Biomedical Engineering Reference
In-Depth Information
18.1 Hückel
π
-Electron Theory
Molecules such as benzene, pyridine and naphthalene that have extensive π -bonding sys-
tems are not well described by classical valence theory. Such molecules are often planar,
or largely planar, and two types of bond contribute to their structure: σ and π bonds.
The σ bonds are thought of as largely localized between atoms, whilst the π bonds are to
be thought of as delocalized over large portions of the molecule. Much of the interesting
chemistry of such compounds appears to relate to the so-called π -electrons, and we visu-
alize these π -electrons as moving under the influence of the nuclear framework and the
σ -electrons. The σ -electrons are thought to form an inert core.
We meet such compounds every day; for example, β-carotene is found in carrots and is
known to be a biological precursor of vitamin A. In 1933, the compound benzo[a]pyrene
was isolated from coal tar; this compound and 7,12-dimethylbenz[a]anthracene are two of
the most potent carcinogens known. Such materials are found in soot and cigarette smoke.
On the other hand, methyl orange is a prototype for of a class of commercially important
compounds known as
azo dyes
. The structures of β-carotene, benzo[a]pyrene and methyl
orange are shown in Figure 18.1.
β
-carotene
N
NMe
2
O
3
SNa
N
methyl orange
benzo[a]pyrene
Figure 18.1
Some conjugated systems
Probably for all these reasons, people have striven from the early days of quantum
chemistry to give simple models of such compounds in an effort to:
•
explain their special structure, stability and properties;
•
predict new properties of related but undiscovered molecules (without the labour of
having to actually synthesize them).
One of the earliest models for π -electron molecules is the Hückel π -electron model,
which dates from 1931 (Hückel 1931). The ideas are simple and chemically appealing, and
