Page 1061 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

to a stage of solid mechanistic understanding as well as synthetic utility. The common 1045
mechanistic pattern of S RN 1 involves electron transfer to the reactant that generates
a radical anion, which then expels the leaving group. A chain process occurs if the SECTION 11.6
radical generated by expulsion of the leaving group can react with the nucleophile to S RN 1 Substitution
Processes
give a radical anion capable of sustaining a chain reaction.

initiation R X + e – R X – R + X –

propagation R + Nu – R Nu –
R Nu – + R X R Nu + R X –

R X – R + X –

A key to the efficiency of S RN 1 reactions is the electron transfer to the alkylating
reagent. 200 This process can be stepwise if the radical anion that is formed is sufficiently
stable, but can also be concerted. The concerted path is the most likely one for alkyl
halides. The combination reaction between the radical and nucleophile is very fast.

Nu – –
R Nu – Nu – R Nu
R X R X
R
R X – R Nu R R Nu
+ –
X – X
concerted mechanism
stepwise mechanism for alkylation stage
for alkylation stage

The S RN 1 reaction was first discovered and developed for nitroalkane anions, but
it is applicable to several other types of nucleophiles. The S RN 1 reaction is applicable
to various aryl and tertiary alkyl halides and has also been extended to other leaving
groups. The reaction has found a number of synthetic applications, especially in
substitution of aryl and bridgehead alkyl halides that are resistant to other substitution
mechanisms. 201

11.6.1. S 1 Substitution Reactions of Alkyl Nitro Compounds
RN
The S RN 1 mechanism of this type permits substitution of certain aromatic and
aliphatic nitro compounds by a variety of nucleophiles. These reactions were discovered
as the result of efforts to understand the mechanistic basis for high-yield carbon
alkylation of the 2-nitropropane anion by p-nitrobenzyl chloride. The corresponding

200 J.-M. Saveant, J. Phys. Chem., 98, 3716 (1994); R. A. Rossi, A. B. Pierini, and G. L. Borosky, J. Chem.
Soc., Perkin Trans. 2, 2577 (1994).
201
R. A. Rossi, Current Org. Chem., 7, 747 (2003); R. A. Rossi, A. B. Pierini, and A. N. Santiago, Org.
React., 54, 1 (1999); R. A. Rossi, A. B. Pierini and A. B. Penenory, Chem. Rev., 103, 71 (2003).