How many isomers has C$_3$H$_6$CI$_2$?

Analyze the question: C₃H₆Cl₂ is a dichloropropane (propane, C₃H₈, with two H atoms replaced by Cl). We need to count the structural isomers.

Concept: Propane has the carbon skeleton C–C–C. Number the carbons 1, 2, 3. Carbons 1 and 3 are equivalent (the end carbons), and carbon 2 is the middle one. We place two Cl atoms in every distinct way.

Solution:

1. Both Cl on the same carbon: • On C1: CHCl₂–CH₂–CH₃ is 1,1-dichloropropane • On C2: CH₃–CCl₂–CH₃ is 2,2-dichloropropane

2. Cl on different carbons: • C1 and C2: CH₂Cl–CHCl–CH₃ is 1,2-dichloropropane • C1 and C3: CH₂Cl–CH₂–CH₂Cl is 1,3-dichloropropane • C2 and C3 is the same as C1 and C2 by symmetry, so it is not a new isomer.

Total = 4 structural isomers: 1,1-, 1,2-, 1,3- and 2,2-dichloropropane.

(If you also counted optical isomers, 1,2-dichloropropane has a chiral carbon, so it would have two enantiomers.)

Answer: C (4)

Tip: For dichloro-substituted alkanes, fix the first Cl on each different carbon, then place the second Cl while avoiding repeats caused by symmetry. Always check that your "new" structure is not just the same molecule flipped over.