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Hyperconjugation
- Devyani Joshi
Resonance involves delocalization of π electrons, leaving
the σ bond untouched.
However in some cases, a σ bond and an a...
•Stabilizing effect of the
molecule
•Interaction of sigma (σ) bond
(eg. C – H, C – C) with a pi
(π) network.
When carbon possessing atleast one Hydrogen is attached
to another carbon bearing an unshared orbital or to an
unsaturate...
 The shared pair of electrons is now borne by carbon alone and
hydrogen is in its close proximity as proton.
 The negati...
Hyperconjugation: No bond resonance
 The electrons of the sigma bond between C and H are involved in delocalization.
 In...
 Such an interaction is also referred to as ‘Heterovalent’ or ‘Sacrificial
hyperconjugation’.
 This is so named because ...
Effect of
Hyperconjugation on
the chemical
properties:
Alkyl cations and their relative stability
 Carbocations have an electron deficient (positively charged) carbon.
 The em...
 The more the number of alkyl groups on the carbocation, more is the number of α C – H σ
bonds and hence more are the pos...
Alkyl radicals and their relative stability:
 Alkyl radicals have p-orbital of carbon occupied by an odd electron which g...
The order of stability depends on the
extent of delocalization. The greater the
delocalization, the more is the stability...
Alkenes and their stability
 Overlap of σ orbital of C – H bond with π orbital of adjacent C – C
double bond gives rise t...
 α C – H possesses partial ionic character and its electrons get delocalized
into the adjacent π system.
 More the subst...
 Stability of alkenes will increase with increase in number of Hydrogen α to unsaturated system.
 Increasing order of st...
Bond length
 Hyperconjugation leads to shortening of sigma (σ) bond.
 Eg. C – C bond in 1,3-butadiene and methylacetylen...
Anomeric effect
 The general tendency of anomeric substituents to prefer an axial orientation is called as
anomeric effec...
Baker – Nathan Effect
 Alkyl groups are electron donating. The relative magnitude of this
inductive effect of alkyl group...
This is known as a Baker – Nathan effect and was explained
by John W. Baker and W. S. Nathan on the basis of
Hyperconjuga...
 In –CH3 there are 3 hyperconjugable hydrogen atoms, thus more
stabilization by hyperconjugation.
 In –CH2CH3 there are ...
Hyperconjugation - organic chemistry
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Hyperconjugation - organic chemistry

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This presentation describes the concept of Hyperconjugation in simple words, gives definition of hyperconjugation, explains why it is called as 'No bond Resonance' and gives the effects of hyperconjugation on the chemical properties of compounds: alkyl cations and their relative stability, alkyl radicals and their relative stability, alkenes and their relative stability, bond length, anomeric effect and Baker - Nathan effect.

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Hyperconjugation - organic chemistry

