Isomerism in organic compounds

Isomerism in Organic Compounds

Structural isomerism for aliphatic compounds containing up to six carbon atoms, to include branched structures and position of the C=C double bonds in alkenes. (Cyclic compounds excluded).

Isomerism (greek isos, equal; meros, parts) occurs when there are two or more compounds (called isomers) with the same molecular formula but different arrangements of their atoms.

Isomers have different physical and chemical properties but the differences may be great or small depending on the type of isomerism.
There are two main classes of isomerism;


(i) structural isomerism

(ii) stereoisomerism

which are themselves sub-divided.

ISOMERISM

structural isomerism stereoisomerism
chain positional functional group geometric optical
isomerism isomerism isomerism isomerism isomerism

Structural isomerism

Structural isomers are molecules with the same molecular formula but with different structural arrangements of the atoms.

Chain isomers

This occurs where isomers have different arrangements of the carbon chain. There is only one alkane corresponding to each of the formulae CH4, C2H6 and C3H8.



For butane C4H10 two arrangements are possible.

n- butane
butane methylpropane
b.pt. 273K b.pt. 261K

As the number of carbon atoms increases the number of possible isomers increases rapidly.

Formula
Number of structural isomers
C5H12-3
C6H14-5
C7H16-9

Positional isomers

This occurs when isomers have the same carbon skeleton but the functional group is in different positions in the molecule.
propan-1-ol
propan-2-ol

Exercise 1
(a) Draw and name the five structural isomers of C6H14.

(b) Write the structural formulae of all the alcohols of molecular formula C4H10O. Name the isomers.


Stereoisomerism : cis-trans isomers for compounds containing one C=C bond, the energy barrier to rotation in these compounds.

Stereoisomerism
Stereoisomers have identical molecular formulae, and the atoms are linked together in the same order, but have different relative positions in space.
The two types of stereoisomerism are
(i) Geometric (or cis-trans ) isomerism
(ii) Optical isomerism [See Module 4.5]

· Geometric isomers
Geometric or cis-trans isomers exist because the π bond of the C=C bond prevents free rotation .

H H Cl H
C=C C=C
Cl Cl H Cl
cis-1,2-dichloroethane trans-1,2-dichloroethane

Cis-trans isomerism can also occur in inorganic complexes about a single bond with square planar or octahedral structures.

e.g. diaminedichloro platinum (II)


Exercise 2
1. Draw and name all the structural isomers, including geometric isomers, of C4H8.