Patent GB-819313-A (GB)
Title: Improvements in and relating to the production of esters
Inventor: DEAN FRED
IPC: C07C69/22
Filed: 1956-05-15
Abstract: Substantially colourless esters are prepared by esterifying, in the absence of sulphur dioxide, a saturated aliphatic monohydric alcohol or an aromatic monohydric alcohol free from aliphatic unsaturation at an elevated temperature with a saturated aliphatic carboxylic acid or an aromatic carboxylic acid free from aliphatic unsaturation, or an anhydride thereof, at least one of the reactants containing impurities of a type which in the presence of oxygen result in the formation of undesirable colour during esterification, and subsequently removing if necessary any readily removable colour by known processes, the esterification being carried out in the presence of an organic antioxidant as hereinafter defined.  Suitable antioxidants are: (1) antioxidant monohydric phenols, e.g. 4-methyl-2,6-di-t-butyl phenol and 2,4-dimethyl-6-t-butyl phenol; (ii) antioxidant dihydric phenols, e.g. hydroquinone or 2,5-di-t-butyl hydroquinone; (iii) antioxidant amines, e.g. N,N1-di-sec. butyl-p-phenylenediamine, 4-isopropylaminodiphenylamine, phenyl a -naphthylamine and phenyl b -naphthylamine; (iv) antioxidant condensation products comprising those formed between: (a) acetone and diphenylamine; (b) 2,4- and 2,5-xylenols and butyraldehyde; (c) di-o-cresylol propane [i.e. di (3-methyl, 4-hydroxyphenyl) dimethyl methane], piperidine and formaldehyde; (d) ethylenediamine and salicylaldehyde; (v) di-o-cresylol sulphide; (vi) bis-(2-hydroxy-3-a -methylcyclohexyl-5-methylphenyl) methane; (vii) sodium diethyl dithiocarbamate.  Suitable acids include aliphatic mono- or di-carboxylic acids such as sebacic or adipic, aromatic acids such as phthalic or acid anhydrides such as phthalic anhydride.  The alcohols usually contain several carbon atoms, generally 6 or more, e.g. isooctanol, 2-ethylhexanol, nonanol (3,5,5-trimethyl-hexanol-1) and oxo alcohols, and also aromatic alcohols such as benzyl alcohol.  An acidic catalyst is preferably used, e.g. sulphuric acid or p-toluene sulphonic acid. Water may be removed from the reaction by means of an entrainer, e.g. toluene.  Volatile antioxidants may be removed from the crude reaction product by steam distillation and recycled along with recovered excess alcohol.  Antioxidants may sometimes be removed by extraction, e.g. acidic compounds by a solution of aqueous alkali and bases by an aqueous mineral acid solution.  In some cases a stronger colour may be formed in the presence of an antioxidant than in its absence, but such colour is often more easily removed, e.g. by bleaching earths or charcoal.  Other precautions which may be taken to prevent colour formation include the use of an enamelled or glass- or inert-metal lined vessel, the use of an inert atmosphere, e.g. CO2 or N21 and predistillation of the alcohol reactant.  In examples: phthalic anhydride is reacted with (1) iso-octanol, 2-ethylhexanol, nonanol, or (2) an oxo alcohol mixture in the presence of H2SO4 and toluene in a chrome-steel reactor, while passing air through the mixture; reaction is effected both without an antioxidant and with 2,4 - dimethyl - 6 - t - butyl phenol and with and without predistillation of the alcohol; (3) hydroquinone is used in a similar test to Example (2); (3) most of the antioxidants specified in the above definition are used in a similar reaction to (3); (4) N,N1di-sec. butyl-p-phenylenediamine, 4 - isopropylamino - diphenylamine, hydroquinone and 2,5-di-t-butylhydroquinone are used in a similar test to (3) and the effect of charcoal or alumina treatment on the ester after neutralization, extraction of the antioxidant and steaming is evaluated.