Organic Chemistry

Introduction 61

where nis any integer. So, if an alkane has one carbon atom, it must have

four hydrogen atoms; if it has two carbon atoms, it must have six hydrogens.

We have seen that carbon forms four covalent bonds and hydrogen forms only one

covalent bond (Section 1.4). This means that there is only one possible structure for an

alkane with molecular formula (methane) and only one structure for an alkane

with molecular formula (ethane). We examined the structures of these com-

pounds in Section 1.7. There is also only one possible structure for an alkane with

molecular formula (propane).

As the number of carbons in an alkane increases beyond three, the number of pos-

sible structures increases. There are two possible structures for an alkane with molec-

ular formula In addition to butane—a straight-chain alkane—there is a

branched butane called isobutane. Both of these structures fulfill the requirement that

each carbon forms four bonds and each hydrogen forms only one bond.

Compounds such as butane and isobutane that have the same molecular formula

but differ in the order in which the atoms are connected are called constitutional

isomers—their molecules have different constitutions. In fact, isobutane got its name

because it is an “iso”mer of butane. The structural unit—a carbon bonded to a hydro-

gen and two groups—that occurs in isobutane has come to be called “iso.”Thus,

the name isobutane tells you that the compound is a four-carbon alkane with an iso

structural unit.

CH 3 CH 2 CH 2 CH 3

butane

CH 3 CHCH 3

isobutane

CH 3

CH 3 CH

an “iso”

structural unit

CH 3

CH 3

C 4 H 10.

C 3 H 8

C 2 H 6

CH 4

CnH 2 n+ 2 ,

C

H

H

name Kekulé structure condensed structure ball-and-stick model

methane H HCH 4

C

H

H

C

H

HCH 3 CH 3

H

ethane H

C

H

H

C

H

C

H

H

HCH 3 CH 2 CH 3

H

propane H

C

H

H

C

H

C

H

H

C

H

H

HCH 3 CH 2 CH 2 CH 3

H

butane H

3-D Molecules:

Methane; Ethane;

Propane; Butane

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