Organic Chemistry

Table 10.1 Relative Rates of SN2 Reactions for Several Alkyl Halides

SN 2

RBr+ Cl− RCl+ Br−

CH 3 C

CH 3

CH 3

CH 3

Br

CH 3 Br

CH 3 CH 2 Br

CH 3 CH 2 CH 2 Br

CH 3 CH Br

Alkyl halide Class of alkyl halide Relative rate

methyl 1200

primary 40

primary 16

secondary

tertiary too slow to measure

1

Section 10.2 The Mechanism of an SN 2 Reaction 363

Movie:

Bimolecular reaction

Edward Davies Hughes

(1906–1963)was born in North

Wales. He earned two doctoral

degrees: a Ph.D. from the University

of Wales and a D.Sc. from the

University of London, working with

Sir Christopher Ingold. He was a

professor of chemistry at University

College, London.

The rate law tells us which molecules are involved in the transition state of the rate-
determining step of the reaction. From the rate law for the reaction of methyl bromide
with hydroxide ion, we know that both methyl bromide and hydroxide ion are involved
in the rate-determining transition state.
The reaction of methyl bromide with hydroxide ion is an example of an reac-
tion, where “S”stands for substitution,“N”for nucleophilic, and “ 2 ”for bimolecu-
lar. Bimolecularmeans that two molecules are involved in the rate-determining step.
In 1937, Edward Hughes and Christopher Ingold proposed a mechanism for an
reaction. Remember that a mechanism describes the step-by-step process by which
reactants are converted into products. It is a theory that fits the experimental evi-
dence that has been accumulated concerning the reaction. Hughes and Ingold based
their mechanism for an reaction on the following three pieces of experimental
evidence:

1. The rate of the reaction depends on the concentration of the alkyl halide andon
   the concentration of the nucleophile. This means that both reactants are involved
   in the rate-determining step.


2. When the hydrogens of methyl bromide are successively replaced with methyl
   groups, the rate of the reaction with a given nucleophile becomes progressively
   slower (Table 10.1).


3. The reaction of an alkyl halide in which the halogen is bonded to an asymmetric
   carbon leads to the formation of only one stereoisomer, and the configuration of
   the asymmetric carbon is inverted relative to its configuration in the reacting
   alkyl halide.

Hughes and Ingold proposed that an reaction is a concertedreaction—it takes
place in a single step, so no intermediates are formed. The nucleophile attacks the car-
bon bearing the leaving group and displaces the leaving group.

Because a productive collision is a collision in which the nucleophile hits the car-
bon on the side opposite the side bonded to the leaving group, the carbon is said to un-
dergo back-side attack. Why does the nucleophile attack from the back side? The
simplest explanation is that the leaving group blocks the approach of the nucleophile
to the front side of the molecule.

HO−−+CH 3 Br CH 3 OH + Br

mechanism of the SN2 reaction

SN 2

SN 2

SN 2

SN 2

Sir Christopher Ingold

(1893–1970)was born in Ilford,

England. In addition to determining

the mechanism of the reaction,

he was a member of the group that

developed nomenclature for

enantiomers. (See p. 188.) He also

participated in developing the theory

of resonance.

SN 2