||Organic Chemistry 4e Carey||
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Chapter 23: Aryl Halides
Aryl Halides |
Preparations of Aryl Halides |
Reactions of Aryl Halides |
Self Assessment |
||Benzyne is an example
of an aryne (-yne = triple bond)
This is no ordinary triple bond as the second p
interaction results from a weak interaction of sp2 hybrid orbitals
lying in the plane of the ring.
The triple bond is non-linear due to the constraints of the 6-membered
Benzyne is strained and highly reactive.
||The image shows the
electrostatic potential for benzyne.
The more red an area is, the higher
the electron density and the more blue
an area is, the lower the electron density.
Here we see the
triple bond as a region of high electron density (red).
As a result of the non-linear triple bond, benzyne is highly reactive.
Benzyne is a reactive intermediate, an tends to undergo addition reactions.
Addition Reactions of Benzyne
Reaction type : elimination
- The strained triple
bond in benzyne makes it reactive towards addition reactions.
- Additions of H2O
and NH3 are commonly encountered are a result of the methods of
formation (see above).
- Substituted benzynes
can also be formed, where the subsequent addition reaction typically gives
mixtures of products:
MECHANISM OF THE
ADDITION OF AMMONIA TO BENZYNE
The N in amide functions as the nucleophile and attacks the reactive triple
bond C in benzyne creating the new C-N bond and an intermediate carbanion.
An acid/base reaction. Rapid protonation of the reactive carbanion from
the ammonia forms the aniline and another molecule of the amide ion.
Reactions of Benzyne
- Benzyne can undergo
leading to the formation of fused ring systems with an aromatic component:
- A milder, more convenient
method for the generation of benzyne involves treating 1,2-bromofluorobenzyne
with magnesium metal:
- The strained triple
bond makes benzyne a good dienophile
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