T
Bonding without Tears
F. Michael Akeroyd Bradford College England BD7 I A Y There has been a debate recently' concerning the relative merits of the n - s model of ethylene as a teaching aid vis i \.is the Pauling bent-bond model: In the present author's opinion, the a - r model (when divorced from quantum-mechanical justifications) is the more valuable for beginning students of organic chemistry. The following empirical approach may, therefore, be of interest to readers of THISJOURNAL. In the student's experience, ethylene is formed from an elimination reaction involving saturated compounds, e.g., CHsCHzBR, CHaCHzOH and CnH2,,+z (cracking). Following this line of approach, let us consider the structure of the newly formed ethylene molecule after the departure of the leaving groups. I t seems reasonable to assume that the 5 remaining covalent (a) bonds remain essentially the same, the remaining hydrogen atoms move up to a planarpnsition (due to rrpulsinn of elecrron clouds and Ve nuclei1 and the remaining valence electrons on each carbon atom form a weak diffuse bond!
Markovnikoff Addition If it is true that a bondsexist and if it is true that the methyl group is electron releasing (cf., hyperconjugation), then it is reasonable to assume that the CH2 "end" of the propylene molecule is the "negative e n d and attack by H6+-Br6- on the a-bond will lead to Markovnikoff Addition.
It also seems reasonable to expect that ethylene will he attacked by electron seeking reagents, i.e., electropbiles. This non-mathematical treatment of a - a bonding can be extended to cope with following phenomena: conjugation, hyperconjugation, Markovnikoff addition, aromaticity, and aromatic substitution.
T h e molecule is undoubtedly hexagonal, but when i t is synthesized from cyclohexane the free valency electron on each carbon atom goes into a central pool. Although this pool attracts electrophiles readily ( s complex), great energy is required to break the pool, and substitution reactions are the rule.
Conjugation The formula of butadiene can be written as
Aromatic Substitution Many textbooks in the 1955-60 era provided a reasonable explanation of a-M-P substitution based on the boosting or withdrawal of electron density a t certain preferential sites of the benzene nucleus, for example,
+
H,T~=CH,
a
A
(classical) or H ~ c % + ~ H ,
I
I
(new notation)
Aromaticity The surprising stability of the benzene ring can be described thus:
However, if diffuse a bonds do exist in butadiene, it is again reasonable to suppose that they delocalize over the entire carbon chain, for example,
There is sound experimental evidence to support this hypothesis. Hypercon]ugatlon This notation provides support for an explanation of hyperconjugation first proposed by Kreevoy and Eyring2 (1957).
The author was influenced in his undergraduate days a t Cambridge hy the lectures of Peter Sykes and the classic book written hv Edwin G ~ u l dHe . ~ develo~edthe ~recediuaideas 1964-67. The when teaihing pure chemistry during the advantaaes - claimed for the method are that it: (1) provides a pattern for mastering the fadual material in organic chernistrv r21 intn,duws the student into the mechanistic apprunch r 3 1 prep:+re*the student for inwe advanced urbitai t l w r y I 1, n l i o w I ~ lrcturrr P t o drrnomlrate that ~hmmic.iltItwrie%sre the servants of the observed facts
' F. A. Robinson and R. J. Gillespie, J. CHEM.Eouc. 57,5,329 (May
1980).
Hyperconjugation is regarded as a-hydrogen bonding between a (very slightly) 6 H atom and a electron cloud.
+
Kreevoy, and Eyring. J. Amer. Chem. Soc.. 79, 5121 (1957). E. S. Gould, "Mechanism and Structure in Organic Chemistry," Holt, Rinehart and Winston. New York, 1959. Volume 59 Number 5
May 1982
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