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Difficulty: HardStructural Isomerism and Stereoisomerism

How many of the acyclic structural isomers with the molecular formula C4H8Cl2C_4H_8Cl_2 possess at least one chiral carbon atom?

  1. 3Answer
  2. B
    2
  3. C
    4
  4. D
    5

Answer

3 acyclic structural isomers of C4H8Cl2C_4H_8Cl_2 possess at least one chiral carbon atom.
The correct response is 3. A chiral carbon atom is defined as a carbon atom bonded to four different atoms or functional groups. Among the nine constitutional acyclic isomers of dichlorobutane (C4H8Cl2C_4H_8Cl_2), only 1,2-dichlorobutane (at carbon-2), 1,3-dichlorobutane (at carbon-2), and 2,3-dichlorobutane (at carbon-2 and carbon-3) possess carbon atoms meeting this criterion.

Step-by-Step Solution

1
Identify all acyclic structural (constitutional) isomers of C4H8Cl2C_4H_8Cl_2.
There are 9 total acyclic structural isomers: 1,1-dichlorobutane, 1,2-dichlorobutane, 1,3-dichlorobutane, 1,4-dichlorobutane, 2,2-dichlorobutane, 2,3-dichlorobutane, 1,1-dichloro-2-methylpropane, 1,2-dichloro-2-methylpropane, and 1,3-dichloro-2-methylpropane.
Structural isomers must be systematically generated based on butane and methylpropane carbon skeletons.
2
Examine each structural isomer for the presence of a chiral (asymmetric) carbon atom.
A chiral carbon must be bonded to four completely different atoms or groups.
Chirality requires tetrahedral asymmetry around a carbon atom.
3
Determine which specific isomers contain chiral carbons.
1. 1,2-dichlorobutane: Carbon-2 is bonded to −H-H, −Cl-Cl, −CH2Cl-CH_2Cl, and −CH2CH3-CH_2CH_3 (Chiral).
2. 1,3-dichlorobutane: Carbon-2 is bonded to −H-H, −Cl-Cl, −CH3-CH_3, and −CH2CH2Cl-CH_2CH_2Cl (Chiral).
3. 2,3-dichlorobutane: Carbon-2 and Carbon-3 are both bonded to −H-H, −Cl-Cl, −CH3-CH_3, and −CH(Cl)CH3-CH(Cl)CH_3 (Chiral).
All other structural isomers have identical groups attached to each carbon (e.g., two −Cl-Cl atoms on C-1 in 1,1-dichlorobutane, or two methyl groups on C-2 in methylpropane derivatives).

Key Concept

Chirality and Structural Isomerism in Haloalkanes
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Topics

Structural Isomerism and Stereoisomerism

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