Esters

Esters, R-COO-R', are the acid-derivative class formed by replacing a carboxylic acid's -OH with an alkoxy group -OR' -- covered here for their own preparation, physical character, and (unusually rich) chemical reactivity, extending well beyond the simple hydrolysis/esterification equilib …

(1) Esterification: as already covered in Section 12.12.B.2, treatment of an alcohol with a carboxylic acid in the presence of a mineral-acid catalyst gives an ester; because the reaction is reversible (an equilibrium), it is driven to completion in practice either by using a large excess of one reactant or by continuously removing the water formed (e.g. by distillation) as the reaction proceeds. (2) Alcoholysis of an acid chloride or acid anhydride: treating either derivative with an alcohol also gives an ester directly (Sections 12.14.2.3 and 12.14.3.2) -- and, since neither of THOSE reactions is reversible in the same troublesome way esterification is, this r …

Esters are colourless liquids or solids with a characteristic, often pleasant fruity smell -- a property exploited directly in food flavouring and perfumery, since many natural fruit and flower scents are themselves ester mixtures, each individual fruit's characteristic aroma typically arising from one or two dominant esters among many trace components. Unlike carboxylic acids, esters cannot hydrogen-bond to themselves (having no O-H of their own), so they boil distinctly lower than the acid or alcohol of comparable molecular mass that they are built from, and most simple, low-molar-mass esters are volatile enough to be smelled directly at r …

(1) HYDROLYSIS: as already covered (Section 12.10.3), acidic hydrolysis of an ester gives back the parent alcohol and carboxylic acid. (2) TRANSESTERIFICATION (reaction with a different alcohol): an ester can react with a SECOND alcohol, in the presence of a mineral acid, to give the ester of that second alcohol -- the alcohol PORTIONS of the two esters are simply interchanged. Example: ethyl acetate + propyl alcohol (H+) gives propyl acetate + ethyl alcohol. This reaction is generally used to prepare the ester of a HIGHER (often less accessible or more expensive) alcohol starting from the ester of a lower, more readily available one. (3) AMMONOLYSIS (reaction with ammonia): esters react slowly with ammonia to form an amide and the corresponding alcohol. Example: ethyl acetate + NH3 gives acetamide + ethanol. (4) CLAISEN CONDENSATION: an ester possessing at least one alpha-hydrogen undergoes self-condensation in the presence of a strong base such as sodium ethoxide to give a beta-keto ester -- the ester's OWN alpha-carbanion (generated by the base) attacks the carbonyl carbon of a SECOND ester molecule, displacing ethoxide as the leaving group (rather than adding permanently, since ethoxide is a good enough leaving group here that the tetrahedral intermediate collapses back to a new carbonyl, unlike the aldol reaction's simple addition). Example: 2 ethyl acetate (C2H5ONa catalyst) gives ethyl acetoacetate, CH3-CO-CH2-COOC2H5, + ethan …