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Nucleophilicities and Nucleofugalities of Organic Carbonates
Author(s) -
Streidl Nicolas,
Branzan Ramona,
Mayr Herbert
Publication year - 2010
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201000414
Subject(s) - chemistry , acetonitrile , heterolysis , propylene carbonate , nucleophile , aqueous solution , reaction rate constant , electrophile , medicinal chemistry , kinetics , inorganic chemistry , nucleophilic substitution , carbonate , ion , organic chemistry , electrochemistry , catalysis , physics , electrode , quantum mechanics
The kinetics of the reactions of the methyl carbonate ion with benzhydrylium ions in acetonitrile have been studied by UV/Vis spectrophotometry. Substitution of the resulting second‐order rate constants and the electrophilicity parameters E of the benzhydrylium ions into the linear free energy relationship log k = s ( N + E ) yielded the nucleophilicity parameters N 25 = 16.03 and s 25 = 0.64 for methyl carbonate in acetonitrile. The kinetics of the reverse reactions, i.e., of the solvolyses of ring‐substituted benzhydryl alkyl carbonates in different aqueous solvents were followed by conductimetry. The obtained first‐order rate constants and the known electrofugality parameters E f of benzhydrylium ions were used to determine the nucleofugality parameters N f and s f of the ROCO 2 – groups by using the linear free energy relationship log k = s f ( N f + E f ). The leaving group abilities of carbonates decrease by a factor of about 300 from PhOCO 2 – over MeOCO 2 – and i BuOCO 2 – to t BuOCO 2 – in various alcoholic and aqueous solvents. tert ‐Butyl carbonates ( t BocO‐R) are, thus, considerably more stable with respect to heterolytic cleavage of the O–R bond than other organic carbonates.