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The ammonolysis of esters in liquid ammonia

Griffin, Joseph, Atherton, John and Page, Michael I. (2013) The ammonolysis of esters in liquid ammonia. Journal of Physical Organic Chemistry, 26 (12). pp. 1032-1037. ISSN 1099-1395

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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 βlg values of −1.18 and −1.34, respectively, when plotted against the aqueous pKa of the alcohol. The Brønsted βlg obtained using the pKa 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 βlg 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 T0 which is compatible with the rate-limiting step for the uncatalysed reaction being the formation of the neutral T0 by a ‘proton switch

Item Type: Article
Subjects: Q Science > Q Science (General)
Q Science > QD Chemistry
Schools: School of Applied Sciences
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Depositing User: Sara Taylor
Date Deposited: 18 Aug 2015 14:03
Last Modified: 04 Nov 2015 17:48


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