Page 44 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

H H + H 23
H C H H C H H C H SECTION 1.1
– + –
H C C H C C H C C Description of Molecular
H H H H H H Structure Using Valence
Bond Concepts
.22
H
–.67
.20
.22 H C –.20 H
H C C –.44
.22
H H
.21 .20
Fig. 1.6. Electron density distribution for propene.

(no-bond) resonance structures. The implication of these resonance structures is that
some electron density is transferred from the C−H bond to the empty orbital.
∗
This is in accord with the calculated electron density distribution for propene shown
in Figure 1.6. Carbon-1, which is negatively charged in the resonance structure, has
a higher electron density than carbon-2, even though the latter carries the methyl
substituent.
There is also hyperconjugation across the double bond. Indeed, this interaction
may be even stronger because the double bond is shorter than a corresponding single
bond, permitting better orbital overlap. 33 Because these resonance structures show
equivalent compensating charge transfer, there is no net charge separation, but struc-
tural features, such as bond length, and spectroscopic properties are affected.

H
H
C C
C C
H
H

Hyperconjugation also can describe the electron-releasing effect of alkyl groups
on aromatic rings.

H H + H + H +
H H H H H H H H
C C C C
– –

–

While part of the electron-releasing effect of alkyl groups toward double bonds and
3
aromatic rings can be attributed to the electronegativity difference between sp and
2
sp carbon, the fact that the -carbon of alkenes and the ortho and para positions of
aromatic rings are selectively affected indicates a resonance component.
33
I. V. Alabugin and T. A. Zeidan, J. Am. Chem. Soc., 124, 3175 (2002).