Organic Chemistry

Section 10.3 Factors Affecting SN 2 Reactions 367

The weaker the base,

the better it is as a leaving group.

10.3 Factors Affecting Reactions

The Leaving Group
If an alkyl iodide, an alkyl bromide, an alkyl chloride, and an alkyl fluoride (all with
the same alkyl group) were allowed to react with the same nucleophile under the same
conditions, we would find that the alkyl iodide is the most reactive and the alkyl fluo-
ride is the least reactive.

The only difference among these four reactions is the nature of the leaving group.
From the relative reaction rates, we can see that the iodide ion is the best leaving
group and the fluoride ion is the worst. This brings us to an important rule in organic
chemistry—one that you will see frequently:The weaker the basicity of a group, the
better is its leaving ability.The reason leaving ability depends on basicity is because
weak bases are stable bases—they readily bear the electrons they formerly shared
with a proton (Section 1.18). Because weak bases don’t share their electrons well, a
weak base is not bonded as strongly to the carbon as a strong base would be and
a weaker bond is more easily broken (Section 1.13).
We have seen that the halide ions have the following relative basicities (or relative
stabilities) because larger atoms are better able to stabilize their negative charge
(Section 1.18):

Because stable (weak) bases are better leaving groups, the halides have the following
relative leaving abilities:

Therefore, alkyl iodides are the most reactive of the alkyl halides, and alkyl fluorides are
the least reactive. In fact, the fluoride ion is such a strong base that alkyl fluorides
essentially do not undergo reactions.

In Section 10.1, we saw that it is the polar carbon–halogen bond that causes alkyl
halides to undergo substitution reactions. Carbon and iodine, however, have the same
electronegativity. (See Table 1.3 on p. 10.) Why, then, does an alkyl halide undergo a
substitution reaction? We know that larger atoms are more polarizable than smaller
atoms. (Recall from Section 2.9 that polarizability is a measure of how easily an
atom’s electron cloud can be distorted.) The high polarizability of the large iodine
atom causes it to react as if it were polar even though, on the basis of the electronega-
tivity of the atoms, the bond is nonpolar.

relative reactivities of alkyl halides in an SN2 reaction

most reactive RI > RBr > RCl > RF least reactive

SN 2

relative leaving abilities of the halide ions

I− > Br− > Cl− > F− worst leaving

group

best leaving

group

relative basicities of the halide ions

I− < Br− < Cl− < F−

strongest base,

least stable base

weakest base,

most stable base

HO− ++RCH 2 I RCH 2 OH I−

relative rates of reaction

30,000

HO− ++RCH 2 Br RCH 2 OH Br− 10,000

HO− ++RCH 2 Cl RCH 2 OH Cl− 200

HO− ++RCH 2 F RCH 2 OH F− 1

SN 2

Stable bases are weak bases.