This document discusses carbonium ion rearrangement reactions. It defines key terms like migrating group and migration terminus. It describes two types of rearrangements: intermolecular, where the migrating group detaches completely before reattaching, and intramolecular, where the migrating group remains attached. It provides Wagner-Meerwein rearrangement as an example and outlines its general reaction, mechanism, and applications. Specifically, it is used to prepare pinacolones from pinacols (diols with adjacent hydroxyl groups) through the Pinacol-Pinacolone rearrangement.

1. CARBONIUM ION REARRANGEMENT
Rearrangement reactions are quite common in organic
chemistry. In a rearrangement reaction the atoms or groups
with in a molecule move from one atom to another thus
forming a structural isomer of the starting compound or the
product. The product is rearranged molecule.
MIGRATING GROUP:
The atom or group which migrates is called the migrating
group
MIGRATION TERMINUS:
The atom to which the migrating group finally gets attached is
called the migration terminus

2. TYPES OF REARRANGEMENT
There are two types of rearrangements
INTERMOLECULAR REARRANGEMENT
INTRAMOLECULAR REARRANGEMENT
If in any rearrangement the migrating group
actually becomes free even for a small fraction of
time is called INTERMOLECULAR
REARRANGEMENT. In such reactions the
migrating group gets completely detached from the
molecule during the rearrangement and then it may
go to migration terminus of the other molecule.

3. If in any rearrangement the migrating group remains attached
to the molecule in one or the other way throughout the
process of rearrangement then such a rearrangement is called
INTRAMOLECULAR REARRANGEMENT.
WAGNER-MEERWEIN REARRANGEMENT
INTRODUCTION:
In wagner-meerwein rearrangement the alkyl halides,
alcohols, amines and paraffin's are involved. First the
electron carbon center is formed and the carbon with its

4. Bonding pairs migrates to stabilize the electron deficient
carbon center, then attack of nucleophile take place and
corresponding product is formed.
GENERAL REACTION
The rearrangement reactions can be represented generally
as Nu-
M- A- B - Nu- A- B- M + L-
L
A is migrating terminal or origin. B is electron deficient
carbon. M is migrating group & L is leaving group & Nu

5. is the attacking nucleophile and if there is no nucleophile
then product is as under
M
A B A= B M + L-
L
(PRODUCT)

6. The mechanism involves carbocation/ carbonium ion
which is an electron-deficient atom. Migration is due to
stabilization of carbonium ion. As primary carbonium
ion is less stable than secondary and secondary is less
stable than tertiary carbonium ion. Thus if primary
carbonium is formed migration takes place to form
secondary carbonium ion and secondary is converted to
tertiary carbonium ion as it is the most stable, then the
attack of nucleophile takes place.

7. MECHANISM
STEP#1
Formation of carbonium ion OR
The formation of electron deficient carbon
STEP#2
Carbon with its bonding pair migrates to stabilize
the electron deficient carbon Centre
STEP#3
Attack of nucleophile takes place and rearranged
product is formed

11. PINACOL-PINACOLONE REARRANGEMENT
The diols in which the hydroxyl groups are
attached to the adjacent carbons are called
“Pinnacol's”
The treatment of pinacol with acids leads to the
rearrangement and ketones are formed which are
called “Pinacolones.” that’s why it is called Pinacol-
Pinacolone rearrangement.

16. APPLICATIONS
It is used in the preparation of pinacolones.
Pinacolones are highly branched ketones and these
cannot be easily obtained by other methods