Organic Chemistry

Section 2.9 Physical Properties of Alkanes, Alkyl Halides, Alcohols, Ethers, and Amines 83

The boiling points of these compounds, however, are also affected by the polar char-

acter of the bond (where Z denotes N, O, F, Cl, or Br) because nitrogen, oxy-

gen, and the halogens are more electronegative than the carbon to which they are

attached.

The magnitude of the charge differential between the two bonded atoms is indicated

by the bond dipole moment (Section 1.3).

Molecules with dipole moments are attracted to one another because they can align

themselves in such a way that the positive end of one dipole is adjacent to the negative

end of another dipole. These electrostatic attractive forces, called dipole–dipole interac-

tions, are stronger than van der Waals forces, but not as strong as ionic or covalent bonds.

Ethers generally have higher boiling points than alkanes of comparable molecular

weight because both van der Waals forces and dipole–dipole interactions must be

overcome for an ether to boil (Table 2.5).

As the table shows, alcohols have much higher boiling points than alkanes or ethers

of comparable molecular weight because, in addition to van der Waals forces and the

dipole–dipole interactions of the bond, alcohols can form hydrogen bonds. A

hydrogen bond is a special kind of dipole–dipole interaction that occurs between a hy-

drogen that is bonded to an oxygen, a nitrogen, or a fluorine and the lone-pair elec-

trons of an oxygen, nitrogen, or fluorine in another molecule.

The length of the covalent bond between oxygen and hydrogen is The hy-

drogen bond between an oxygen of one molecule and a hydrogen of another molecule

is almost twice as long (1.69–1.79 ), which means that a hydrogen bond is not as

strong as an O¬Hcovalent bond. A hydrogen bond, however, is stronger than other

Å

0.96 Å.

C¬O

cyclopentane

bp = 49.3 °C

tetrahydrofuran

bp = 65 °C

O

−+ −+ −+

+ +−

− +

+− −

H 3 CNH 2

0.2 D

H 3 CF

1.6 D

H 3 CBr

1.4 D

H 3 CCl

1.5 D

H 3 CCHO 3

0.7 D

H 3 CI

1.2 D

H 3 COH

0.7 D

RCZZ = N, O, F, Cl, or Br

δ+ δ−

C¬Z

Table 2.5 Comparative Boiling Points (°C)

Alkanes Ethers Alcohols Amines

78 16.6

10.8 97.4 47.8

36.1 34.5 117.3 77.8

CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CH 2 OCH 2 CH 3 CH 3 CH 2 CH 2 CH 2 OH CH 3 CH 2 CH 2 CH 2 NH 2

0.5

CH 3 CH 2 CH 2 CH 3 CH 3 OCH 2 CH 3 CH 3 CH 2 CH 2 OH CH 3 CH 2 CH 2 NH 2

42.1 23.7

CH 3 CH 2 CH 3 CH 3 OCH 3 CH 3 CH 2 OH CH 3 CH 2 NH 2

The dipole moment of a bond is equal to

the magnitude of the charge on one of

the bonded atoms times the distance

between the bonded atoms.

More extensive tables of physical prop-

erties can be found in Appendix I.

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