The reaction of a ketone with a LiAlH4 reagent will yield a. b. an aldehyde a primary alcohol a secondary alcohol a tertiary alcohol C. d. The reaction of an aldehyde with two moles of methanol in the presence of acid catalysis will produce a. acetal b. hemiacetal c. ketal d. hemiketal
The Correct Answer and Explanation is :
Reaction 1: Ketone with LiAlH₄
Answer: c. secondary alcohol
When a ketone reacts with lithium aluminum hydride (LiAlH₄), a strong reducing agent, the ketone undergoes reduction to form a secondary alcohol. In the reduction process, LiAlH₄ donates a hydride ion (H⁻) to the carbonyl carbon of the ketone. This results in the breaking of the double bond in the carbonyl group (C=O), leading to the formation of a C-H bond and a C-OH bond. This results in the formation of a secondary alcohol, as the carbon that was part of the ketone is now attached to two other carbons, typical of a secondary alcohol.
Reaction 2: Aldehyde with Two Moles of Methanol and Acid Catalysis
Answer: a. acetal
When an aldehyde reacts with two moles of methanol (CH₃OH) in the presence of an acid catalyst (usually H₂SO₄), the reaction produces an acetal. This occurs in two steps:
- Formation of a hemiacetal: The aldehyde first reacts with one mole of methanol. The methanol adds to the carbonyl carbon of the aldehyde, forming an intermediate hemiacetal. This step is catalyzed by the acid, which protonates the carbonyl oxygen, making it more electrophilic and enhancing the attack by the methanol.
- Formation of the acetal: In the second step, another mole of methanol adds to the hemiacetal. The protonated hydroxyl group (OH) of the hemiacetal is eliminated as water (H₂O), resulting in the formation of an acetal. The acetal is characterized by two -OCH₃ (methoxy) groups attached to the same carbon atom.
This process is known as acetal formation and is widely used in organic synthesis to protect aldehydes and ketones from nucleophilic attack.
Thus, the correct answers are:
- Ketone with LiAlH₄ → secondary alcohol
- Aldehyde with two moles of methanol → acetal