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The ammonolysis of esters in liquid ammonia
Author(s) -
Griffin Joseph,
Atherton John,
Page Michael I.
Publication year - 2013
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.3148
Subject(s) - chemistry , solvolysis , tetrahedral carbonyl addition compound , deprotonation , protonation , leaving group , alkyl , alcohol , sn2 reaction , medicinal chemistry , ammonia , ammonium , aqueous solution , catalysis , inorganic chemistry , organic chemistry , ion , nucleophile , hydrolysis
The rates of ammonolysis of alkyl benzoate and phenylacetate esters in liquid ammonia increase with the acidity of the leaving group alcohol and show relatively large Brønsted β lg values of −1.18 and −1.34, respectively, when plotted against the aqueous pK a of the alcohol. The Brønsted β lg obtained using the pK a of the leaving group alcohol in liquid ammonia is significantly reduced to ~ −0.7, which indicates that the rate‐limiting step involves a reaction of the tetrahedral intermediate with little C–OR bond fission in the transition state. The solvolysis reaction is subject to significant catalysis by ammonium ion, which, surprisingly, generates a similar Brønsted β lg indicating little interaction between the ammonium ion and the leaving group. It is concluded that the rate‐limiting step for the ammonium‐ion‐catalysed solvolysis of alkyl esters in liquid ammonia is the diffusion‐controlled protonation of the zwitterionic tetrahedral intermediate T +‐ to give T + , which is rapidly deprotonated to give T 0 which is compatible with the rate‐limiting step for the uncatalysed reaction being the formation of the neutral T 0 by a ‘proton switch’. Copyright © 2013 John Wiley & Sons, Ltd.