To solve the question step by step, we will analyze the information provided and deduce the structure of the dihalogen derivative.
### Step 1: Understand the Reaction
The problem states that a dihalogen derivative reacts with alcoholic potash (KOH) to form an unsaturated hydrocarbon that gives a red precipitate with ammoniacal cuprous chloride. Additionally, the same dihalogen derivative produces propanal when reacted with aqueous potash.
### Step 2: Identify the Dihalogen Derivative
Since the dihalogen derivative gives propanal upon treatment with aqueous KOH, we can deduce that the carbon skeleton of the dihalogen compound must have three carbon atoms (since propanal has three carbons).
### Step 3: Determine Possible Structures
The possible structures for a three-carbon dihalogen derivative (with two halogens) include:
1. 1,1-dihalopropane
2. 1,2-dihalopropane
3. 1,3-dihalopropane
4. 2,2-dihalopropane
### Step 4: Analyze the Reaction with Aqueous KOH
When treated with aqueous KOH, the dihalogen derivative undergoes nucleophilic substitution, where the halogens are replaced by hydroxyl groups (OH). For propanal to be formed, the reaction must lead to a carbonyl compound.
- In the case of 1,1-dihalopropane, the reaction would yield a geminal diol which can dehydrate to form propanal.
- In the case of 1,2-dihalopropane, it would yield propanol, which is not the desired product.
- 1,3-dihalopropane would not lead to propanal.
- 2,2-dihalopropane would lead to a ketone, not an aldehyde.
Thus, the only viable candidate is **1,1-dihalopropane**.
### Step 5: Confirm the Unsaturated Hydrocarbon
When 1,1-dihalopropane reacts with alcoholic KOH, it undergoes dehydrohalogenation to form an alkyne. The alkyne formed from 1,1-dichloropropane would be propyne (CH≡C-CH3), which is a terminal alkyne.
### Step 6: Red Precipitate with Ammoniacal Cuprous Chloride
Terminal alkynes react with ammoniacal cuprous chloride to give a red precipitate, confirming that the product from the dihalogen derivative is indeed a terminal alkyne.
### Conclusion
Based on the analysis, the dihalogen derivative that fits all the criteria is **1,1-dichloropropane**.
### Final Answer
The dihalogen derivative is **1,1-dichloropropane**.
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