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Aqueous NaBH4 reduces aldehydes but does not reduce alkenes - AQA - A-Level Chemistry - Question 13 - 2019 - Paper 2
Question 13
Aqueous NaBH4 reduces aldehydes but does not reduce alkenes. 1. Show the first step of the mechanism of the reaction between NaBH4 and 2-methylbutanal. You shoul... show full transcript
Worked Solution & Example Answer:Aqueous NaBH4 reduces aldehydes but does not reduce alkenes - AQA - A-Level Chemistry - Question 13 - 2019 - Paper 2
Step 1
Show the first step of the mechanism of the reaction between NaBH4 and 2-methylbutanal
Answer
The first step of the mechanism involves the nucleophilic attack of the hydride ion () from NaBH4 on the carbonyl group (C=O) of 2-methylbutanal. This is represented with two curly arrows: one showing the movement of the electron pair from the nucleophile (the hydride ion) to the carbon atom of the carbonyl, and the second showing the movement of the double bond from the C=O bond to the oxygen, resulting in the formation of an alkoxide intermediate.
Step 2
Explain why NaBH4 reduces 2-methylbutanal but has no reaction with 2-methyl-1-ene
Answer
NaBH4 effectively reduces aldehydes, such as 2-methylbutanal, because aldehydes possess a carbonyl group that is readily attacked by the nucleophile. In the case of 2-methylbutanal, the electrophilic carbon in the carbonyl is sufficiently polarized to undergo nucleophilic addition.
In contrast, 2-methyl-1-ene is an alkene with a carbon-carbon double bond (C=C) that does not have a carbonyl group. Therefore, it is not susceptible to nucleophilic attack in the same manner, and NaBH4 does not react with it.
Step 3