How many of the following are vic-dihalides?
1,2-Dibromoethane, 1,1-Dichloroethane, 1,2-Dichloropropane, 2,2-Dichloropropane, 1,3-Dibromopropane, 2,3-Dibromobutane, 1,4-Dichlorobutane
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 3
In a dihaloalkane the two halogen atoms may sit on the same carbon (geminal, gem-dihalide) or on adjacent carbons (vicinal, vic-dihalide). Halogens farther apart than adjacent carbons give neither kind. The numbers in the IUPAC name show where the halogens are.
vic-Dihalides:
1,2-Dibromoethane, \( \ce{BrCH2-CH2Br} \) (carbons 1 and 2)
1,2-Dichloropropane, \( \ce{ClCH2-CHCl-CH3} \) (carbons 1 and 2)
2,3-Dibromobutane, \( \ce{CH3-CHBr-CHBr-CH3} \) (carbons 2 and 3)
Not vic-dihalides:
1,1-Dichloroethane, \( \ce{CH3-CHCl2} \) — both on carbon 1: gem-dihalide.
2,2-Dichloropropane, \( \ce{CH3-CCl2-CH3} \) — both on carbon 2: gem-dihalide.
1,3-Dibromopropane, \( \ce{BrCH2-CH2-CH2Br} \) — carbons 1 and 3 are not adjacent.
1,4-Dichlorobutane, \( \ce{ClCH2-CH2-CH2-CH2Cl} \) — carbons 1 and 4 are not adjacent.
vic-Dihalides: 3 compounds.
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. In common name system, gem-dihalides are named as alkylidene halides and vic-dihalides are named as alkylene dihalides. In IUPAC system, they are named as dihaloalkanes.”