Page 194 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Organic Chemical Systems, Theory 441

frequently written with auxiliary symbols such as circles, H H 3 C
often with dashed lines, particularly to indicate the delo-
calization of charge.
CH 3 H
21 22
D. Isomerism
cyclohexane. Since rotations about single bonds are nor-
Molecules that do not differ in overall molecular formula
mally very facile, it is usually impossible to separate
(specified by the number of atoms of each kind and the net
conformers from one another and to handle them sepa-
charge) but only in internal structure are called isomers.
rately at room temperature.
1. Constitutional Isomers c. Chirality. Another important classification of
stereoisomers into two groups is related to optical activity,
Constitutional isomers are compounds that differ in con-
manifested by the rotation of a plane of polarized light on
nectivity, that is, in the way in which the constituent atoms
passage through a sample.
are connected to one another. Graph theory is an important
A pair of stereoisomers that are related to one another in
tool for their enumeration.
the same way as an object and its mirror image are called
An important class of constitutional isomers are posi-
enantiomers (e.g., 23 and 24):
tional isomers, in which the functional groups are the same
but differ in their location within the molecule (e.g., 6 and
7, or the ortho, meta, and para isomers 18–20): H F F H
Cl Cl
F H H F
Cl
23 24
Any pair of stereoisomers that are not related in this way
Cl Cl
are called diastereomers (e.g., 6 and 8 or 21 and 22).
18 19 Cl
A molecule that is not identical with its mirror image
20
is called chiral and occurs as a pair of enantiomers. The
2. Stereoisomers necessary and sufficient condition for chirality is the ab-
sence of an improper rotational axis of symmetry in the
Stereoisomers have the same connectivity but differ in the
molecule (this includes a center of inversion and mirror
way in which the constituent atoms are oriented in space.
plane symmetry elements).
They can be divided into configurational stereoisomers
A mixture of equal amounts of two enantiomers is
and conformational stereoisomers. The precise specifica-
known as a racemic modification and is optically inac-
tion of the spatial arrangement of the groups in a con-
tive. A pure enantiomer or an unbalanced mixture of two
figurational isomer is called its configuration, and in a
enantiomers is optically active; the two enantiomers have
conformational isomer, its conformation.
opposite handedness and cause the plane of polarization
to rotate in opposite directions.
a. Configurational stereoisomers. These stereo-
isomers cannot be made superimposable by any rotations
about single bonds. In order to make them superimpos- II. QUALITATIVE MOLECULAR ORBITAL
able, rotation about a double bond or a dissociation of one MODEL OF ELECTRONIC STRUCTURE
or more single bonds, or both, is necessary (e.g., 6 and
8). Since these processes normally require considerable Although the classical structural theory as outlined so far
energy, they usually do not occur at a measurable rate at accounts for much of organic chemistry, it leaves quite
room temperature. Configurational stereoisomers can nor- a few observations unexplained. In order to proceed and
mally be isolated from one another and stored essentially relate the chemical behavior of molecules to fundamental
indefinitely at room temperature. physical principles it is necessary to use quantum theory.
This can be done at a more or less rigorous quan-
b. Conformational stereoisomers. Often called titative level. A fairly direct translation of the struc-
conformers for short, these stereoisomers can be made tural concepts just outlined into mathematical terms then
superimposable by rotations about single bonds. Exam- leads to the quantum mechanical valence–bond theory
ples are the axial (21) and equatorial (22) conformers of a of electronic structure. Although conceptually appealing,
monosubstituted computationally this method is quite unwieldy and has