Methods of Preparation of Carboxylic Acids

Primary alcohols and aldehydes are both readily oxidised straight through to the corresponding carboxylic acid, using oxidising agents such as potassium permanganate (in either acidic or alkaline medium) or potassium dichromate (in acidic medium) -- this is simply the FULL, unstopped version of the same oxidation that PCC alone halts at the aldehyde stage (Section 12.3.A.1). Worked example, showing both stages of the same overall two-electron-times-two oxidation: ethyl alcohol, oxidised (H+/K2Cr2O7), giv …

Nitriles (alkyl cyanides) yield carboxylic acids when subjected to hydrolysis with either an acid or an alkali catalyst -- the nitrile carbon, already at the correct (carboxylic) oxidation level, is simply converted via an amide intermediate to the free acid, releasing ammonia. Example: methyl cyanide (acetonitrile), CH3-C#N, treated with 2 H2O (H+ catalyst), gives acetic acid, CH3COOH, + NH3. This route is a recurring building block across the chapter's reaction-sequence questions, since a nitrile itself is commonly reached first from a haloalkane via the Williamson-style substitution R-X + KCN gives R-CN + KX (Unit 11), making 'haloalkane -> nitrile -> acid' a …

Esters, on hydrolysis with dilute mineral acid, yield the corresponding carboxylic acid together with the parent alcohol -- the exact reverse of the esterification reaction of Section 12.12.B.2, and, like esterification, an equilibrium reaction (so an excess of water, or removal of the alcohol as it forms, is needed to drive the hydrolysis to completion). Example: ethyl acetate, CH3-COO-C2H5, treated with …

A Grignard reagent reacts with carbon dioxide (typically as solid dry ice, to keep the reaction anhydrous and well-controlled) to form the magnesium salt of a carboxylic acid, which on subsequent acidification with a mineral acid gives the free carboxylic acid -- this is the standard, reliable route for building a carboxylic acid with exactly ONE MORE carbon than the starting halide, since the new -COOH carbon comes entirely from CO2. Worked examples: methylmagnesium bromide + CO2 (dry ether), then H3O+/H+ work-up, gives acetic acid + Mg(OH)Br; phenylmagnesium bromide + CO2 (dry ether), then H3O+/H+, gives benzoic acid + Mg(OH)Br -- exactly the pathway used in Question 13 (from bromocyclopropane) and Question 13's short-answer twin (from benzyl bromide). One structural exception is worth noting explicitly: formic acid (HCOOH) CANNOT be prepared by this Grignard-plus-CO2 route, since the method necessarily builds a NEW car …

(a) Acid chlorides, hydrolysed with water, give carboxylic acids directly: acetyl chloride + H2O gives acetic acid + HCl. (b) Acid anhydrides, hydrolysed with water, likewise give the corresponding carboxylic acid -- but since an anhydride is built from TWO acyl groups, hydrolysis releases TWO molecules of the acid: acetic anhydride + H2O gives 2 acetic acid; benzoic anhydride + H2O gives 2 benzoic acid. Both hydrolyses are simply the acid-halide/anhydride-specific versions of the general nucleophilic acyl substitution that these derivatives undergo with water as the nucleophile -- the same …

Aromatic carboxylic acids can be prepared by the vigorous oxidation of an alkylbenzene, using chromic acid or acidic/alkaline potassium permanganate. A crucial, distinctive feature of this specific oxidation is that the ENTIRE side chain is oxidised down to a single -COOH group, IRRESPECTIVE of how long that side chain originally was -- a propyl-, butyl- or any longer n-alkyl substituent on the ring is degraded all the way back to just -COOH, since the oxidant attacks and cleaves the chain repeatedly at the benzylic position until nothing but the ring-attached carboxyl carbon remains. Example: toluene, oxidised with alkaline KMnO4 (a triple (O) oxidation) then acidified, gives benzoic acid, C6H5COOH. This is exactly why toluene, ethylbenzene, and n-propylbenzene (Evaluate Yourself, below) all converge on the SAME single product, benzoic acid, under …