How many structural isomers are possible for the dichloro derivative…

The correct answer is Option (2) → 4

Propane has a chain of three carbons. The two chlorine atoms can sit on the same carbon (a gem-dihalide), on adjacent carbons (a vic-dihalide), or on the two end carbons. Each different placing is one structural isomer.

Isomers:

• 1,1-Dichloropropane, \( \ce{Cl2CHCH2CH3} \) (both Cl on C-1, gem)

• 2,2-Dichloropropane, \( \ce{CH3CCl2CH3} \) (both Cl on C-2, gem)

• 1,2-Dichloropropane, \( \ce{ClCH2CHClCH3} \) (Cl on C-1 and C-2, vic)

• 1,3-Dichloropropane, \( \ce{ClCH2CH2CH2Cl} \) (Cl on C-1 and C-3)

Not new:

• Cl on C-2 and C-3, or both on C-3 — the same compounds as 1,2- and 1,1-dichloropropane, numbered from the other end.

Structural isomers of C₃H₆Cl₂: 4.

NCERT (2026-27 edition):
6.2 Nomenclature – “…The dihalo-compounds having both the halogen atoms are further classified as geminal halides or gem-dihalides when both the halogen atoms are present on the same carbon atom of the chain and vicinal halides or vic-dihalides when halogen atoms are present on adjacent carbon atoms.”

Answers to Some Intext Questions – “6.3 (i) ClCH₂CH₂CH₂Cl (ii) ClCH₂CHClCH₃ (iii) Cl₂CHCH₂CH₃ (iv) CH₃CCl₂CH₃”