Page 68 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

ψ 4 +.283 47
∗
LUMO π .+243
LUMO ψ 3 +.127 SECTION 1.2
LUMO Molecular Orbital
HOMO π –.371 HOMO ψ 2 –.358 Theory and Methods
ψ
HOMO –.473 1 –.477

–.516
CH 2 CH 2 CH 2 CHCH CH 2
CH 2 CHCH O

π MO energy levels in au for ethylene with a π-acceptor substituent. From Ref 54.
The LUMO, which is the frontier orbital in reactions with nucleophiles, has a larger
coefficient on the ß-carbon atom, whereas the two occupied orbitals are distorted in
such a way as to have larger coefficients on the oxygen. The overall effect is that
the LUMO is relatively low lying and has a high coefficient on the ß-carbon atom.
Frontier orbital theory therefore predicts that nucleophiles will react preferentially at
the ß-carbon atom.
MO orbital calculations at the HF/6-31G level have been done on both propenal
∗∗
and ethenamine. The resulting MOs were used to calculate charge distributions.
Figure 1.21 gives the electron densities calculated for butadiene, propenal, and
aminoethyene. 58 We see that the C(3) in propenal has a less negative charge than the
terminal carbons in butadiene. On the other hand, C(2), the -carbon in ethenamine,
is more negative.
The MO approach gives the same qualitative picture of the substituent effect as
described by resonance structures. The amino group is pictured by resonance as an
electron donor, indicating a buildup of electron density at the ß-carbon, whereas the
formyl group is an electron acceptor and diminishes electron density at the ß-carbon.

CH 2 CH NH 2 CH 2 CH NH 2 + CH 2 CH CH O + CH 2 CH CH O –

The chemical reactivity of these two substituted ethenes is in agreement with the MO
and resonance descriptions. Amino-substituted alkenes, known as enamines, are very
reactive toward electrophilic species and it is the ß-carbon that is the site of attack.
For example, enamines are protonated on the ß-carbon. Propenal is an electrophilic
alkene, as predicted, and the nucleophile attacks the ß-carbon.

+0.21 + 0.24 + 0.21
+ 0.23
H H H – 0.35 H
+ 0.21 + 0.04
H H O H H + 0.40
– 0.41 H – 0.34 + 0.48 – 0.56 N – 0.91
H H H H H H
+ 0.22
(a) butadiene (b) propenal (c) ethenamine

Fig. 1.21. Charge distribution in butadiene, acrolein, and aminoethylene based on
HF/6-31G calculations. From J. Org. Chem., 59, 4506 (1994).
∗
58
M. A. McAllister and T. T. Tidwell, J. Org. Chem., 59, 4506 (1994).