The structure of n-butane is CH3−CH2−CH2−CH3.
Monochlorination of n-butane gives two structural isomers: 1-chlorobutane and 2-chlorobutane.
Among these, 2-chlorobutane (CH3−CH2−C∗HCl−CH3) contains a chiral carbon atom and is optically active. Thus, the optically active compound "P" is 2-chlorobutane.
Further chlorination of 2-chlorobutane can take place at any of the four carbon atoms, yielding the following dichloro compounds (ignoring stereoisomers):
Chlorination at C-1: CH3−CH2−CHCl−CH2Cl (1,2-dichlorobutane)
Chlorination at C-2: CH3−CH2−CCl2−CH3 (2,2-dichlorobutane)
Chlorination at C-3: CH3−CHCl−CHCl−CH3 (2,3-dichlorobutane)
Chlorination at C-4: CH2Cl−CH2−CHCl−CH3 (1,3-dichlorobutane)
Therefore, 4 structurally distinct dichloro compounds are obtained.
Answer: 4