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. H 888 R1 C V A % 5 B Which atom is attacked by the hydride ion source H in the first step? O A B O C OD T F5 R2 D G B A 1. H- 2. H+ 6 MacBook Air Y F6 H & 7 N R1 Ad F7 U J * CO 8 H M 1 R2 DII F8 Which atom is protonated in the second step of the reaction by H+? A B K D 9 9€ V DD F9 O ) O L F10 P A F11 { [ ? 11 + F12 } 1
The Correct Answer and Explanation is :
To clarify, the reaction you’re describing involves the reduction of a ketone to a secondary alcohol using a hydride ion (often from a reducing agent like lithium aluminum hydride, LiAlH₄) followed by protonation (usually from water or H⁺) to yield the alcohol. Here’s how the process works:
Step 1: Hydride Attack
The ketone consists of a carbonyl group (C=O). The hydride ion (H⁻) acts as a nucleophile and attacks the electrophilic carbonyl carbon, which is partially positive due to the electron-withdrawing effect of the oxygen. The carbonyl carbon is the site of attack because it has a partial positive charge, making it vulnerable to nucleophilic attack.
Atom attacked by the hydride (H⁻) in the first step: The carbonyl carbon (C).
Step 2: Protonation
After the hydride attack, the resulting intermediate is a tetrahedral alkoxide ion (R₂C-O⁻), where the oxygen is now negatively charged. This intermediate needs to be protonated to form the alcohol. In the second step, the alkoxide ion is protonated by a proton source (e.g., H₂O or H⁺), which adds a proton to the negatively charged oxygen atom.
Atom protonated by H⁺ in the second step: The oxygen (O) atom.
Explanation:
In the first step of the reaction, the hydride ion (H⁻), which is a source of nucleophilic electrons, attacks the carbonyl carbon of the ketone. This step transforms the ketone into an alkoxide ion, an intermediate in the reaction. The carbonyl carbon is the electrophilic center, making it the site of the hydride attack.
In the second step, the alkoxide ion (which has a negatively charged oxygen) is protonated by a proton source such as water or an acid (H⁺). This protonation adds a hydrogen to the oxygen atom, neutralizing the charge and forming a secondary alcohol.
Thus, the hydride attacks the carbonyl carbon (C), and the oxygen (O) is protonated in the second step.