In the Cannizzaro reaction given below:
`2Ph-CHO overset(overset(Θ)OH)rarrPh-CH_(2)OH+PhCO_(2)^(-)` the slowest step is:

Options

A. the attack of `overset((-))(OH)` at the carboxyl group
B. the transfer of hydride to the carbonly group
C. the abstraction of proton from the carboxylic group
D. the deportonation of `Ph " " CH_(2)OH`

Answer

To determine the slowest step in the Cannizzaro reaction, we need to analyze the reaction mechanism. The Cannizzaro reaction involves the disproportionation of non-enolizable aldehydes in the presence of a strong base, such as hydroxide ion (OH-).

### Step-by-Step Solution:

1. **Identify the Reactants and Products**:
The reaction involves two moles of benzaldehyde (PhCHO) reacting in the presence of hydroxide ion (OH-) to produce benzyl alcohol (PhCH2OH) and benzoate ion (PhCOO-).

2. **Mechanism Overview**:
The Cannizzaro reaction proceeds through several steps:
- The hydroxide ion attacks the carbonyl carbon of one of the benzaldehyde molecules.
- This leads to the formation of a tetrahedral intermediate.
- A hydride ion is transferred from the tetrahedral intermediate to the carbonyl carbon of the second benzaldehyde molecule.
- Finally, the intermediate collapses to form the products.

3. **Determine the Rate-Determining Step**:
The rate-determining step (RDS) is the slowest step in the reaction mechanism, which controls the overall rate of the reaction.

- **Step A**: The attack of OH- at the carbonyl group is a fast step as it is a nucleophilic attack.
- **Step B**: The transfer of hydride to the carbonyl group is typically the slowest step because it involves breaking a bond and forming a new one, which is often the rate-limiting step in many reactions.
- **Step C**: The abstraction of a proton from the carboxylic acid is also a fast step.
- **Step D**: The deprotonation of PhCH2OH is not the slowest step as it occurs relatively quickly.

4. **Conclusion**:
Based on the analysis, the slowest step in the Cannizzaro reaction is the transfer of hydride to the carbonyl group.

### Final Answer:
The slowest step in the Cannizzaro reaction is **Option B: the transfer of hydride to the carbonyl group**.

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