Chemistry for Pharmacy Students : General, Organic and Natural ...

dimensional orientation of the hydroxyl group at C–1 is different in each
case. Similarly, cis- and trans-cinnamic acid only differ in the three
dimensional orientation of the atoms or groups.
HO
6 HO
6
O
HO O
4 5
HO
1
3
2
OH
OH
α-Glucose
HO O
4 5
HO
3
2 1
OH
β-Glucose
OH OH
trans-Cinnamic acid
O OH
cis-Cinnamic acid
There are two major types of stereoisomer: conformational isomers and
configurational isomers. Configurational isomers include optical isomers,
geometrical isomers, enantiomers and diastereomers.
Conformational isomers
Atoms within a molecule move relative to one another by rotation around
single bonds. Such rotation of covalent bonds gives rise to different conformations
of a compound. Each structure is called a conformer or conformational
isomer. Generally, conformers rapidly interconvert at room temperature.
Conformational isomerism can be presented with the simplest example,
ethane (C2H6), which can exist as an infinite number of conformers by the
rotation of the C–C s bond. Ethane has two sp 3 -hybridized carbon atoms,
and the tetrahedral angle about each is 109.5 . The most significant
conformers of ethane are the staggered and eclipsed conformers. The
staggered conformation is the most stable as it has the lowest energy.
H H
H H
H H
Rotation about the C-C bond in ethane
Visualization of conformers There are four conventional methods for
visualization of three-dimensional structures on paper. These are the ball
and stick method, the sawhorse method, the wedge and broken line method
and the Newman projection method. Using these methods, the staggered and
eclipsed conformers of ethane can be drawn as follows.
Eclipsed Staggered
H
H
H H
H
H
H
H H
H
H
Ball and stick method
3.2 ISOMERISM 37
H
Eclipsed Staggered
H
H
H
H
H
H
H
H
H
H
H
H
Sawhorse method