Post on 25-Sep-2020
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
• 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
• 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.
• 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.
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:
(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
(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
(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
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.
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.
• 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
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
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.
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.
• 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
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:
• 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.
• 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
• 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
• 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
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
• 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
• 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.
• 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.
• 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
• 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