Organic Chemistry

370 CHAPTER 10 Substitution Reactions of Alkyl Halides

An aprotic solvent does not contain a
hydrogen bonded to either an oxygen
or a nitrogen; it is not a hydrogen bond
donor.

an iodide ion (a weak base) and the solvent than between a fluoride ion (a stronger

base) and the solvent. As a result, iodide ion is a better nucleophile than fluoride ion in

a protic solvent (Table 10.2).

Fluoride ion would be a better nucleophile in a nonpolar solventthan in a polar sol-

vent because there would be no ion–dipole interactions between the ion and the nonpo-

lar solvent. However, ionic compounds are insoluble in most nonpolar solvents, but they

can dissolve in aprotic polar solvents, such as dimethylformamide (DMF) or dimethyl-

sulfoxide (DMSO). An aprotic polar solvent is not a hydrogen bond donor because it

does not have a hydrogen attached to an oxygen or to a nitrogen, so there are no posi-

tively charged hydrogens to form ion–dipole interactions. The molecules of an aprotic

polar solvent have a partial negative charge on their surface that can solvate cations, but

the partial positive charge is on the insideof the molecule, which makes it less accessi-

ble. The relatively “naked”anion can be a powerful nucleophile in an aprotic polar sol-

vent. Fluoride ion, therefore, is a better nucleophile in DMSO than it is in water.

PROBLEM 5

Indicate whether each of the following solvents is protic or aprotic:

a. chloroform c. acetic acid

b. diethyl ether (CH 3 CH 2 OCH 2 CH 3 ) d. hexane [CH 3 (CH 2 ) 4 CH 3 ]

(CHCl 3 ) (CH 3 COOH)

δδ−−

δ−

δ−

δ+

δ+

δ+

δ+

δ−

δ+

δ+

δ+

δ+ H

Y− HO

H

H

O

H

H

O

H

O H

ion–dipole interactions

between a nucleophile

and water

3-D Molecules:

N,N-Dimethylformamide;

Dimethyl sulfoxide

N,N-dimethylformamide

DMF

C

H CH 3

Oδ−

CH 3

N

dimethyl sulfoxide

DMSO

H 3 CCH 3

the δ− is on the surface

of the molecule

S

Oδ−

δ+

δ+

the δ+ is not

very accessible

δ+

δ+

δ+

δ−

δ+ δ−

δ−

CH 3

CH 3 CH 3

CH 3 CH 3

CH 3 CH 3

CH 3

S

S

O

δ−

OOK+

O

SS

DMSO can solvate a cation better

than it can solvate an anion

increasing nucleophilicity

Table 10.2 Relative Nucleophilicity Toward CH 3 l in Methanol

RS− > I−^ > −C N > CH 3 O− > Br−^ > NH 3 > Cl− > F−^ > CH 3 OH