Hyperconjugation

By Dr. Neelam

1. Presented by
Dr. Neelam
Department of chemistry

2. Itisadelocalizationofσ-electronsofC–Hbondofanalkylgroupdirectly
attachedtoanatomofunsaturatedsystemoranatomwithunsharedp-
orbital.
Thisphenomenoniscalledhyperconjugation.Itmaytakeplacein
alkene,alkynes,carbocationsandcarbonfreeradicals.

3. *HyperconjugationisalsocallednobondresonanceorBakerNathan
Effect.
•Numberofhyperconjugativestructure=Numberofα-hydrogenatoms
atsp
3
hybridisedα-carbonatoms.

4. •Condition:sp
3
hybridC–HorC–Dmustbepresentadjacenttothe
• C+/C•/C=C.
Importantpoints:
(i)Itisdistanceindependent.
(ii)Notapplicableatcarbanion.
(iii)Hybridisationofatomsremainsunchanged.
(iv)Itisapermanenteffect.

5. ApplicationsofHyperconjugationeffect
(i)StabilityofAlkenes:Hyperconjugationexplainsthestabilityof
certainalkenesoverotheralkenes.
(ii)Heatofhydrogenation:Greaterthenumberofα-hydrogenatoms,
greaterwillbestabilityofalkene.Thusgreaterextentof
hyperconjugationresultslowervalueofheatofhydrogenation
(ΔH
hydrogenation)
CH
2=CH
2>CH
3–CH=CH
2>CH
3–CH=CH–CH
3(Heatof
Hydrogenation)

6. (iv)Stabilityofcarbocation:Greaternumberof‘α’hydrogenatoms,
greaterwillbestabilityofcarbocations.
(v)Stabilityoffreeradical:Morethenumberofα-hydrogenatoms,
morewillbestabilityofcarbonfreeradical.

7. Itisatemporaryeffect.
Itisdefinedasthecompletetransferofasharedpairofπ-electronsto
oneoftheatomjoinedbyamultiplebondonthedemandofanattacking
reagent.
Theorganiccompoundhavingamultiplebond(doubleortriplebond)
showthiseffectinthepresenceofanattackingreagentonly.
ItisrepresentedbyEandtheshiftingoftheelectronsisshownbya
curvedarrow.

8. Therearetwotypesofelectromericeffect
(i)+Eeffect:Inthiseffectπ-electronofthemultiplebondtransferredto
thatatomatwhichthereagentgetsattached.
(ii)–Eeffect:Inthiseffecttheπ-electronofthemultiplebond
transferredtothatatomatwhichtheattackingreagentdoesnotget
attached.
Wheninductiveandelectromericeffectsoperateinoppositedirections
thentheelectromericeffectdominates.

9. 9