  1. 1. Hyperconjugation - Devyani Joshi
  2. 2. Resonance involves delocalization of π electrons, leaving the σ bond untouched. However in some cases, a σ bond and an adjacent π bond may get involved in resonance. Such a delocalization is called as Hyperconjugation.
  3. 3. •Stabilizing effect of the molecule •Interaction of sigma (σ) bond (eg. C – H, C – C) with a pi (π) network.
  4. 4. When carbon possessing atleast one Hydrogen is attached to another carbon bearing an unshared orbital or to an unsaturated carbon, there are more than one structure (canonical forms) possible for the molecule. These result from the overlap of the bonding electrons of C – H σ bond with 2p or π orbital of the adjacent carbon atom. As a result, the bond between carbon and hydrogen does not exist anymore.
  5. 5.  The shared pair of electrons is now borne by carbon alone and hydrogen is in its close proximity as proton.  The negative charge developed on the carbon gets delocalized by overlap with adjacent p orbital.
  6. 6. Hyperconjugation: No bond resonance  The electrons of the sigma bond between C and H are involved in delocalization.  In structure to the right: No bond between C and H due to migration of the sigma bond. Hence Hyperconjugation is also called as ‘NO BOND RESONANCE’.  This does not indicate that hydrogen is completely detached from the structure, but some degree of ionic character in the C – H bond and some single bond character between carbon – carbon double bond.
  7. 7.  Such an interaction is also referred to as ‘Heterovalent’ or ‘Sacrificial hyperconjugation’.  This is so named because the contributing structure contains one two-electron bond less than the normal Lewis formula of the compound.  At present, there is no evidence of sacrificial hyperconjugation with neutral hydrocarbons
  8. 8. Effect of Hyperconjugation on the chemical properties:
  9. 9. Alkyl cations and their relative stability  Carbocations have an electron deficient (positively charged) carbon.  The empty p orbital of this sp2 carbon can overlap with σ orbital of C – H bond of adjacent alkyl group (α C – H bond).  This overlap permits individual electrons to help bind together three nucleus: 2 carbon and 1 hydrogen.  The positive charge thus gets dispersed over large volume of space and is stabilized.
  10. 10.  The more the number of alkyl groups on the carbocation, more is the number of α C – H σ bonds and hence more are the possibilities for hyperconjugation which makes the carbocation more stable.  The order of stability of the Carbocations is: 30alkyl carbocation > 20alkyl carbocation > 10alkyl carbocation > methyl carbocation 9 Hyperconjugable H 6 Hyperconjugable H 3 Hyperconjugable H No Hyperconjugable H
  11. 11. Alkyl radicals and their relative stability:  Alkyl radicals have p-orbital of carbon occupied by an odd electron which gets delocalized over three nuclei (2 carbon and 1 hydrogen) by the overlap with the σ orbital of C – H bond of adjacent alkyl groups.  The bond is formed between 2 carbons and odd electron is held by the hydrogen atom.
  12. 12. The order of stability depends on the extent of delocalization. The greater the delocalization, the more is the stability. 30alkyl radical > 20alkyl radical > 10alkyl radical > methyl radical
  13. 13. Alkenes and their stability  Overlap of σ orbital of C – H bond with π orbital of adjacent C – C double bond gives rise to canonical structures.  Delocalization of electrons occurs over three nuclei and thus stabilizes the alkene.
  14. 14.  α C – H possesses partial ionic character and its electrons get delocalized into the adjacent π system.  More the substituent, more is the opportunity for hyperconjugation and more stable is the alkene.
  15. 15.  Stability of alkenes will increase with increase in number of Hydrogen α to unsaturated system.  Increasing order of stability of alkenes because of Hyperconjugation: Alkene Number of Hyperconjugable Hydrogen (α to unsaturated function) CH2=CH2 0 (CH3)CH=CH2 3 (CH3)2C=CH2 (CH3)CH=CH(CH3) 6 (CH3)2C=CH(CH3) 9 (CH3)2C=C(CH3)2 12
  16. 16. Bond length  Hyperconjugation leads to shortening of sigma (σ) bond.  Eg. C – C bond in 1,3-butadiene and methylacetylene is 1.46 A0 in length, much less as compared to 1.54 A0 found in saturated hydrocarbons. For butadiene, this can be explained by the normal conjugation of 2 alkenyl parts. But for methylacetylene, hyperconjugation between alkyl and alkynyl parts.
  17. 17. Anomeric effect  The general tendency of anomeric substituents to prefer an axial orientation is called as anomeric effect.  α-methyl glucoside is more stable than β-methyl glucoside due to hyperconjugation.  In β-methyl glucoside the methoxy group is at equatorial position and cannot involve in hyperconjugation. Therefore β-methyl glucoside is less stable than the α-methyl glucoside. α-methyl glucoside Β-methyl glucoside
  18. 18. Baker – Nathan Effect  Alkyl groups are electron donating. The relative magnitude of this inductive effect of alkyl groups is: 30 > 20 > 10 > CH3  This order is reversed when an alkyl group is directly attached to an unsaturated system, where the electron release is maximum for CH3 & the order is: CH3 > 10 > 20 > 30
  19. 19. This is known as a Baker – Nathan effect and was explained by John W. Baker and W. S. Nathan on the basis of Hyperconjugation.
  20. 20.  In –CH3 there are 3 hyperconjugable hydrogen atoms, thus more stabilization by hyperconjugation.  In –CH2CH3 there are only 2 In –CHR there is only 1.  This explains the reversal of order where electron release is not by inductive effect but by Hyperconjugation.

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