Treatment of benzene with CO/HCl in the presence of anhydrous `"AlCl"_(3)//"CuCl" ` followed by reaction with `Ac_(2)"O"//"NaOAc"` gives compound X as the major product. Compound X upon reaction with `Br_(2)//Na_(2)CO_(3)`, followed by heating at 473 K with moist KOH furnishes Y as the major product. Reaction of X with `H_(2)//Pd-C`, followed by `H_(3)PO_(4)` treatment gives Z as the major product.
The compound Z is

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Answer

To solve the problem step by step, let's break down the reactions and identify the compounds formed at each stage.

### Step 1: Reaction of Benzene with CO/HCl and Anhydrous AlCl3/CuCl
- **Reaction**: Benzene reacts with carbon monoxide (CO) and hydrochloric acid (HCl) in the presence of anhydrous aluminum chloride (AlCl3) and cuprous chloride (CuCl).
- **Product**: This reaction is known as the Gatterman-Koch reaction, which introduces a formyl group (-CHO) into the benzene ring, resulting in the formation of benzaldehyde (C6H5CHO).

### Step 2: Reaction of Benzaldehyde with Ac2O/NaOAc
- **Reaction**: The benzaldehyde obtained from Step 1 is then treated with acetic anhydride (Ac2O) and sodium acetate (NaOAc).
- **Product**: This reaction leads to the formation of an acetylated product, which is 4-acetylbenzaldehyde (C6H4(COCH3)CHO or CH3C6H4CHO).

### Step 3: Bromination of Compound X
- **Reaction**: The compound X (4-acetylbenzaldehyde) is then treated with bromine (Br2) in the presence of sodium carbonate (Na2CO3).
- **Product**: This leads to bromination at the para position, resulting in the formation of 4-bromo-4-acetylbenzaldehyde.

### Step 4: Heating with Moist KOH
- **Reaction**: The brominated product is then heated at 473 K with moist potassium hydroxide (KOH).
- **Product**: This reaction leads to the elimination of the bromine atom and the formation of an alkyne, specifically phenylacetylene (C6H5C≡CH).

### Step 5: Hydrogenation of Compound X
- **Reaction**: The compound X (4-acetylbenzaldehyde) is subjected to hydrogenation in the presence of palladium on carbon (Pd-C) followed by treatment with phosphoric acid (H3PO4).
- **Product**: This results in the formation of 4-hydroxyacetophenone (C6H4(OH)(COCH3)), which is compound Z.

### Conclusion
- **Final Product (Z)**: The compound Z is 4-hydroxyacetophenone.

### Summary of Compounds:
- **X**: 4-acetylbenzaldehyde (C6H4(COCH3)CHO)
- **Y**: Phenylacetylene (C6H5C≡CH)
- **Z**: 4-hydroxyacetophenone (C6H4(OH)(COCH3))