How many of the following reagents react with arene diazonium slats to form aryl halides?
i. `HBF_(4)//Delta`
ii. `KHS`
iii. `CuCN//KCN`
iv. `CuBr//HBr`
v. `Cu//HCl`
vi. `KI//Delta`
vii. `CH_(3)CH_(2)OH`
viii. `H_(3)PO_(2)//H_(2)O`

Answer

To determine how many of the given reagents react with arene diazonium salts to form aryl halides, we will analyze each reagent one by one.

### Step-by-Step Solution:

1. **Identify the nature of arene diazonium salts**:
Arene diazonium salts have the general formula Ar-N₂⁺X⁻, where Ar is an aryl group and X is a halide (like Cl⁻). These salts can undergo various reactions to form different products.

2. **Reagent i: HBF₄/Δ**
- Reaction: Arene diazonium salts react with HBF₄ upon heating to form aryl fluorides.
- Product: Yes, it forms aryl halide (fluorobenzene).

3. **Reagent ii: KHS**
- Reaction: This reagent does not typically react with arene diazonium salts to form aryl halides.
- Product: No, it does not form aryl halide.

4. **Reagent iii: CuCN/KCN**
- Reaction: Arene diazonium salts react with CuCN to form aryl cyanides (not halides).
- Product: No, it does not form aryl halide.

5. **Reagent iv: CuBr/HBr**
- Reaction: Arene diazonium salts react with CuBr in the presence of HBr to form aryl bromides.
- Product: Yes, it forms aryl halide (bromobenzene).

6. **Reagent v: Cu/HCl**
- Reaction: Arene diazonium salts react with Cu in the presence of HCl to form aryl chlorides.
- Product: Yes, it forms aryl halide (chlorobenzene).

7. **Reagent vi: KI/Δ**
- Reaction: Arene diazonium salts react with KI upon heating to form aryl iodides.
- Product: Yes, it forms aryl halide (iodobenzene).

8. **Reagent vii: CH₃CH₂OH**
- Reaction: This reagent does not react with arene diazonium salts to form aryl halides.
- Product: No, it does not form aryl halide.

9. **Reagent viii: H₃PO₂/H₂O**
- Reaction: This reagent reduces arene diazonium salts to form aryl compounds, but not halides.
- Product: No, it does not form aryl halide.

### Summary of Reactions:
- Reagents that form aryl halides:
- HBF₄ (i)
- CuBr/HBr (iv)
- Cu/HCl (v)
- KI (vi)

### Total Count:
Out of the eight reagents, four reagents (i, iv, v, vi) react with arene diazonium salts to form aryl halides.

**Final Answer**: 4 reagents react with arene diazonium salts to form aryl halides.