Hyperconjugation

Hyperconjugation

1. Hyperconjugation
BakerandNathansuggestedthatanalkylgroup
withatleastoneH-atomonthe-carbonatom
whenattachedtoanunsaturatedC-atom,isable
toreleaseelectronbyamechanismsimilarto
thatofelectromericeffect.C
H
CC

2. •Thistypeofelectronreleaseduetopresenceof
thesystemH–C–C=C isknownas
hyperconjugation.Forexample,Propylenemay
beregardedastheresonancehybridofthe
followingstructuresduetohyperconjugation.C
H
H
CHH CH
2
C
H
CHH CH
2
I II
H C
H
H
CH CH
2
C
H
CHH CH
2
III IV
H
H

3. •Thevarioushyperconjugationformsof
propylenearecalledcontributingstructures.
Infacthyperconjugationeffectissimilarto
resonanceeffect.
•SincestructureII,III&IVhaveno
definitebondbetweenthe-C–atomand
oneoftheH–atom,hencehyperconjugation
isalsoknownasnobondresonance.Itis
alsoknownassecondorderresonance
orBakerNathaneffect.

4. •Theorbitalconceptofhyperconjugation
maybeexplainedwiththehelpof
propylene.Inthisconcept,theelectron
pairofC–Hbond(bond)isinvolvedin
conjugationwiththeelectronpairofthe
doublebond.Therefore,hyperconjugation
involvesdelocalizationofelectronsof
C–Hbondthroughoverlappingofp-orbital
ofdoublebondasshownbelow:C
H
H
C C
H
 bond
CH  bond
 hyperconjugation
Orbital picture of  hyperconjugation.

5. Structural requirements of
hyperconjugation:
•Any organiccompound can show
hyperconjugationifitwillfulfillthefollowing
conditions:
1.Compoundshouldhavesp2hybridcarbonof
alkene,arenes,carbocationsandfreeradicals.
2.-Carbonatomwithrespecttosp2hybridC–
atomshouldhaveatleastonehydrogenatom
and-carbonatomshouldbesp3hybridized.
•Thus,hyperconjugationisoffollowingtypes:

6. (i) (C–H), conjugation:
•This type of conjugation occurs in alkenes
and alkyl substituted aromatic compounds. C
H
H
CHH CH
2

CCH
3
H
H


7. (ii) (C–H), positive charge
(vacant p-orbital) conjugation:
•Thistypeofconjugationoccursinalkyl
carbocations.C CH
2
H
H
H

C CHH
H
H
C H
H
H

8. (iii)(C–H),oddelectron(incomplete
p-orbitalconjugation):
•This type of conjugation occurs in alkyl
free radicals. C CH
2
H
H
H

C CHH
H
H
C H
H
H
 

9. Applications:
1. Stability of alkenes:
•Heatofhydrogenationshowsthatthe
greaterthenumberofalkylgroups
attachedtothedoublebondedC–atom,
greateristhestabilityofalkene(loweris
theheatofhydrogenation).Thusonthe
basisofheatofhydrogenation,theorder
ofstabilityofdifferentalkeneshavebeen
foundtobe:Tetrasubstituted>
Trisubstituted> Disubstituted>
Monosubstituted>Ethylene.

10. Alkenes
No. of
-H atoms
12 9 6 3
No. of
Resonating
structures
13 10 7 4
•The above order of stability of substituted
alkenes can also be explained on the basis of
hyperconjugation.C C
CH
3
CH
3
CH
3
CH
3 C C
CH
3
H
CH
3
CH
3 CH CH CH
3
CH
3 CHCH
2
CH
3
Thegreaterthenumberofresonatingstructures
ofamolecule,greaterwillbeitsstability.

11. •Trans-2-Buteneismorestablethancis-2-
butene,inwhichtwomethylgroupsareclose
togetherandhencetheirelectroniccloud
(sterichindrace)repeleachother.C C
CH
3
H
CH
3
H
C C
H
CH
3
CH
3
H
Steric hendrace
in cis-2-butene
No Steric hendrace
in trans-2-butene

12. 2. Abnormal bond lengths:
•Inhyperconjugationasinglebondacquiresa
doublebondcharacterandviceversa,hence
abnormalityinbond,lengthsisobservedinthe
compoundsshowinghyperconjugation.For
example,Ethaneandethylene,C–CandC=C
bondsshownormallength1.54Aºand1.33Aº,
respectivelyduetonohyperconjugationinthe
compoundsbutinpropene,thebondlengths
are1.47Aºand1.35AºforC–CandC=C
bonds,respectively.Thischangeinbond
lengthsmaybeexplainedbyhyperconjugation.1.54 Aº
Ethane
1.33 Aº
Ethylene
CH
3
CH CH
2
1.47 Aº
Propylene
1.35 Aº
CH
2
CH
2
CH
3
CH
3

13. 3. Directive influence of alkyl groups:
•The–CH3andotheralkylgroupsareo-and
p-directingwhichcanbeexplainedonthe
basisofhyperconjugationasfollows:C HH
H
CH
H
H CH
H
H CH
H
H
Six more such structures
are possible due to other
two -H atoms.
Asaresultofhyperconjugation,electrondensity
ato-andp-position(w.r.t.methylgroup)
increasesandthereforeelectrophilicsubstitution
intoluenetakesplaceato-andp-positions.
Thusalkylgroupsareo-andp-directing.

