Carbanion Hybridization

[shefv]

Doesn't the carbon here have a sp3 hybridization? I am counting the 3 sigma bonds and the lone pair of electrons.

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[Labrat07]

The lone pair from C would be delocalize around C-N. That would be sp2.

 

[ElectricNoogie]

Resonance effects allows some carbanions to affect an sp2 hybridization though without that resonance capability (e.g. a methyl carbanion H3C-) you can consider it sp3.

For the MCAT a quick way to determine hybridization is to tally the # electron domains and subtract 1, where a domain can be a bond ( single, double or triple) or a lone pair.

Example: A carbonyl oxygen C=O.

The oxygen has 1 double bond + 2 lone pairs = 3 domains - 1 = 2 (sp2).

Example 2: Hydrogen Cyanide HCN.

The carbon has 1 triple bond + 1 single bond = 2 domains - 1 = 1 (sp).
The Nitrogen has 1 triple bond + 1 lone pair = 2 domains - 1 = 1 (sp)

This strategy is reliable enough for the MCAT but with practice you could get good enough to spot the hybridization just by looking. Efficiency is the name of the game on the MCAT, work as short a time as possible to arrive at the right answer.

 

[shefv]

Thanks for the response.

I am trying to use your method (# domains - 1) for the negatively changed carbon in the compound above but I am confused

(3 single bonds + 1 lone pair of electrons) - 1 = 3 which would mean sp3 hybrid?

 

[ElectricNoogie]

Thanks for the response.

I am trying to use your method (# domains - 1) for the negatively changed carbon in the compound above but I am confused

(3 single bonds + 1 lone pair of electrons) - 1 = 3 which would mean sp3 hybrid?

Hi, for the carbanion above the lone pair on carbon has the ability to resonate, which is what results in the sp2 designation. Think of the strategy above as a good rule of thumb.