The unsaturated hydrocarbon obtained by the reaction of a dihalogen derivative with alcoholic potash gives a red precipitate with ammoniacal cuprous chloride. The dihalogen derivative gives propanal on heating with aqueous potash. The dihalogen derivative is:

Answer

To solve the problem step by step, we need to analyze the information given in the question and deduce the correct dihalogen derivative.

### Step 1: Understanding the Reaction
The question states that a dihalogen derivative reacts with alcoholic potash (alcoholic KOH) to form an unsaturated hydrocarbon. This unsaturated hydrocarbon gives a red precipitate with ammoniacal cuprous chloride.

**Hint:** Identify the type of reaction that occurs with alcoholic KOH.

### Step 2: Identifying the Unsaturated Hydrocarbon
The unsaturated hydrocarbon formed must have a double or triple bond, as it is referred to as unsaturated. The red precipitate indicates that the unsaturated hydrocarbon is likely an alkyne, as alkynes react with ammoniacal cuprous chloride to form a red precipitate.

**Hint:** Recall the characteristic reactions of alkynes with ammoniacal cuprous chloride.

### Step 3: Analyzing the Dihalogen Derivative
The dihalogen derivative also gives propanal when heated with aqueous potash (aqueous KOH). This suggests that the dihalogen derivative must be a compound that can undergo dehydrohalogenation to form propanal.

**Hint:** Consider the structure of propanal and how it can be formed from a dihalogen derivative through the elimination of halogens.

### Step 4: Structure of the Dihalogen Derivative
To yield propanal (C3H6O), the dihalogen derivative must be a 3-carbon compound with two halogen atoms. The most likely candidate is 1,2-dichloropropane (C3H6Cl2).

1. When 1,2-dichloropropane reacts with alcoholic KOH, it undergoes dehydrohalogenation to form propene (C3H6), which is an unsaturated hydrocarbon.
2. Propene can then react with ammoniacal cuprous chloride to give a red precipitate.

**Hint:** Visualize the structure of 1,2-dichloropropane and how it can lead to propene through elimination.

### Step 5: Conclusion
Thus, the dihalogen derivative that fits all the criteria given in the question is **1,2-dichloropropane**.

**Final Answer:** The dihalogen derivative is **1,2-dichloropropane**.