14. 4. Orienting power of methyl
group in p-substituted toluene:
•Inp-t-butyltoluene,furthersubstitution
occursato-positionwithrespectto
methylgroupeventhoughthe(+)
inductiveeffectoft-butylgroupisfar
greaterthanmethylgroup.Thisanomaly
isduetogreaterhyperconjugativeeffect
ofmethylgroup(having3-Hatoms)
whichincreasestheelectrondensityato-
positionw.r.t.methylgroupthant-butyl
groupswhichhaveno-Hatomsand
hencenohyperconjugationoccurs.Here
hyperconjugationoverweightinductive
effect.

15. •Hyperconjugation in t-butyl tolueneCH
H
C CH
3
H
3
C
CH
3
H HCH
C CH
3
H
3
C
CH
3
C H
H
C CH
3
H
3
C
CH
3
H
C CH
3
H
3
C
CH
3
C HH
H H

16. 5. Stability of free radicals andcarbocations:
1.Stability of alkyl carbocations:
•The order of stability of different alkyl
carbocations is: C
CH
3
CH
3
CH
3
C
H
CH
3
CH
3
C
H
H
CH
3
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:

17. •hyperconjugationstatesthatgreaterthenumberofα-
hydrogenatomsonacarbocationgreateristhe
numberofhyperconjugativestructures(greaterthe
dispersionofpositivecharge)andhencemoreisthe
stabilityofcarbocation.
•Thust-butylcarbocation(3º)withnineα-hydrogen
atomshasoneusualandninehyperconjugative
structuresismorestablethanisopropylcarbocation
(2º)withsixα-hydrogenatoms,havingoneusualand
sixhyperconjugativestructureswhich,inturn,ismore
stablethanethylcarbocation(1º)withthreeα-
hydrogens,havingoneusual&threehyperconjugative
structures,whilemethylcarbocationwithnoα-
hydrogen,hasoneusualandnohyperconjugative
structure,isleaststable.

18. •One usual and nine hyperconjugative
structures of t-butyl carbocation (3º)HCCCH
3
H
H
CH
3



HCCCH
3
HCH
3
CCCH
3
H
H
CH
3
HCCCH
3
H
CH
3
etc.
t Butyl Carbocation
H
H
H

19. •One usual and six hyperconugative structures
of isopropyl carbocation (2º)HCCCH
H
H
HH
H

I
HCCCH
HHH
H
II
CCCH
H
H
HH
H
III
HCCCH
H
HH
H
IV
HCCCH
H
H
H
V
HCCC
H
H
HH
H
VI
HCCCH
H
H H
VII
H
H
H
H
H
H

20. •One usual & three hyperconjugative
structures of ethyl carbocation (1º) HCCH
2
H
H
I
HCCH
2
H
II
CCH
2
H
H
III
HCCH
2
H
IV
H
H
H

21. Stability of alkyl free radicals:
•The order of stability of different alkyl free
radicals is as follows:C
CH
3
CH
3
CH
3
C
H
CH
3
CH
3
C
H
H
CH
3
C
H
H
H
t-Butyl
free radical
( 3º )
Isoprppyl
free radical
( 2º )
Ethyl
free radical
( 1º )
Methyl
free radical

22. •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

23. •Ingeneral,greaterthenumberofα-hydrogens,
greateristhenumberofhyperconjugative
structuresandhencemorestableisthefree
radical.Thust-butylfreeradicals(3º)withnine
α-hydrogenshasoneusualandnine
hyperconjugativestructures,ismorestable
thanisopropylfreeradical(2º)withsixα-
hydrogenshasoneusual& six
hyperconjugativestructureswhichinturn,is
morestablethanethylfreeradical(1º)with
threeα-hydrogenshasoneusual&three
hyperconjugativestructureswhilemethylfree
radicalwithnoα-hydrogenshasoneusual&no
hyperconjugativestructuresisleaststable.

24. •Oneusual&ninehyperconjugative
structuresoft-butylfreeradical(3º)HCCCH
3
H
H
CH
3
HCCCH
3
HCH
3
CCCH
3
H
H
CH
3
HCCCH
3
H
CH
3
 

H
H
H
etc.

25. •One usualandsixhyperconjugative
structuresofisopropylfreeradical(2º)HCCC
H
H
HH
H
H
 
HCCC
HHH
H
H
H
CCC
HHH
H
HH
H
HCCC
HH
H
HH
H
HCCC
HH
H
H
H
CCC
HH
H
H H
H HCCC
H
H
H H
H

26. •Oneusualandthreehyperconjugative
structuresofethylfreeradical(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