Question detail
Which of the following best describes the general mechanism for the nucleophilic addition of a hydride ion to a ketone?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Aldehydes and ketones (A-level only)
Question
- A. The hydride ion attacks the carbonyl carbon, forming a tetrahedral alkoxide intermediate that is protonated to give a secondary alcohol.
- B. The hydride ion attacks the carbonyl oxygen, forming a protonated alkoxide that rearranges to an alcohol.
- C. The hydride ion attacks the carbonyl carbon, forming a carbocation intermediate that is stabilized by resonance.
- D. The hydride ion attacks the carbonyl carbon, forming a carbanion intermediate that is protonated to give a tertiary alcohol.
Answer
The hydride ion attacks the carbonyl carbon, forming a tetrahedral alkoxide intermediate that is protonated to give a secondary alcohol.
Explanation
In a nucleophilic addition to a ketone, the hydride nucleophile attacks the electrophilic carbonyl carbon, creating a tetrahedral alkoxide. Protonation of the alkoxide during work‑up gives the corresponding secondary alcohol. This sequence matches the textbook mechanism for hydride addition to carbonyls.
Common mistake
Misidentifying Nucleophiles
Students often confuse nucleophiles with electrophiles, thinking that nucleophiles are positively charged species.
Remember that nucleophiles are electron-rich species that donate a pair of electrons to form a bond. They are typically negatively charged or neutral molecules with lone pairs.
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