Electrophilic Addition

The Cannizzaro reaction is a type of redox reaction where aldehydes without alpha-hydrogen atoms undergo disproportionation in the presence of a strong base. In this reaction, one molecule of aldehyde is reduced to alcohol, while another is oxidized to the corresponding carboxylate anion.

Let's analyze each compound:
- (A) 2-Chlorobutanal: This compound has a chlorine atom attached to the second carbon, which makes it unable to undergo the Cannizzaro reaction because it lacks the alpha-hydrogen required for the reaction to proceed. Therefore, 2-Chlorobutanal does not undergo Cannizzaro's reaction.
- (B) 2,2-Dimethylpropanal: This compound has no alpha-hydrogens as the two methyl groups at the alpha position prevent the formation of the enolate ion required for the Cannizzaro reaction. Hence, this compound would not undergo Cannizzaro's reaction either.
- (C) Benzaldehyde: Benzaldehyde contains an aldehyde group attached to a phenyl group, and it has an alpha-hydrogen. Therefore, it can undergo the Cannizzaro reaction, so it does not belong to the pair that cannot undergo the reaction.
- (D) 2-Phenyl ethanol: This compound is an alcohol, not an aldehyde, and thus cannot undergo the Cannizzaro reaction, which requires an aldehyde group. Thus, the two compounds that do not undergo Cannizzaro's reaction are (A) 2-Chlorobutanal and (D) 2-Phenyl ethanol.

This question involves the reaction of 1,1,2,2-Tetrabromopropane with zinc in ethanol, followed by the addition of HBr. The reactions can be broken down as follows: 1. First
Step (Reduction with Zinc in Ethanol):
When 1,1,2,2-Tetrabromopropane reacts with zinc in ethanol, a reduction occurs, and two bromine atoms are removed, creating a bromoalkene. This intermediate compound formed in step 1 will have a double bond and two bromine atoms on adjacent carbons.
2. Second Step (Reaction with HBr):
The product of the first reaction then reacts with HBr. In this step, the alkene undergoes an electrophilic addition, where the hydrogen (H) from HBr adds to one carbon, and the bromine (Br) adds to the other carbon of the double bond. Since the two bromine atoms were initially on adjacent carbons in the original tetrabromopropane, the final product is 2,2-Dibromopropane. Thus, the final product is 2,2-Dibromopropane.