P.1
Isomerism
A.
Introduction
The molecular formula (C4H10) has two display structural formulae:
These two molecules are isomers, that is they have the same molecular formula but their
atoms are arranged in a different way.
For more example, propanoic acid and ethyl methanoate have the same molecular
formula, C3H6O2. However, their structures and properties are different:
There are many examples of isomerism in chemistry and these may be divided into two
main classes:
Structural isomerism
Isomers whose molecules have
their atoms linked together in
different bonding arrangements
B.
Steroisomerism
Isomers whose molecules have
their atoms linked together in the
same bonding arrangement but
are arranged differently in space.
Structural isomerism
There are three main types : chain isomerism, positional isomerism and functional group
isomerism.
i. Chain isomers have different length of main hydrocarbon chain: For example C5H12
has two structural isomers
2-methylbutane
2,2-dimethylpropane
Chain isomers have similar chemical properties but different physical properties as
shown below as an example.
properties
pentane
melting pt.
boiling pt.
density (g/cm3)
-130
36
0.63
2-methylbutane
-160
28
0.62
2,2-dimethylpropane
-16
10
0.59
P.2
ii. Positional isomers have a particular atom, or bond, or group of atoms, in different
molecular positions. For example, there are two positional isomers of molecular
formula C3H7Br.
Positional isomers have different physical properties. They take part in similar
chemical reactions, but often at different rates.
iii. Functional group isomers contain different organic functional groups. For example,
propanoic acid and ethyl methanoate have the same molecular formula, C3H6O2.
However, they have different functional groups, (acid and ester respectively).
C.
Stereoisomerism
There are two types of stereoisomerism, geometrical isomerism and optical isomerism
(enantiomerism).
Stereoisomers are molecules which have different properties even though the same
atoms are bonded together in the same way.
Consider the molecular models of 1,2-dichloroethene.
Each molecule has the same atoms linked together in the same way. However, their 3-D
structures differ because the π electron cloud prevents rotation about the double bond.
They are called geometrical isomers of but-2-ene.
Consider the molecules (I) and (II) where W, X, Y and Z are all different.
X
X
W
W
Z
Z
I
II
They are called enantiomers (mirror images of each other). Such stereoisomerism is
called enantiomerism.
P.3
D.
Nomenclature of enantiomers:
The name of the enantiomer is (R)-butan-2-ol. The carbon attaching with four different
groups is called “chiral” carbon (or chiral centre).
E.
Optical activity and enantiomerism
The following are some physical properties of (R)- and (S)-2-butanol
Physical property
Boiling point (oC at 1 atm)
Density (g/cm3 at 20oC)
Index of refraction (at 20oC)
Specific rotation ([α] 25D )
(R) )-2-butanol
99.5
0.808
1.397
-13.52o
Plane polarized light and optical activity:
(S)-2-butanol
99.5
0.808
1.397
+13.52o
P.4
Class work
1.
Draw structural formulae for
a. the four structural isomers of formula C3H6Cl2;
b. the five structural isomers of formula C4H8;
c. the four structural isomers of C4H9Br
2.
Several compounds have the molecular formula C2H4O2.
a. Three of these are functional group isomers containing a carbonyl group, C=O. Give
their structures.
b. Three other isomers contain a C=C bond. Draw their structural formulae.
3.
Give the structural formulae for the isomers having the molecular formulae
b. C5H10O (contains C=O)
a.
C3H6O2 (esters)
4.