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Organic reactivity in liquid ammonia.
MedLine Citation:
PMID:  22538452     Owner:  NLM     Status:  Publisher    
Liquid ammonia is a useful solvent for many organic reactions including aliphatic and aromatic nucleophilic substitution and metal-ion catalysed reactions. The acidity of acids is modified in liquid ammonia giving rise to differences from conventional solvents. The ionisation constants of phenols and carbon acids are the product of those for ion-pair formation and dissociation to the free ions. There is a linear relationship between the pK(a) of phenols and carbon acids in liquid ammonia and those in water of slope 1.68 and 0.7, respectively. Aminium ions exist in their unprotonated free base form in liquid ammonia. The rates of solvolysis and aminolysis by neutral amines of substituted benzyl chlorides in liquid ammonia show little or no dependence upon ring substituents, in stark contrast with the hydrolysis rates of substituted benzyl halides in water which vary 10(7) fold. However, the rates of the reaction of phenoxide ions and amine anions with 4-substituted benzyl chlorides gives a Hammett ρ = 1.1 and 0.93, respectively. The second order rate constants for the substitution of benzyl chlorides by neutral and anionic amines show a single Brønsted β(nuc) = 0.21 whereas those for substituted phenoxide ions generate a Brønsted β(nuc) = 0.40. The rates of aromatic nucleophilic substitution reactions in liquid ammonia are much faster than those in protic solvents indicating that liquid ammonia behaves like a typical dipolar aprotic solvent in its solvent effects on organic reactions. Nitrofluorobenzenes (NFB) readily undergo solvolysis in liquid ammonia but oxygen nucleophiles, such as alkoxide and phenoxide ions, displace the fluorine of 4-NFB in liquid ammonia to give the corresponding substitution product with little or no competing solvolysis product. The Brønsted β(nuc) for the reaction of 4-NFB with para-substituted phenoxides is 0.91, indicative that the decomposition of the Meisenheimer σ-intermediate is rate limiting. The aminolysis of 4-NFB occurs without general base catalysis by the amine and the second order rate constants generate a Brønsted β(nuc) of 0.36, which is also interpreted in terms of rate limiting breakdown of the Meisenheimer σ-intermediate.
Pengju Ji; John Atherton; Michael I Page
Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2012-4-26
Journal Detail:
Title:  Organic & biomolecular chemistry     Volume:  -     ISSN:  1477-0539     ISO Abbreviation:  -     Publication Date:  2012 Apr 
Date Detail:
Created Date:  2012-4-27     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  101154995     Medline TA:  Org Biomol Chem     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
IPOS, The Page Laboratories, Department of Chemical and Biological Sciences, The University of Huddersfield, Queensgate, Huddersfield HD1 3DH, UK.
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