Natural Science Forum / Chemistry / Organic Synthesis / September 2003

Look it up yourself, books or Google.  The effect is neither real nor
imaginary - it is a rationalization of observation (curve fitting).

see answers below

First, conjugation is generally described as the sharing of electrons
among three or more p-orbitals.  Hyperconjugation is almost the same
thing, but uses one or more sigma molecular orbitals from neighboring
sigma bonds.  The neighboring sigma bonds must be in the correct
orientation, however, meaning approximately parallel with the p-orbital
that they are conjugated with.

One simple answer is that conjugation, of any kind, offers extra
stability by allowing more resonance structures.  In the case of
carbocation/carboanion, there is added benefit of diffusing the charge,
which also increases the stability.

<stepping on soapbox>  The answer depends on what you mean by resonance.
Individual resonance structures are imaginary.  They are a convenient
way of describing a phenomenon that is hard to put down on paper.  There
is only one structure, even though that structure is constantly changing
as the molecule undergoes vibration and rotation in various directions.
Any drawing that we try to make concerning a molecule is imaginary.
There are no lines connecting atoms together.  There is no fixed
interatomic distance between two atoms of a molecule.  Even though you
will be able to obtain interatomic distances from X-ray analysis, these
are only averages, as each atom is constantly in motion.  Also keep in
mind that the atomic theory is still just that:  a theory.  It is no
more a proven fact than the big bang.  Both of these just happen to be
the theories that best fit the data available at the moment.  A hundred
years down the road, the students of the time may laugh at us for
believing such rubbish, just as we laugh at those who believed in
phlogiston 200-300 years ago, or polywater 30 only years ago.  Regarding
cold fusion, they may look at us in the same light as we now look at the
establishment at the time of Galileo.  Always try to remember that
chemistry is a science, not a religion.  Take EVERYTHING with a grain of
salt.
</soapbox>

Dick

Hi,

Hyperconjugation is stabilizing interaction of electrons in a s-bond
with an adjacent empty or partially filled p-orbital. It is delocalization
of sigma electrons. It offen ocurs, when a carbon is attached to an
unsaturated atom or atom with an unshared orbital.
                         _
R2C-CR'=CR''2 <--> R2C=CR-CR2
 H                  H+

in carbanions:

H3C-CH2+ <--> CH2=CH2 <--> ...
              H+  

Google search will tell you more (nice pictures).

Increasing number of alkyl substituents on carbocation or radical centre
leads to an increase in stability because of hyperconjugation. There are
two hyperconjugative forms for a primary ion and 6 for a tertiary one. The
more equivalent forms, the greater stability.

Imaginary. Resonance is representation of a real structure as a weighted
average of more canonical forms (lewis strutures)- J.March. You can draw
many lewis structures for benzene, but which is real?
The lewis structures exist only in our imagination (a lot of things in
chemistry).

Ivana.