Q.15 the major product of the following reaction is:
I have read that $\ce{NaBH4}$ reduces only aldehyde, ketone and acid chlorides. In some cases it may reduce double bonds conjugated with carbonyl groups. So shouldn't the answer here be option 1?
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1$\begingroup$ Sodium borohydride readily reduces isolated C=N bonds, read up on reductive amination. $\endgroup$– WaylanderCommented Jan 16, 2019 at 8:28
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$\begingroup$ @Waylander Can you provide some reference in the literature? $\endgroup$– SohamCommented Jan 16, 2019 at 9:49
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1$\begingroup$ try this commonorganicchemistry.com/Rxn_Pages/Reductive_Amination/… and this sciencedirect.com/science/article/pii/S1001841709003167 $\endgroup$– WaylanderCommented Jan 16, 2019 at 10:02
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1 Answer
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Waylander's comments answer the title question as well as I could. Sodium borohydride, and even modified versions where some of the hydride ligands are replaced by others like $\ce{NaBH3CN}$, can reduce imines.
But, if we have only a limited amount of borohydride we will likely see the carboxyl group primarily reduced, as it has a more polar pi bond and thus it is a more reactive electrophile. So you may in principle get mostly 1 with limited sodium borohydride, or 4 if you have enough sodium borohydride to attack both the carboxyl and less reactive imine functions.
The carbon-carbon bond here, not conjugated to either electronegatove function, is interactive to borohydride and thus products 2 and 3 are not reached.
answered Jan 16, 2019 at 11:02
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$\begingroup$ If the question is written exactly as I had uploaded in the question (nowhere excess or anything is mentioned) do you think I should be penalized for writing 1 and not 4 ? Note the question has specifically one correct answer and there are no other details than that I have given in my original question... please reply $\endgroup$– SohamCommented Jan 16, 2019 at 13:19
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$\begingroup$ Then either 1or 4 might be formed. I really can't decide because I do not have ESP and cannot figure whether they mean one or possibly two moles of borohydride reacting. Always ask precise questions! $\endgroup$ Commented Jan 16, 2019 at 13:30
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$\begingroup$ Thanks for your reply. The question is exactly as written. No other details given. Then what do you think? Please share your opinions, it matters a lot $\endgroup$– SohamCommented Jan 16, 2019 at 13:33
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$\begingroup$ The examination commitee has decided that 4 is correct and decided to penalize option 1 . However they are ready to get their answers challenged if proper citations are provided. Please help me if you think that $1$ should not be penalized for the question written with no other specific details. Please sir. (This question is from an undergraduate entrance examination for high school students) $\endgroup$– SohamCommented Jan 16, 2019 at 13:35
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$\begingroup$ Then they apparently decided to assume excess borohulydride. I have no more to say. $\endgroup$ Commented Jan 16, 2019 at 14:00