Isomers can be broadly divided into two major classes: constitutional isomers and
stereoisomers. In Chapter 25 we discussed isomerism in coordination compounds, and in
Chapter 27 we learned about some isomeric organic compounds. In this chapter we will
take a more systematic look at some three-dimensional aspects of organic structures—a
subject known as stereochemistry(“spatial chemistry”).CONSTITUTIONAL ISOMERS
Constitutional(or structural) isomersdiffer in the orderin which their atoms are
bonded together.In our studies of hydrocarbons in Sections 27-1 through 27-7, we saw some examples of
constitutional isomerism. Recall that there are three isomers of C 5 H 12.28-1
1108 CHAPTER 28: Organic Chemistry II: Shapes, Selected Reactions, and Biopolymers
HHHHHC CHHHHC CHHC H HHHHHHC CHHCCH 3HC HHHHCHHCCH 3CH 3Cpentane, C 5 H 12 methylbutane
isopentane, C 5 H 12dimethylpropane
neopentane, C 5 H 12CH 2 UCHCHUCHCH 2 CH 3 CH 2 UCHCH 2 CHUCHCH 3 CH 2 UCHCH 2 CH 2 CHUCH 2
1,3-hexadiene, C 6 H 10 1,4-hexadiene, C 6 H 10 1,5-hexadiene, C 6 H 10HHHOH OHC CHHC H1,2-propanediol, C 3 H 8 O 2HHHOH OHC CHHC H1,3-propanediol, C 3 H 8 O 2Cl
ClClClo-dichlorobenzene, C 6 H 4 Cl 2 m-dichlorobenzene, C 6 H 4 Cl 2ClCl
p-dichlorobenzene, C 6 H 4 Cl 2These three isomers differ in the lengths of their base chains but not in the functional
groups present (i.e., only alkyl groups are present in this case). As a result, they differ
somewhat in their melting and boiling points but differ only very slightly in the reactions
they undergo.
In one kind of constitutional isomerism, the compounds have the same numberand
kindof functional groups on the same base chainor the same ringbut in different positions.
Isomers that vary only in the location of groups of atoms usually have very similar chem-
ical and physical properties differing mainly in physical properties such as melting and
boiling points. The following groups of isomers are examples in which the type of func-
tional group does not change.