Organic Chemistry

82 CHAPTER 2 An Introduction to Organic Compounds

Relatively weak forces hold alkane molecules together. Alkanes contain only car-

bon and hydrogen atoms. Because the electronegativities of carbon and hydrogen are

similar, the bonds in alkanes are nonpolar. Consequently, there are no significant par-

tial charges on any of the atoms in an alkane.

It is, however, only the average charge distribution over the alkane molecule that is

neutral. Electrons are moving continuously, so at any instant the electron density on

one side of the molecule can be slightly higher than that on the other side, giving the

molecule a temporary dipole.

A temporary dipole in one molecule can induce a temporary dipole in a nearby

molecule. As a result, the negative side of one molecule ends up adjacent to the posi-

tive side of another molecule, as shown in Figure 2.1. Because the dipoles in the mol-

ecules are induced, the interactions between the molecules are called induced-dipole–

induced-dipole interactions. The molecules of an alkane are held together by these

induced-dipole–induced-dipole interactions, which are known as van der Waals

forces. Van der Waals forces are the weakest of all the intermolecular attractions.

In order for an alkane to boil, the van der Waals forces must be overcome. The

magnitude of the van der Waals forces that hold alkane molecules together depends

on the area of contact between the molecules. The greater the area of contact, the

stronger are the van der Waals forces and the greater is the amount of energy needed

to overcome those forces. If you look at the homologous series of alkanes in

Table 2.1, you will see that the boiling points of alkanes increase as their size in-

creases. This relationship holds because each additional methylene group increases

the area of contact between the molecules. The four smallest alkanes have boiling

points below room temperature (room temperature is about 25 °C), so they exist as

gases at room temperature. Pentane is the smallest alkane that is a

liquid at room temperature.

Because the strength of the van der Waals forces depends on the area of contact be-

tween the molecules, branching in a compound lowers its boiling point because it re-

duces the area of contact. A branched compound has a more compact, nearly spherical

shape. If you think of the unbranched alkane pentane as a cigar and branched neopen-

tane as a tennis ball, you can see that branching decreases the area of contact between

molecules: Two cigars make contact over a greater area than do two tennis balls. Thus,

if two alkanes have the same molecular weight, the more highly branched alkane will

have a lower boiling point.

The boiling points of the compounds in any homologous series increase as their

molecular weights increase because of the increase in van der Waals forces. So the

boiling points of the compounds in a homologous series of ethers, alkyl halides,

alcohols, and amines increase with increasing molecular weight. (See Appendix I.)

CH 3 CH 2 CH 2 CH 2 CH 3

pentane

bp = 36.1 °C

CH 3 CHCH 2 CH 3

isopentane

bp = 27.9 °C

CH 3

CH 3 CCH 3

neopentane

bp = 9.5 °C

CH 3

CH 3

(bp=36.1 °C)

Johannes Diderik van der Waals

(1837–1923)was a Dutch physicist.

He was born in Leiden, the son of a

carpenter, and was largely self-taught

when he entered the University of

Leiden, where he earned a Ph.D. He

was a professor of physics at the

University of Amsterdam from 1877

to 1903. He won the 1910 Nobel

Prize for his research on the gaseous

and liquid states of matter.

δ− δ+

δ− δ+

δ− δ+

δ− δ+

δ− δ+

δ− δ+

δ− δ+

δ− δ+

Figure 2.1N

Van der Waals forces are induced-

dipole–induced-dipole interactions.

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