Carbonyl carbon electropositivity?

Question

Is the carbon in a carbonyl functional group electropositive? I had a professor tell me so, but I immediately refuted him, because he defined electronegativity earlier as the ability to stabilize negative charge ... and taking electronegativity to be a converse property, the strong partial positive character on the carbon probably isn't able to stabilize any further positive charge.

Similarly, I know that lone pairs are defined as having an electronegativity of 0 because electrons cannot stabilize other electrons.

The guy agreed with me. So, to confirm, am I correct, and, as an extension, can we describe the electron deficient carbon as electronegative instead? I could very well see a electron-deficient carbon stabilizing negative charge.

However, if we stick with the IUPAC definition of electronegativity - which is the ability to attract electrons, the carbonyl carbon still seems to lose to oxygen. A charge-separated resonance structure can be drawn for any carbonyl group with a + on carbon and a - on oxygen.

So that leads me to another question - is electronegativity the ability to stabilize negative charge or the ability to attract electrons within a covalent framework? I guess the two go hand in hand - i.e. you wouldn't attract electrons if you weren't able to stabilize them ...

As a side question: can we calculate EN for ions?

Answer 1

Is the carbon in a carbonyl functional group electropositive?

Electronegativity and electropositivity are usually used in a relative sense, X is more electronegative than Y. The carbonyl carbon in acetone is more electropositive than the central carbon atom in isobutane, and less electropositive than the methyl carbocation carbon.

because he defined electronegativity earlier as the ability to stabilize negative charge

Weak definition, wouldn't you say; that's why you won that argument. Atoms can be electronegative in the abstract, charge doesn't need to be involved. The statement, "fluorine is more electronegative than carbon according to the Pauling Electronegativty scale" is accurate and understandable, yet no one is describing a situation where charge stabilization is involved.

because electrons cannot stabilize other electrons.

I understand what you mean, but you should know by now that you can't make such sweeping statements. Don't the electrons in one of the double bonds in 1,3-butadiene interact with and stabilize the electrons in the other double bond?

Can we describe the electron deficient carbon as electronegative

Again, I understand what you mean, but... Yes, it is more electronegative than the carbon in an alkane, but not as electronegative as $\ce{F+}$.

Is electronegativity the ability to stabilize negative charge or the ability to attract electrons within a covalent framework? I guess the two go hand in hand - i.e. you wouldn't attract electrons if you weren't able to stabilize them

Yes, the two go hand in hand. More electronegative atoms are better able to stabilize negative charge than less electronegative atoms. It's all about lowering the overall system energy.

As a side question: can we calculate EN for ions?

I'm sure there is a way. For example, using studying the $\ce{^{13}C}$-nmr spectrum of various carbocations leads to a relationship of (somewhere around) 180 ppm shift/ electron. So you can estimate the fractional positive charge on any carbon atom, the carbonyl carbon in acetone for example. Once you've estimated the amount of charge on an atom, I suspect there is a way to relate it to EN.