights thionyl chloride specifically as the SUPERIOR choice: acetic acid + SOCl2 gives acetyl chloride + HCl + SO2, and this method is superior to the others precisely because BOTH by-products (HCl and SO2) are gases that simply escape the reaction mixture, le …

Acid chlorides emit pale, visible fumes of hydrogen chloride gas when exposed to open air, on account of their ready reaction with atmospheric water vapour. They are formally insoluble in water, but in practice slowly begin to dissolve/decompose as that same reaction with water (hydrolysis, Section 12.14.2.3) proceeds -- so an acid chloride is not genuinely stable in the presence o …

Acid chlorides react readily with a range of weak nucleophiles -- water, alcohols, ammonia, and amines -- each giving a different corresponding derivative product, since the chloride is an excellent leaving group (Section 12.14's reactivity order) and the acid chloride's carbonyl carbon is correspondingly highly electrophilic. (1) HYDROLYSIS: acid chloride + H2O gives the parent carboxylic acid + HCl, e.g. acetyl chloride + H2O gives acetic acid + HCl. (2) Reaction with ALCOHOLS (alcoholysis) gives ESTERS: acetyl chloride + ethanol gives ethyl acetate + HCl. (3) Reaction with AMMONIA (ammonolysis) gives ACID AMIDES: acetyl chloride + NH3 gives acetamide + HCl. (4) Reaction with PRIMARY and SECONDARY amines gives substituted amides: a 1-degree amine (R'NH2) gives an N-alkylamide, R-CO-NHR'; a 2-degree amine (R'2NH) gives an N,N-dialkylamide, R-CO-NR'2. (5) REDUCTION -- two named, mutually exclusive outcomes depending on the reducing agent: (a) hydrogenation over 'poisoned' Pd/BaSO4 gives the ALDEHYDE (this is exactly the Rosenmund reduction of Section 12.3.B.1, restated here from the acid-chloride side); (b) reduction with LiAlH4 instead gives the fully-reduced PRIMARY ALCOHOL, e.g. acetyl chloride + 4[H] (LiAlH4) gives ethanol + HCl - …