The Kinetics and Mechanisms of Aromatic Nucleophilic Substitution Reactions in Liquid Ammonia

Michael Page, John Atherton, Pengju Ji

Research output: Contribution to journalArticle

37 Citations (Scopus)

Abstract

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 (NFBs) readily undergo solvolysis in liquid ammonia and 2-nitrofluorobenzene is about 30 times more reactive than the 4-substituted isomer. Oxygen nucleophiles, such as alkoxide and phenoxide ions, readily displace fluorine of 4-NFB in liquid ammonia to give the corresponding substitution product with little or no competing solvolysis product. Using the pKa of the substituted phenols in liquid ammonia, the Brønsted βnuc for the reaction of 4-NFB with para-substituted phenoxides is 0.91, indicative of the removal of most of the negative charge on the oxygen anion and complete bond formation in the transition state and therefore suggests 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 βnuc of 0.36 using either the pKa of aminium ion in acetonitrile or in water, which is also interpreted in terms of rate limiting breakdown of the Meisenheimer σ-intermediate. Nitrobenzene and diazene are formed as unusual products from the reaction between sodium azide and 4-NFB, which may be due to the initially formed 4-nitroazidobenzene decomposing to give a nitrene intermediate, which may then give diazene or be trapped by ammonia to give the unstable hydrazine which then yields nitrobenzene.
Original languageEnglish
Pages (from-to)3286-3295
Number of pages10
JournalJournal of Organic Chemistry
Volume76
Issue number9
DOIs
Publication statusPublished - 2011

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Ammonia
Substitution reactions
Kinetics
Liquids
hydrazine
Ions
Oxygen
Sodium Azide
Nucleophiles
Fluorine
Phenols
Isomers
Catalysis
Amines
Anions
Rate constants
Decomposition
Water

Cite this

Page, Michael ; Atherton, John ; Ji, Pengju. / The Kinetics and Mechanisms of Aromatic Nucleophilic Substitution Reactions in Liquid Ammonia. In: Journal of Organic Chemistry. 2011 ; Vol. 76, No. 9. pp. 3286-3295.
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The Kinetics and Mechanisms of Aromatic Nucleophilic Substitution Reactions in Liquid Ammonia. / Page, Michael; Atherton, John; Ji, Pengju.

In: Journal of Organic Chemistry, Vol. 76, No. 9, 2011, p. 3286-3295.

Research output: Contribution to journalArticle

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AB - 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 (NFBs) readily undergo solvolysis in liquid ammonia and 2-nitrofluorobenzene is about 30 times more reactive than the 4-substituted isomer. Oxygen nucleophiles, such as alkoxide and phenoxide ions, readily displace fluorine of 4-NFB in liquid ammonia to give the corresponding substitution product with little or no competing solvolysis product. Using the pKa of the substituted phenols in liquid ammonia, the Brønsted βnuc for the reaction of 4-NFB with para-substituted phenoxides is 0.91, indicative of the removal of most of the negative charge on the oxygen anion and complete bond formation in the transition state and therefore suggests 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 βnuc of 0.36 using either the pKa of aminium ion in acetonitrile or in water, which is also interpreted in terms of rate limiting breakdown of the Meisenheimer σ-intermediate. Nitrobenzene and diazene are formed as unusual products from the reaction between sodium azide and 4-NFB, which may be due to the initially formed 4-nitroazidobenzene decomposing to give a nitrene intermediate, which may then give diazene or be trapped by ammonia to give the unstable hydrazine which then yields nitrobenzene.

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