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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. Show the first step of the mechanism of the reaction between NaBH4 and 2-methylbutanal. You should incl... 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
First step of mechanism
Answer
The first step of the mechanism involves the nucleophilic attack of the hydride ion (H-) from NaBH4 on the carbonyl carbon of 2-methylbutanal. This can be represented as follows:
H H
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H-C-C-C-C=O + NaBH4
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H H
The curly arrows indicate the movement of the electrons: one arrow points from the H- ion to the carbonyl carbon, while another arrow points from the C=O double bond to the oxygen.
Step 2
Explanation
Answer
NaBH4 reduces 2-methylbutanal because the carbonyl carbon is electrophilic, making it susceptible to nucleophilic attack. The hydride ion is attracted to the partially positive carbon within the carbonyl group. In contrast, 2-methyl-1-ene, being an alkene, possesses a double bond without a carbonyl functional group. This lack of electrophilic character means it does not undergo a nucleophilic attack by the hydride ion, thereby explaining the absence of reactivity with NaBH4.
Step 3
Step 4