A ketone may react with a nucleophilic hydride ion source (e.g., LiAlH or “H”) and, subsequently, a proton source (e.g., H₂O or “H+”) to give a secondary alcohol. Given the labeled ketone, determine the atom that is attacked by the hydride and proton. Which atom is attacked by the hydride ion source H in the first step? 1. H- 2. H+ Which atom is protonated in the second step of the reaction by H+?
The Correct Answer and Explanation is:
In the nucleophilic reduction of a ketone to a secondary alcohol, the reaction proceeds in two main steps involving a hydride ion source (like LiAlH₄ or NaBH₄) and then a proton source (like H₂O or H₃O⁺). Understanding which atoms are involved in each step is crucial to predicting the mechanism and outcome.
Correct Answers:
Step 1 – Nucleophilic attack by H⁻:
→ The hydride ion (H⁻) attacks the carbon atom of the carbonyl group (C=O).
Step 2 – Protonation by H⁺:
→ The negatively charged oxygen atom (O⁻) formed after hydride attack is protonated.
Explanation (Textbook Style):
Ketones contain a carbonyl group (C=O), where the carbon atom is electrophilic due to the electron-withdrawing nature of the double-bonded oxygen. This makes the carbon susceptible to attack by nucleophiles. When a nucleophilic hydride ion source like lithium aluminum hydride (LiAlH₄) or sodium borohydride (NaBH₄) is used, the hydride ion (H⁻) donates a pair of electrons to the electrophilic carbon of the carbonyl group.
In the first step, the hydride ion acts as a nucleophile and attacks the carbon atom of the C=O group. This breaks the π bond of the carbonyl group, resulting in the formation of a tetrahedral alkoxide intermediate. The oxygen now bears a negative charge (O⁻), as it has accepted both bonding electrons from the broken double bond.
In the second step, the reaction mixture is treated with a proton source, such as water or dilute acid. This provides a proton (H⁺), which is accepted by the negatively charged oxygen atom (O⁻) of the alkoxide intermediate. This step neutralizes the charge and yields a secondary alcohol as the final product.
Thus, the overall transformation involves nucleophilic attack at the carbon atom by H⁻ followed by electrophilic protonation of the oxygen atom by H⁺. This two-step process is a common method for reducing ketones into alcohols in organic synthesis.
Summary:
- Hydride attacks: Carbon (C) of the carbonyl group.
- Protonation occurs at: Oxygen (O⁻) of the intermediate alkoxide.
