Which types of isomerism is shown by 2,3-dichlorobutane?

2,3-Dichlorobutane has the condensed formula $$CH_3-CH(Cl)-CH(Cl)-CH_3$$. The second and third carbon atoms each bear four different groups: $$H$$, $$Cl$$, $$CH_3$$ and $$CHClCH_3$$, so both $$C_2$$ and $$C_3$$ are chiral centres.

With two chiral centres, the molecule can be arranged in three distinct stereoisomers:

(i) the $$d$$-(+) form, (ii) the $$l$$-(-) form and (iii) the meso form (internally compensated because of an internal mirror plane).

The $$d$$ and $$l$$ forms are non-superimposable mirror images, i.e. an enantiomeric pair. Enantiomers rotate the plane of polarised light in opposite directions and therefore come under optical isomerism. The meso form, being achiral, does not rotate light but is still a stereoisomer of the other two.

2,3-Dichlorobutane does not show:

• Geometric (cis-trans) isomerism - no rigid double bond or ring with suitable substituents is present.
• Structural (constitutional) isomerism - the connectivity of atoms is fixed; only spatial arrangement changes.
• Pure “diastereo” as a separate category in this question - although the $$d/l$$ pair and the meso form are diastereomers of one another, the examination classifies the phenomenon primarily under optical isomerism.

Hence the compound is best described, in the context of the given options, as exhibiting optical isomerism.

Option B which is: Optical