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Isomerism:
Chain isomer
Positional isomer
Functional isomer
Geometrical isomer
Enantiomer / Optical isomer
Structural isomer
Isomer
Stereoisomer
A. Structural isomer
1. Chain isomer:
Example:
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
HH
H
pentane
2-methylbutane
How to draw chain isomer:
C6H14:
Main carbon chain with 6 Cs:
H
H
HH
H
H
H
2,2-dimethylpropane
Main carbon chain with 5 Cs:
Main carbon chain with 4 Cs:
2. Positional isomer:
Example:
OH
OH
hexan-1-ol
3. Functional isomer:
Example:
hexan-2-ol
OH
hexan-3-ol
O
OH
butan-1-ol
hex-1-ene
ethoxyethane
cyclohexane
B. Stereoisomer
1. Geometrical isomer:
Isomerism / page 1
Example:
Cl
Cl
Cl
H
H
H
H
Cl
cis-1,2-dichloroethene
trans-1,2-dichloroethene
less stable
more stable
2. Enantiomer (Optical isomer):
Example:
F
F
H
H
Br
Cl
Cl
Br
 -bromochlorofluoromethane
dextrorotatory, d, +
laevorotatory,  , They have the same physical properties (including melting point, boiling point, density, solubility) but
different specific optical rotation.
Br *C is an asymmetric center (chiral centre), so the compound is optically active.
No of optical isomer = 2 n
d-bromochlorofluoromethane
*
Cl
H
F
where n is the no. of asymmetry
Racemic mixture: A mixture of 50% d- and 50%  - isomer by mole.
* Polarimeter: an instruement is used to measure the optical rotation.
Na lamp
specific rotation:
polarizer
cell with the
sample solution
analyzer
eye

 25D 

c
100
where  is the optical rotation measured,
 is the length of the cell, default is 1 dm,
c is the mass of the solute in 100 cm3 solution,
 25D
is the specific optical rotation at 25C by using sodium D-line as light source.
Isomerism / page 2