[chipmunk88]

Hi,
just starting to look at organic chemistry practice exams for a class I will be taking - exam has answers but no explanations! I would appreciate help with this question:

There are 5 different carbon atoms in these molecules, labeled a-e. Which carbon is the most electron rich, and which carbon is the second most electron rich? (picture attached)

thank you!

[billnotgatez]

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[kamiyu]

A is carbonyl carbon, this should be pretty trivial

Pay attention to resonance structures and electron-withdrawing effect of atoms and you will get the answers

[chipmunk88]

billnotgatez, sorry about that! new to the forum!

kamiyu and anyone else, would you mind looking at my reasoning.

For A, the oxygen is more electronegative than the carbon (would it be correct to call that double bond polar?) so it draws electron density away from that carbon. The resonance structure for this molecule, with a single bond between C and O, makes this even more apparent.
For B, there is a resonance structure which gives the top carbon a -1 formal charge. I'm not sure how to deal with the central carbon though.
For C, there are no resonance structures, but no additional electron-withdrawing groups compared to the other molecules either. I guess in all three molecules the central carbon is having some of its electron density drawn away by the nitrogen?

where do I go from here - total beginner, thanks for your patience!

[AWK]

This is rather impossible task without knowledge of R and quantum chemical calculations.
You probably know that in case of CO (carbon monoxide) a small negative charge is located at carbon atom, though carbon shows lower electronegativity than oxygen and expected polarisation on the basis of simple rule is reversed.

[orgopete]

billnotgatez, sorry about that! new to the forum!

kamiyu and anyone else, would you mind looking at my reasoning.

For A, the oxygen is more electronegative than the carbon (would it be correct to call that double bond polar?) so it draws electron density away from that carbon. The resonance structure for this molecule, with a single bond between C and O, makes this even more apparent.
For B, there is a resonance structure which gives the top carbon a -1 formal charge. I'm not sure how to deal with the central carbon though.
For C, there are no resonance structures, but no additional electron-withdrawing groups compared to the other molecules either. I guess in all three molecules the central carbon is having some of its electron density drawn away by the nitrogen?

where do I go from here - total beginner, thanks for your patience!

"Which carbon is the most electron rich?"
I thought you had it.

[chipmunk88]

Ah, the highlighting is quite helpful! Thank you.