CARBOCATIONS
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CARBOCATIONS VAMSI RAJASEKHARA REDDY M.PHARMACY 1 st year (Pharmaceutical Chemistry)
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Transcript of CARBOCATIONS
CARBOCATIONS
VAMSI RAJASEKHARA REDDYM.PHARMACY 1st year
(Pharmaceutical Chemistry)
CONTENTS
INTRODUCTION CARBOCATIONS
DEFINITION
STABILITY
GENERATION
REACTIONS
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CARBOCATIONS
In Heterolytic Fission Of A C-x Bond In An Organic Molecule The Positively Charged Carbon Species Is Called Carbocation
A carbon atom which is trivalent, contains an even number of electrons and carries a positive charge is known as carbocation.
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H
H
R X R C
H
H
+X-:
STABILITY OF CARBOCATIONS
Tertiary carbocation is more than secondary carbocation which in turn more stable than a primary carbocation.
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R C
R
R
+ > C
R
R
H
+> R C
H
H+
tertiary secondary primary
carbocation carbocation carbocation
STABILITY OF CARBOCATIONS
Increase in the number of electron releasing groups increases carbocation stability.
The more the canonical structures for carbocation, thae more stable it will be.
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CH2+
CH2 CH2 CH2
+
+
+
ORDER OF STABILITY
tropylium cation >trimethyl carbocation>benzyl carbocation> allyl carbocation> tertiary butyl carbocation> isopropyl carbocation> ethyl carbocation> methyl carbocation
> > > >
> >
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+C
+CH2
+ CH2 CH CH2
+CH3 C
CH3
CH3
+
CH3 CH+
CH3
CH3 CH2+ CH3
+
GENETRATION OF CARBOCATIONS
• FROM ALKENES
• FROM ALKYLHALIDES (X=I,Cl,Br) • FROM DIAZONIUM IONS • FROM ALCHOLS • FROM ACYL HALIDES
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CH3CH3
CH2 CH3
CH3CH3
CH3 CH3
H+
++
R N+
N R N2++
R OH +H+
[ROH2]+
R+H2O-
RCOCl RCO ClAlCl3...+ _
RCO+ + AlCl4
_
Carbocation(acylium ion)
RR X X+ _
+
REACTIONS OF CARBOCATIONS
ELIMINATION OF A PROTON
Rearrangement
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CH3CH2CH2NH2 CH3CH2CH2N N+
1 Aminopropane Propyldiazonium cation
NaNO2
HClCH3CH2CH2
+HC CH2
CH3
1 Propylcation(primary cation)
propene
CH3CHCH3
+
Secondary cation
REACTION WITH NUCLEOPHILES
CH3 CH2 CH2 :Br: CH3 CH2 CH2Br
CH3 CH CH2 :Cl: CH3 HC CH3
Cl
..
..
..
..
+
+
+
+
_
_
Secondary propyl carbocation
Propyl carbocationn-Propyl bromide
isopropyl chloride
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CH3 C
CH3
CH3
O
H
H CH3 C
CH3
CH3
O+
H
H CH3 C
CH3
CH3
OH++ ..
Tertiary butyl carbocation
water
(neutral nucleophile)t-butyl alcohol
-H+
ADDITION TO UNSATURATED COMPOUNDS
CH3 C CH2
CH3
+ C CH3
CH3
CH3
CH3 C CH2 C CH3
CH3
CH3CH3
+
+
2-methylpropene
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MOLECULAR REARRANGEMENT
CH3CH2CH2
+
(primary carbocation)CH3 CH CH3
+
(secondary carbocation)
Rearrangement
CH3 C
CH3
CH3
CH CH3 CH3 C C CH3
CH3
H
CH3+
+
Rearrangement
3,3-Dimethyl-2-butyl
carbocation2,3-Dimethyl-2-butylcarbocation(secondary carbocation) (tertiary carbocation)
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HO CH2NH2HO CH2N2 OH CH2
+ ..
..
+
-N2NaNO2/HCl
0 to 5
OH+ O
-H+
1Aminomethylcyclohexanol cycloheptanone
Stereochemistry of rearrangements
R
CH3
H
YCH3
H
CH3
H
H
CH3
RH
CH3 Z
H
R
CH3
H
CH3 Z
R
CH3
H
Z
_:
+
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Friedel-craft reaction
It carried out by treating aromatic with the alkyl halide in the presence of Lewis acid.
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CH2H R
+ AlCl4-
CH2
R
+ HCl + AlCl3
+ R CH2 Cl+
AlCl3-
CH2H RCH2H R CH2H R
+ +
+
Wagner-meerwein rearrangementIn these rearrangements the change in the carbon
skeleton occurs through the rearrangement of carbocation intermediate.
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CH3
CH3
CH3
CH2OHH2SO4
-H2OCH3
CH3
CH3
CH2
+ 1,2methylshiftCH3 C
CH3
C2H5
+
CH3 C
CH3
C2H5
+
CH3 C
CH3
C2H5
CH2 C
CH3
C2H5
+
minor
major
REFERENCES
ADVANCE ORGANIC CHEMISTRY REACTIONS, MECHANISMS AND STRUCTURE BY JERRY MARCH , PG NO:165-185.
Organic name reactions and molecular rearrangements by Gurdeep raj pgno: A-158,A332.
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