Hyperconjugation...•This type of electron release due to presence of the system H–C–C=C is...
Transcript of Hyperconjugation...•This type of electron release due to presence of the system H–C–C=C is...
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Hyperconjugation
Baker and Nathan suggested that an alkyl groupwith at least one H-atom on the -carbon atomwhen attached to an unsaturated C-atom, is ableto release electron by a mechanism similar tothat of electromeric effect.
C
H
CC
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• This type of electron release due to presence ofthe system H–C–C=C is known ashyperconjugation. For example, Propylene maybe regarded as the resonance hybrid of thefollowing structures due to hyperconjugation.
C
H
H
CHH CH2 C
H
CHH CH2
I II
H
C
H
H
CH CH2 C
H
CHH CH2
III IV
H
H
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• The various hyperconjugation forms ofpropylene are called contributing structures.In fact hyperconjugation effect is similar toresonance effect.
• Since structure II, III & IV have nodefinite bond between the -C – atom andone of the H–atom, hence hyperconjugationis also known as no bond resonance. It isalso known as second order resonanceor Baker Nathan effect.
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• The orbital concept of hyperconjugationmay be explained with the help ofpropylene. In this concept, the electronpair of C–H bond ( bond) is involved inconjugation with the electron pair of thedouble bond. Therefore, hyperconjugationinvolves delocalization of electrons ofC–H bond through overlapping of p-orbitalof double bond as shown below:
C
H
H
C C
H bond
CH bond
hyperconjugation
Orbital picture of hyperconjugation.
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Structural requirements of hyperconjugation:
• Any organic compound can showhyperconjugation if it will fulfill the followingconditions:
1. Compound should have sp2 hybrid carbon ofalkene, arenes, carbocations and free radicals.
2. -Carbon atom with respect to sp2 hybrid C–atom should have at least one hydrogen atomand -carbon atom should be sp3 hybridized.
• Thus, hyperconjugation is of following types:
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(i) (C–H), conjugation:
• This type of conjugation occurs in alkenes and alkyl substituted aromatic compounds.
C
H
H
CHH CH2
C CH3
H
H
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(ii) (C–H), positive charge (vacant p-orbital) conjugation:
• This type of conjugation occurs in alkylcarbocations.
C CH2H
H
H
C CHH
H
H
C H
H
H
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(iii) (C–H), odd electron (incomplete
p-orbital conjugation):
• This type of conjugation occurs in alkyl free radicals.
C CH2H
H
H
C CHH
H
H
C H
H
H
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Applications: 1. Stability of alkenes:
• Heat of hydrogenation shows that thegreater the number of alkyl groupsattached to the double bonded C – atom,greater is the stability of alkene (lower isthe heat of hydrogenation). Thus on thebasis of heat of hydrogenation, the orderof stability of different alkenes have beenfound to be: Tetrasubstituted >Trisubstituted > Disubstituted >Monosubstituted > Ethylene.
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Alkenes
No. of
- H atoms
12 9 6 3
No. ofResonating
structures
13 10 7 4
•The above order of stability of substituted alkenes can also be explained on the basis of hyperconjugation.
C C
CH3
CH3
CH3
CH3
C C
CH3
H
CH3
CH3
CH CH CH3CH3 CH CH2CH3
The greater the number of resonating structuresof a molecule, greater will be its stability.
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• Trans-2-Butene is more stable than cis-2-butene, in which two methyl groups are closetogether and hence their electronic cloud(steric hindrace) repel each other.
C C
CH3
H
CH3
H
C C
H
CH3
CH3
H
Steric hendrace
in cis-2-butene
No Steric hendrace
in trans-2-butene
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2. Abnormal bond lengths:• In hyperconjugation a single bond acquires a
double bond character and vice versa, henceabnormality in bond, lengths is observed in thecompounds showing hyperconjugation. Forexample, Ethane and ethylene, C–C and C=Cbonds show normal length 1.54Aº and 1.33Aº,respectively due to no hyperconjugation in thecompounds but in propene, the bond lengthsare 1.47Aº and 1.35Aº for C–C and C = Cbonds, respectively. This change in bondlengths may be explained by hyperconjugation.
1.54 Aº
Ethane
1.33 Aº
Ethylene
CH3 CH CH2
1.47 Aº
Propylene
1.35 Aº
CH2 CH2CH3 CH3
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3. Directive influence of alkyl groups:
• The –CH3 and other alkyl groups are o- andp- directing which can be explained on thebasis of hyperconjugation as follows:
C HH
H
CH
H
H CH
H
H CH
H
H
Six more such structures
are possible due to other
two -H atoms.
As a result of hyperconjugation, electron densityat o- and p- position (w.r.t. methyl group)increases and therefore electrophilic substitutionin toluene takes place at o- and p- positions.Thus alkyl groups are o- and p- directing.
