|MadSci Network: Chemistry|
in methane the electronegativity of carbon is less where as in sp2
hybridized ethylene the electronegativity of carbon is more. why does this
Electronegativity is an empirical quantity. The complete definition is given by Pauling in The Nature of the Chemical Bond, but it is largely based on ionization energy. It is also closely based on Pauling's version of the valence-bond picture of chemical bonding, which gave us hybrid orbitals.
Qualitatively, electronegativity is a measure of how closely electrons are held in an atom. This allows us to give a qualitative theoretical explanation for your question.
In an s atomic orbital, electrons are allowed to get much closer to the nucleus -- and are therefore more tightly held -- than in a p atomic orbital. Hybrid orbitals (for carbon, sp, sp2, sp3) are mixtures of an s atomic orbital with one, two or three p orbitals. As the p proportion of the mixture rises, the amount of "s-character" falls and the electrons in that orbital will be more loosely held by the central atom. Conversely, as the p-character falls, s-character rises and electrons will be more tightly held.
This means that electrons in an sp hybrid orbital are held more tightly than those in an sp2 hybrid, which are in turn more tightly held than electrons in an sp3 hybrid orbital. So we say that the effective electronegativity of sp-hybridized carbon is higher than that of sp2-hybridized carbon, and sp3-hybridized carbon is lower than sp2.
The power of this model is found both in bond strengths (C-H bond strengths go from about 99 kcal/mol in ethane to about 105 in ethene to about 111 in ethyne) and in the acidities of C-H bonds. When a C-H bond is deprotonated, the carbanion lone pair is held, according to the Pauling model, in a hybrid atomic orbital. More s-character means a more stable negative charge because the electrons are able to "snuggle" closer to the nucleus.
When we look at actual acidities, we find that ethyne is about 1010 times more acidic than ethene, which in turn is at least 105 times more acidic than ethane.
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