Question

What are possible structural isomers of \( \text{C}_3\text{H}_6\text{Cl}_2 \)?

Hint:

To avoid missing isomers or counting duplicates, work systematically: try placing multiple substituents on the same carbon first, then move one substituent progressively further down the chain. Always name them to ensure they are unique.

Answer 1

Step 1: Understanding the Concept:
Structural isomers are molecules with the same molecular formula but different bonding patterns (connectivity). To find them systematically, first determine the degree of unsaturation, draw the carbon skeleton, and then place the functional groups (chlorine atoms) in all possible unique positions.

Step 2: Key Formula or Approach:
Calculate the Degree of Unsaturation (DU) to confirm there are no rings or double bonds.
$\text{DU} = C + 1 - \frac{H}{2} - \frac{X}{2} + \frac{N}{2}$
$\text{DU} = 3 + 1 - \frac{6}{2} - \frac{2}{2} = 4 - 3 - 1 = 0$.
Since DU is 0, the carbon skeleton is an open-chain alkane.

Step 3: Detailed Explanation:
The basic carbon skeleton for a 3-carbon alkane is propane: $\text{C}-\text{C}-\text{C}$.
We need to attach two chlorine atoms to this skeleton. We can classify the possibilities based on whether the chlorine atoms are on the same carbon (geminal) or different carbons (vicinal/terminal).
Case 1: Both Cl atoms on the same carbon (Geminal dihalides)
- Place both on an end carbon (C1): $\text{CH}_3-\text{CH}_2-\text{CHCl}_2$
Name: 1,1-dichloropropane
- Place both on the middle carbon (C2): $\text{CH}_3-\text{CCl}_2-\text{CH}_3$
Name: 2,2-dichloropropane
Case 2: Cl atoms on different carbons
- Place them on adjacent carbons (C1 and C2): $\text{CH}_3-\text{CHCl}-\text{CH}_2\text{Cl}$
Name: 1,2-dichloropropane
- Place them on the terminal carbons (C1 and C3): $\text{CH}_2\text{Cl}-\text{CH}_2-\text{CH}_2\text{Cl}$
Name: 1,3-dichloropropane
There are no other unique ways to attach the two chlorine atoms to a 3-carbon chain.

Step 4: Final Answer:
There are 4 possible structural isomers.