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4. Orienting power of methyl group in p-substituted toluene:• In p-t-butyl toluene, further substitution
occurs at o- position with respect tomethyl group even though the (+)inductive effect of t-butyl group is fargreater than methyl group. This anomalyis due to greater hyperconjugative effectof methyl group (having 3 - H atoms)which increases the electron density at o-position w.r.t. methyl group than t-butylgroups which have no - H atoms andhence no hyperconjugation occurs. Herehyperconjugation over weight inductiveeffect.
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• Hyperconjugation in t-butyl toluene
CH
H
C CH3H3C
CH3
H HCH
C CH3H3C
CH3
C H
H
C CH3H3C
CH3
H
C CH3H3C
CH3
C HH
H H
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5. Stability of free radicals andcarbocations:
1. Stability of alkyl carbocations:
• The order of stability of different alkyl carbocations is:
C
CH3
CH3
CH3 C
H
CH3
CH3 C
H
H
CH3 C
H
H
H
t Butyl Isopropyl Ethyl Methyl
Carbocation (3º) Carbocation (2º) Carbocation (1º) Carbocation
This order of stability of alkyl carbocations can be explained on the basis of inductive effect & hyperconjugation as follows:
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• hyperconjugation states that greater the number of α-hydrogen atoms on a carbocation greater is thenumber of hyperconjugative structures (greater thedispersion of positive charge) and hence more is thestability of carbocation.
• Thus t-butyl carbocation (3º) with nine α-hydrogenatoms has one usual and nine hyperconjugativestructures is more stable than isopropyl carbocation(2º) with six α-hydrogen atoms, having one usual andsix hyperconjugative structures which, in turn, is morestable than ethyl carbocation (1º) with three α-hydrogens, having one usual & three hyperconjugativestructures, while methyl carbocation with no α-hydrogen, has one usual and no hyperconjugativestructure, is least stable.
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• One usual and nine hyperconjugative structures of t-butyl carbocation (3º)
H C C CH3
H
H
CH3
H C C CH3
H CH3
C C CH3
H
H
CH3
H C C CH3
H
CH3
etc.
t Butyl Carbocation
H
H
H
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• One usual and six hyperconugative structures of isopropyl carbocation (2º)
H C C C H
H
H
H H
H
I
H C C C H
H H H
H
II
C C C H
H
H
H H
H
III
H C C C H
H
H H
H
IV
H C C C H
H
H
H
V
H C C C
H
H
H H
H
VI
H C C C H
H
H H
VII
H
H
H
H
H
H
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• One usual & three hyperconjugative structures of ethyl carbocation (1º)
H C CH2
H
H
I
H C CH2
H
II
C CH2
H
H
III
H C CH2
H
IV
H
H
H
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Stability of alkyl free radicals:
• The order of stability of different alkyl free radicals is as follows:
C
CH3
CH3
CH3 C
H
CH3
CH3 C
H
H
CH3 C
H
H
H
t-Butyl
free radical
( 3º )
Isoprppyl
free radical
( 2º )
Ethyl
free radical
( 1º )
Methyl
free radical
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• The above order of stability can be explained on the basis of hyperconjugation.
R'
C
R
C
H
H
H
p-orbital bonds of
having odd electronalkyl group
R
C
R
C
H
H
H
overlaping of orbital due to hyperconjugation
R
C
R
C
H
H
H
Structure of a radical
having -H atoms
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• In general, greater the number of α-hydrogens,greater is the number of hyperconjugativestructures and hence more stable is the freeradical. Thus t-butyl free radicals (3º) with nineα-hydrogens has one usual and ninehyperconjugative structures, is more stablethan isopropyl free radical (2º) with six α-hydrogens has one usual & sixhyperconjugative structures which in turn, ismore stable than ethyl free radical (1º) withthree α-hydrogens has one usual & threehyperconjugative structures while methyl freeradical with no α-hydrogens has one usual & nohyperconjugative structures is least stable.
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• One usual & nine hyperconjugativestructures of t-butyl free radical (3º)
H C C CH3
H
H
CH3
H C C CH3
H CH3
C C CH3
H
H
CH3
H C C CH3
H
CH3
H
H
H
etc.
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• One usual and six hyperconjugativestructures of isopropyl free radical (2º)
H C C C
H
H
H H
H
H
H C C C
H H H
H
HH
C C C
H H H
H
HH
H
H C C C
H H
H
HH
H
HCCC
HH
H
HH
CCC
HH
H
H H
H
HCCC
H
H
H H
H
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• One usual and three hyperconjugativestructures of ethyl free radical (1º)
H
C
H
C
H
H
H
H
C
H
C
H
H
H
C
H
C
H
H
H
C
H
C
H
H
H
H
H