This document discusses classical and nonclassical carbocations. Nonclassical carbocations have charge delocalization from neighboring bonds like C=C pi bonds. The main difference is that classical carbocations have charge localized on one carbon, while nonclassical carbocations have charge delocalized by double or single bonds not in the allylic position. Examples like the norbornyl carbocation are given to show how neighboring double bonds can stabilize and delocalize charge through 3-center bonds. Reaction rates and product stereochemistry provide evidence for nonclassical intermediates. While some challenged this view, most chemists accept nonclassical interpretations of carbocation reactions.

2. PRE REQUISITE
EXAMPLES
CH3 O CH2
CH2
C

3. Nonclassical carbocations are stabilized by charge
delocalization from contributions of neighbouring C=C ,
C–C or C–H bonds.
The bridged carbocations, involve delocalization of
electrons and formation of three-center, two-electron
bonds, and are sometimes called nonclassical ions.
Most common example :
Norbornyl carbocation

4. MAIN DIFFERENCE BETWEEN CLASSICAL
AND NONCLASSICAL CARBOCATIONS
CLASSICAL
• The positive charge
is localized on one
carbon atom or
delocalized by
resonance involving
an unshared pair of
electrons or a double
or triple bond in the
allylic position.
NONCLASSICAL
• the positive charge is
delocalized by a
double or triple bond
that is not in the
allylic position or by
a single bond.

5. C=C as a neighbouring
group
OTs H OTs H
A B
When acetolysis of A and B is done, it is
observed that it is 1011 times faster for A than
that for B and the reaction proceeds with a
retention of configuration.
The rate data is not sufficient to prove it but
it is believed that the nonclassical
intermediate C is responsible for such
behavior. In any case it is clear that the C=C
assists in the departure of the OTs.
NMR spectrum of the stable norbornadienyl
cation (D) shows that 2 and 3 protons are not
equivalent with 5 and 6 protons which
indicates an interaction between the charged
carbon and one of the double bond. This
proves the existence of C.
C D

6. The Carbon–Carbon Single Bond as a Neighboring Group
•Winstein and Trifan found that acetolysis of both exo-2 norbornyl brosylate
and endo-2-norbornyl brosylate produce exclusively exo-2-norbornyl
brosylate.
•The exo isomer was found to solvolyze 350 times faster than endo isomer
•Enantiomerically pure exo-brosylate gives a complete racemic exo-
acetate and endo-brosylate gives acetate that is atleast 93% racemic.
exo
endo

8.  Stereo chemistry of the product is explained by using
the nonclassical intermediate.
 The 1 and 2 positions are equivalent and would be
attacked by the nucleophile with equal facility but
only from the exo direction in either case.
•Solvolysis in the case of the endo isomer is not
completely racemic, contains 2 more than 1.
1
2

9. H.C Brown challenged and disapproved the concepts of sigma participation and
nonclassical ion.
He described the intermediate as a rapidly equilibrating classical secondary ion.
The 1,2 shift that interconverts the two ions was presumed to be rapid and he
suggested that the rapid migration would lead to preferential approach of the
nucleophile from the exo direction.
A vast amount of research and work has been done on 2-
norbornyl system and almost (not all) all chemists accept the
the Winstein’s interpretation.

10. References
 Advanced Organic Chemistry Part A:
Structure and Mechanisms, Francis A.
Carey and Richard J. Sundberg, Fifth
edition, Springer
 MARCH’S ADVANCED ORGANIC
CHEMISTRY REACTIONS, MECHANISMS,
AND STRUCTURE,Michael B. Smith, Jerry
March, SIXTH EDITION,WILEY
 www.unacademy.com
 https://chem.libretexts.org/