The Foundations of Chemistry

Two different compounds have the formula C 4 H 10 but different structures and, hence,

different properties. Such isomersresult when two molecules contain the same atoms

bonded together in different orders. The structures of these two isomeric C 4 H 10 alkanes

are shown in Figure 27-5. These two structures correspond to the two ways in which a

hydrogen atom can be removed from a propane molecule and replaced by a XCH 3 group.

If a XCH 3 replaces an H on either of the end carbon atoms, the result is butane. It is

considered to be a straight-chain hydrocarbon because there is no branching. Though

somewhat misleading, the term “straight chain” is widely used. The carbon chains are

linear only in the structural formulas that we write. They are actually zigzag due to the

tetrahedral bond angles at each carbon and are sometimes further kinked or twisted. Think

of such a chain of carbon atoms as continuous.We can trace a single path from one terminal

carbon to the other and pass through every other C atom without backtracking.

If the XCH 3 group replaces an H from the central carbon atom of propane, the result

is the branched-chain hydrocarbon 2-methylpropane, or isobutane. This is the simplest

branched-chain hydrocarbon.

The formulas of the alkanes can be written in general terms as CnH 2 n 2 , where nis

the number of carbon atoms per molecule. The first five members of the series are

Iso“same”; mer“part.” As we
saw in Sections 25-6 and 25-7, isomers
are substances that have the same
numbers and kinds of atoms arranged
differently. Isomerism in organic
compounds is discussed more
systematically in Chapter 28.

1044 CHAPTER 27: Organic Chemistry I: Formulas, Names, and Properties

CH 3 CH 3 or H C

H

H

H

H

C H

(a) (b)

(c)

C

H

H

H

H

C

H

H

(d)

Figure 27-3 Models of ethane, C 2 H 6. (a) The condensed and Lewis formulas for ethane.

(b) A ball-and-stick model, (c) a space-filling model of ethane, and (d) a 3-D representation.

Figure 27-4 Ball-and-stick and
space-filling models and a three-
dimensional representation of
propane, C 3 H 8.

H CCC H

H

H H

H

H

H