Organic Chemistry

Section 10.3 Factors Affecting SN 2 Reactions 369

H

C X

H H

C

H

C X

HH

I−

transition state

H

C X

H H

C

H

C X

HH

F−

transition state

more bonding

little bonding

++

++

δ−

δ−

>Figure 10.5

An iodide ion is larger and more

polarizable than a fluoride ion.

Therefore, the relatively loosely

held electrons of the iodide ion can

overlap from farther away with the

orbital of carbon undergoing

nucleophilic attack. The tightly

bound electrons of the fluoride ion

cannot start to overlap until the

atoms are closer together.

If the reaction is carried out in the gas phase, the direct relationship between basici-
ty and nucleophilicity is maintained—the stronger bases are still the best nucleophiles.
If, however, the reaction is carried out in a protic solvent—meaning the solvent mole-
cules have a hydrogen bonded to an oxygen or to a nitrogen—the relationship between
basicity and nucleophilicity becomes inverted. The largest atom is the best nucleophile
even though it is the weakest base. Therefore, iodide ion is the best nucleophile of the
halide ions in a protic solvent.

PROBLEM 4

a. Which is a stronger base, or

b. Which is a better nucleophile in an aqueous solution?

The Effect of the Solvent on Nucleophilicity
Why, in a protic solvent, is the smallest atom the poorest nucleophile even though it is
the strongest base? How does a protic solvent make strong bases less nucleophilic?
When a negatively charged species is placed in a protic solvent, the ion becomes sol-
vated (Section 2.9). Protic solvents are hydrogen bond donors. The solvent molecules
arrange themselves so that their partially positively charged hydrogens point toward
the negatively charged species. The interaction between the ion and the dipole of the
protic solvent is called an ion–dipole interaction. Because the solvent shields the nu-
cleophile, at least one of the ion–dipole interactions must be broken before the nucle-
ophile can participate in an reaction. Weak bases interact weakly with protic
solvents, whereas strong bases interact more strongly because they are better at shar-
ing their electrons. It is easier, therefore, to break the ion–dipole interactions between

SN 2

RO- RS-?

increasing

basicity

increasing

nucleophilicity

in the gas phase

increasing

size

increasing

nucleophilicity

in a protic

solvent

F−

Cl−

Br−

I−

A protic solvent contains a hydrogen

bonded to an oxygen or a nitrogen;

it is a hydrogen bond donor.