Organic Chemistry

390 CHAPTER 10 Substitution Reactions of Alkyl Halides

SOLVATION EFFECTS

The tremendous amount of energy that is provid-

ed by solvation can be appreciated by considering

the energy required to break the crystal lattice of sodium chlo-

ride (table salt) (Figure 1.1). In the absence of a solvent, sodium

chloride must be heated to more than 800°C to overcome the

forces that hold the oppositely charged ions together. However,

sodium chloride readily dissolves in water at room temperature

because solvation of the and ions by water provides

the energy necessary to separate the ions.

Na+ Cl-

Stabilization of charges by solvent interaction plays an important role in organic re-

actions. For example, when an alkyl halide undergoes an reaction, the first step is

dissociation of the carbon–halogen bond to form a carbocation and a halide ion. Ener-

gy is required to break the bond, but with no bonds being formed, where does the en-

ergy come from? If the reaction is carried out in a polar solvent, the ions that are

produced are solvated. The energy associated with a single ion–dipole interaction is

small, but the additive effect of all the ion–dipole interactions involved in stabilizing a

charged species by the solvent represents a great deal of energy. These ion–dipole in-

teractions provide much of the energy necessary for dissociation of the carbon–halo-

gen bond. So in an reaction, the alkyl halide does not fall apart spontaneously, but

rather, polar solvent molecules pull it apart. An reaction, therefore, cannot take

place in a nonpolar solvent. It also cannot take place in the gas phase, where there are

no solvent molecules and, consequently, no solvation effects.

SN 1

SN 1

SN 1

Table 10.7 The Dielectric Constants of Some Common Solvents

Solvent Structure Abbreviation Dielectric constant

(ε, at 25°C)

Boiling point

(°C)

Water H 2 O79

Formic acid HCOOH 59

Methanol CH 3 OH 33

Ethanol CH 3 CH 2 OH 25

tert-Butyl alcohol (CH 3 ) 3 COH 11

Acetic acid CH 3 COOH 6

Dimethyl sulfoxide (CH 3 ) 2 SO 47

Acetonitrile CH 3 CN 38

Dimethylformamide (CH 3 ) 2 NCHO 37

Hexamethylphosphoric acid triamide [(CH 3 ) 2 N] 3 PO 30

Acetone (CH 3 ) 2 CO 21

Eth

Dichloromethane

Tetrahydrofuran

yl acetate CH

CH 2 Cl 2

3 COOCH 2 CH 3 6

9.1

Diethyl ether CH 3 CH 2 OCH 2 CH 3 4.3

Benzene 2.3

Hexane CH 3 (CH 2 ) 4 CH 3 1.9

100

100.6

64.7

78.3

82.3

117.9

189

81.6

153

233

56.3

77.1

40

7.6 66

34.6

80.1

68.7

—

—

MeOH

EtOH

tert-BuOH

HOAc

DMSO

MeCN

DMF

HMPA

Me 2 CO

THF

EtOAc

—

Et 2 O

O

—

—

Protic solvents

Aprotic solvents