In the given reaction sequence
`C_(6)H_(5)CHO underset(H^(o+), Delta)overset(H_(2)N-OH)rarr A overset(P_(2)O_(5))rarrB`
A and B respectively are:

Options

A. `C_(6)H_(5)-CH=N-OH,C_(6)H_(5)CN`
B. `C_(6)H_(5)-CH=N-OH, C_(6)H_(5)overset(O)overset(||)C-NH_(2)`
C. `C_(6)H_(5)-CH=N-OH, C_(6)H_(5)CHO`
D. `C_(6)H_(5)-CH=N-OH, C_(6)H_(5)-COOH`

Answer

To solve the given reaction sequence, we need to identify the compounds A and B formed from the starting compound \( C_6H_5CHO \) (benzaldehyde) through the specified reactions.

### Step 1: Identify the starting compound
The starting compound is benzaldehyde, which has the formula \( C_6H_5CHO \).

### Step 2: Reaction with Hydroxylamine
The first step involves the reaction of benzaldehyde with hydroxylamine (\( NH_2OH \)) in the presence of an acid and heat.

- **Mechanism**: The nitrogen atom in hydroxylamine has a lone pair of electrons that can attack the carbonyl carbon of benzaldehyde. This forms a tetrahedral intermediate, which then loses a water molecule (dehydration) to form an oxime.

- **Product A**: The product formed after this reaction is benzaldehyde oxime, which has the structure \( C_6H_5C=NOH \).

### Step 3: Reaction with Phosphorus Pentoxide
The next step involves treating the oxime (compound A) with phosphorus pentoxide (\( P_2O_5 \)), which is a strong dehydrating agent.

- **Mechanism**: The dehydration of the oxime will lead to the elimination of water, resulting in the formation of a nitrile. In this case, the oxime loses the hydroxyl group and a hydrogen atom from the adjacent carbon, resulting in the formation of a carbon-nitrogen triple bond.

- **Product B**: The product formed in this step is benzyl cyanide (or phenyl cyanide), which has the structure \( C_6H_5C \equiv N \).

### Final Summary
- **Compound A**: Benzaldehyde oxime (\( C_6H_5C=NOH \))
- **Compound B**: Benzyl cyanide (\( C_6H_5C \equiv N \))

### Answer
A and B respectively are:
- A: Benzaldehyde oxime
- B: Benzyl cyanide