Steroisomerism is exhibited by the compounds which have similar attachment of atoms but differ in their arrangement in space. There are two types of stereoisomerism:
Any compound with a carbon-carbon double bond can exhibit cis-trans isomerism, provided that the carbons involved in the double bond do not have two of the same groups or atoms attached to each of them. 2-Butene is the simplest alkene that can have cis-trans isomerism.
A cis-isomer is the one having identical groups on same side of double bond. On the other hand, a trans-isomer has identical groups on opposite side. The two isomers exist because the rotation of groups across C=C bond is not possible.
Trans isomers are generally more stable than their cis counterparts. Alkyl groups are mildly electron-donating toward the double bond. This can lead to polarity. cis-2-butene has a net dipole moment but, in trans-2-butene the net dipole is zero since the dipole moments cancel out.
The optical isomerism is shown by the compounds having at least one carbon atom joined to four different atoms or groups. Such a carbon atom is called asymmetric or chiral carbon atom. Those which are not chiral (do not have four different groups) are called achiral.
A chiral compound can have two different arrangements of groups attached to the chiral carbon.
The wedge sign shows that the direction of the bonds is towards the viewer and dotted line indicates backward direction of the bonds.
The optical isomers have identical physical properties except optical activity. They rotate the plane of plain polarized light in opposite directions.
The plain polarized light is defined as the light that vibrates in one plane only. The rotation of the plane of polarized light is called optical activity. Those substances that can rotate the plane of polarized light are said to be optically active.
Compounds that rotate the plane of plane polarized light to the right (clockwise) are said to be dextrorotatory. The dextrorotatory is denoted by 'd' or (+) before the name of the compound. Similarly, compounds that rotate the plane to the left (anticlockwise) are called laevorotatory and this is denoted by placing 'l' or (-) before the name of the compound.
A mixture containing equal amounts of d- and l- isomers is called a recemic mixture and is optically inactive denoted by dl or ±.