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This is a question from BIOMOLECULES asked in jee 2012 (pls see the attachment for the question) in that why was V(fifth) bit not included in the answer even though it has an amino group. pls find the attchment

This is a question from BIOMOLECULES  asked in jee 2012 (pls see the attachment for the question)
in that why was V(fifth) bit not included in the answer even though it has an amino group.
 
 
 
pls find the attchment
 

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Grade:11

1 Answers

Harishwar IIT Roorkee
askIITians Faculty 50 Points
9 years ago
Bear in mind that both an amine and an amide are Lewis bases as they both have a lone pair of electrons. However, the basicity of amides is far lower than that of amines.

What makes an amide a base is the same thing that makes an amine a base, so the question should be, what is preventing the electrons on the nitrogen from acting in a basic manner?

The nitrogen in an amide is next to a carbonyl, while the amine has no carbonyl next to it. That means the carbonyl has some electron-sucking effect. And to explore that, you need to consider the hybridization of the lone pair on the nitrogen of the amide.

The amide has a lone pair which normally occupies an sp3 hybridized orbital. However, when a lone pair is in bonded proximity (read: an atom next to) to a pi-system (read: any double or triple bond), the orbital reverts to a p-orbital to share electrons with the pi-system. It's resonating with the carbonyl, and that means the electrons on the amide nitrogen are being shared in a pi-resonance system.

If you can, draw a resonance form of the amide where the lone-pair resides on the nitrogen and then resides on the oxygen of the carbonyl. Make sure you move the double bond. This shows the movement of the electrons, which means they are "delocalized"-- they no longer reside on specific point, and that means they are less likely to act as a base at this point.

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