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-1-REACTIVE INTERMEDIATES NOTES
Carbenes
These are neutral, divalent, highly reactive carbon intermediates. The two non-
bonding electrons can be arranged parallel or spin-paired. Spin-paired show both
electrons an sp2 orbital, leaving the p orbital vacant. This is the singlet state. The
triplet state shows both electrons with parallel spins and leaves one electron in an
sp2 orbital and the other in a p orbital, and this is the triplet state.
Structure –
Evidence of this is provided by UV, IR, and particularly ESR.
Stability –
Generation –
There are several methods for generation of carbenes, which are listed below.
These Notes are copyright Alex Moss 2003. They may be reproduced without need for permission.
www.alchemyst.f2o.org
, -2-
The most common method is α-elimination. This can be modified to generate
carbenoids:
Carbenes undergo many reactions.
Cycloaddition –
1,2-addition of carbenes to alkenes to form cyclopropanes. This is basically addition
to a double bond.
The Skell Hypothesis states that a singlet carbene can add to a double bond in a
single step and is stereospecific, while the triplet carbene cannot since it is spin
forbidden:
It should also be noted that in addition to allenes, the singlet carbene will add at the
more substituted double bond, while triplet at the less substituted.
Carbenes will also add to alkynes (although less readily, since alkynes are less
nucleophilic than alkenes), and to arenes. The latter generates a 7-membered ring
with 3 double bonds:
A similar molecule could be attacked by carbenes itself, e.g.
One particular mechanism is the Riemer-Tiemann Reaction:
Insertion –
Characteristic reaction of carbenes to insert into a single bond, often C-H. Again two
mechanisms can be conceived depending on spin:
These Notes are copyright Alex Moss 2003. They may be reproduced without need for permission.
www.alchemyst.f2o.org
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