Important
Concepts
1. The 2-propenyl
(allyl) system is stabilized by resonance. Its molecular-orbital
description shows the presence of three pmolecular levels: one bonding,
one nonbonding, and one antibonding. Its structure is symmetric, any
charges or odd electrons being equally distributed between the two
end carbons.
2. The chemistry
of the 2-propenyl (allyl) cation is subject to both thermodynamic and kinetic control. Nucleophilic trapping may
occur more rapidly at an internal carbon that bears relatively more
positive charge, giving the thermodynamically less stable product. The kinetic
product
may rearrange to
its thermodynamic isomer by dissociation followed by eventual
thermodynamic trapping.
3. The stability
of allylic radicals allows radical halogenations of alkenes at the allylic
position.
4. The SN2 reaction of allylic
halides is accelerated by orbital overlap in the transition state.
5. The special stability
of allylic anions allows allylic
deprotonation by a strong base, such as butyllithium – TMEDA.
6. 1,3-Dienes
reveal the effects of conjugation by
their relative stability (compared to nonconjugated systems) and a
relatively short internal bond (1.47 Å).
7. Electrophilic
attack on 1,3-dienes leads to the preferential formation of allylic cations.
8. Extended conjugated systems are reactive because they have
many sites for attack and the resulting intermediates are stabilized by
resonance.
9. Benzene has
special stability because of cyclic
delocalization.
10. The Diels-Alder reaction is a concerted
stereospecific cycloaddition reaction
of an s-cis diene to a dienophile; it leads to cyclohexene derivatives. It
follows the endo rule.
11. Conjugated
dienes and trienes equilibrate with their respective cyclic isomers by
concerted and stereospecific electrocyclic
reactions.
12. Polymerization of 1,3-dienes results in
1,2- or 1,4-additions to give polymers that are capable of further cross-linking. Synthetic rubbers can be
synthesized in this way. Natural rubber is made by electrophilic carbon –
carbon bond formation involving biosynthetic five-carbon cations derived
from
3-methyl-3-butenyl pyrophosphate.
13. Ultraviolet and visible spectroscopy gives
a way of estimating the extent of conjugation in a molecule. Peaks in electronic spectra are usually broad
and are reported as λmax
(nm). Their relative
intensities are given by the molar
absorptivity (extinction coefficient) ɛ.
From "Organic Chemistry" Textbook of
VOLLHARDT & SCHORE
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