Hyperconjugation is the donation of a sigma bond into an adjacent empty or partially filled p orbital, which results in an increased stability of the molecule. Contributed by: Samuel Redstone (Undergraduate), University of Utah, 2016

1. Organic Pedagogical Electronic
Network
Hyperconjugation
Samuel Redstone
University of Utah

2. Hyperconjugation: Definitions
1. J. Chem. Soc., 1935, 1844-1847.
Hyperconjugation is the donation of a sigma bond into an adjacent empty or
partially filled p orbital, which results in an increased stability of the molecule.
Hyperconjugation contributes to the resonance stabilization
of this tertiary carbocation, where electrons from the C-H
sigma bonding orbital are donated to the empty p orbital of
the cation.
Hyperconjugation was first described by Baker and Nathan in 1935 to describe the
“abnormal behavior” of alkyl-substituted compounds.1

3. Trends in Hyperconjugation
1. Less electronegative atoms make for better sigma bond
donors. This is because more electronegative atoms have
lower lying HOMOs, which have poor overlap with the LUMO
of the electron acceptor.
>
In other words, less electronegative atoms are more willing to share there
electrons with the carbocation. Therefore, the 2-methyl butane is more
stabilized through hyperconjugation than the 3-fluoro 2-methyl butane.

4. Trends in Hyperconjugation
3. The greater the number of substituents on a carbocation, the
more stabilized it will be through hyperconjugation.
2. Lower lying LUMOs make for better electron acceptors, as there is better
orbital overlap with the HOMO of the electron donor.
> > >Atomic orbital sp π* sp2 π*
sp3 π*
> >
Therefore, tertiary carbocations are more stable than secondary or primary.

5. Other types of Hyperconjugation
Occasionally, electron density will be donated from a filled 𝜋 or p orbital into
an adjacent σ* orbital. This is known as negative hyperconjugation.
Other times, electron density will be donated from a p orbital into an adjacent
𝜋* orbital; the result is an overall neutral charge on the molecule. This is
known as neutral hyperconjugation.

6. Consequences of Hyperconjugation
2. Chem. Phys. Lett., 1981, 84, 267.
1. Conformational Preferences of Esters
(E) Conformer
(Z) Conformer
ΔG=+4.8 kcal/mol
Esters exist in two
conformers, E and Z.
(E) Conformer (Z) Conformer
Because the Z conformation
allows for the overlap of the
lone pair with the C—O σ* it
is favored over the E
conformation, where the
lone pair is aligned with the
C—R σ*.2
No resonance, poor orbital overlap!

7. Consequences of Hyperconjugation
J. Chem. Phys., 1951, 19, 342.
2. The Secondary Kinetic Isotope Effect
The kinetic isotope effect (KIE) is the change in rate of a reaction when an atom is replaced with a heavier
isotope. A secondary KIE is when the isotope substitution occurs at an atom where a bond is not being broken
or formed.
Because heavier atoms form stronger bonds, they are less willing to contribute to hyperconjugation. Therefore,
reactions that require the elimination of a leaving group proceed more slowly.
For example:
*Note: TsO, or tosylate, is a leaving group.
1.
2.
Due to the weaker C—H bond, Reaction 1 proceeds faster than
Reaction 2. This is because the C—H σ bond is more willing to push
electrons into the empty orbital of the carbocation and contribute to
hyperconjugation.
KH/KD is the ratio of the rates of the individual reactions. A KH/KD >1
suggests that the reaction occurs more slowly with the isotopic
substitution.
KH/KD = 1.15, as found by Streitwieser, et al.3

8. Problems
1. Rank the ability of the following bonds to contribute to hyperconjugation: C—F, C—O, C—N, C—H.
A. C—F > C—O > C—N > C—H
B. C—H > C—N > C—O > C—F
C. C—H > C—O > C—N > C—F
D. C—F > C—H > C—N > C—O
2. The following molecule will not experience hyperconjugation. Why is that?
A. The hybridization of the molecule does not permit proper orbital overlap.
B. The deuterium does not contribute hyperconjugation very well.
C. There is poor overlap between the LUMO of the N—Me bond and HOMO of the N—D bond.
D. Hyperconjugation does not occur on heteroatoms such as nitrogen.
3. Which of the following most readily ionizes?
A. B. C. D.
4. Identify whether the following molecules have positive, negative, or neutral hyperconjugation.
i.
A. Positive
B. Negative
C. Neutral
ii.
A. Positive
B. Negative
C. Neutral
A. Positive
B. Negative
C. Neutral
iii.

9. Solutions
1. B
2. C
3. A
4. i. B ii. C iii. A

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Contributed by:
Samuel Redstone (Undergraduate)
University of Utah, 2016