The Foundations of Chemistry

The number of constitutional isomers increases rapidly as the number of carbon atoms

in saturated hydrocarbons increases. There are five isomeric C 6 H 14 alkanes (Table 27-3).

Table 27-4 displays the number of isomers of some saturated hydrocarbons (alkanes). Most

of the isomers have not been prepared or isolated. They probably never will be.

As the degree of branching increases for a series of molecules of the same molecular

weight, the molecules become more compact. A compact molecule can have fewer points

of contact with its neighbors than more extended molecules do. As a result, the total

induced dipole forces (dispersion forces) are weaker for branched molecules, and the

boiling points of such compounds are lower. This pattern can be seen in the data in Table

27-3 and in Example 27-1.

EXAMPLE 27-1 Isomeric Alkanes

Write the Lewis formula of each of the three isomers having the molecular formula of C 5 H 12.

The boiling points of the three isomers are 9.5°C, 27.9°C, and 36.1°C. Match each compound

with its boiling point.

Plan

We first write the carbon skeleton of the straight-chain isomer (no branching). Then we remove

one carbon from the straight-chain structure and reposition it to form the skeleton for a

branched isomer. We check to see whether branching is possible at other carbon atoms. Then

1046 CHAPTER 27: Organic Chemistry I: Formulas, Names, and Properties

TABLE 27-2 Some Straight-Chain Hydrocarbons (Alkanes)

Molecular Normal Normal State at Room

Formula IUPAC Name bp (°C) mp (°C) Temperature

CH 4 methane  161  184

C 2 H 6 ethane  88  183

gas

C 3 H 8 propane  42  188

C 4 H 10 butane 0.6  138

C 5 H 12 pentane 36  130

C 6 H 14 hexane 69  94

C 7 H 16 heptane 98  91

C 8 H 18 octane 126  57

C 9 H 20 nonane 150  54

C 10 H 22 decane 174  30

liquid

C 11 H 24 undecane 194.5 25.6

C 12 H 26 dodecane 214.5 9.6

C 13 H 28 tridecane 234 6.2

C 14 H 30 tetradecane 252.5 5.5

C 15 H 32 pentadecane 270.5 10

C 16 H 34 hexadecane 287.5 18

C 17 H 36 heptadecane 303 22.5

C 18 H 38 octadecane 317 28

solid

C 19 H 40 nonadecane 330 32

C 20 H 42 eicosane 205 .(at 15 torr) 36.7