n-Butane on monochlorination under photochemical condition gives an...

The structure of n-butane is $CH_3-CH_2-CH_2-CH_3$.

Monochlorination of n-butane gives two structural isomers: 1-chlorobutane and 2-chlorobutane.

Among these, 2-chlorobutane ($CH_3-CH_2-C^{*}HCl-CH_3$) 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):

1. Chlorination at C-1: $CH_3-CH_2-CHCl-CH_2Cl$ (1,2-dichlorobutane)

2. Chlorination at C-2: $CH_3-CH_2-CCl_2-CH_3$ (2,2-dichlorobutane)

3. Chlorination at C-3: $CH_3-CHCl-CHCl-CH_3$ (2,3-dichlorobutane)

4. Chlorination at C-4: $CH_2Cl-CH_2-CHCl-CH_3$ (1,3-dichlorobutane)

Therefore, 4 structurally distinct dichloro compounds are obtained.

Answer: $4$