In organic chemistry, hyperconjugation is the interaction of the
electrons in a sigma orbitals (e.g. C–H or C–C) with an adjacent
empty (or partially filled) non-bonding orbital, antibonding σ or π
orbital, to give an extended molecular orbital. Increased electron
delocalization associated with hyperconjugation increases the
stability of the system. Only electrons in bonds that are β
to the positively charged carbon can stabilize a carbocation by direct
hyperconjugation. However, extended versions of hyperconjugation (such
as double hyperconjugation) can be important as well. The
Baker–Nathan effect, sometimes used synonymously for
hyperconjugation, is a specific application of it to certain
chemical reactions or types of structures.
Hyperconjugation: a stabilizing overlap between an pi orbital and a
sigma orbital. Ref. McMurry
1.1 Effect on chemical properties
Hyperconjugation in unsaturated compounds
1.3 Stabilization of
1,3-butadiyne and 1,3-butadiene
1.4 Trends in hyperconjugation
Rotational barrier of ethane
2 See also
4 External links
Hyperconjugation can be rationalizing a variety of other chemical
phenomena, including the anomeric effect, the gauche effect, the
rotational barrier of ethane, the beta-silicon effect, the vibrational
frequency of exocyclic carbonyl groups, and the relative stability of
substituted carbocations and substituted carbon centred radicals.
Hyperconjugation is proposed by quantum mechanical modeling to be the
correct explanation for the preference of the staggered conformation
rather than the old textbook notion of steric hindrance.
Effect on chemical properties
Hyperconjugation affects several properties.
Hyperconjugation is suggested as a key factor in
shortening of sigma bonds (σ bonds). For example, the single C–C
1,3-butadiene and methylacetylene are approximately 1.46
angstrom in length, much less than the value of around 1.54 Å
found in saturated hydrocarbons. For butadiene, this can be explained
as normal conjugation of the two alkenyl parts. But for
methylacetylene, hyperconjugation between the alkyl and alkynyl parts.
Dipole moments: The large increase in dipole moment of
1,1,1-trichloroethane as compared with chloroform can be attributed to
The heat of formation of molecules with hyperconjugation are greater
than sum of their bond energies and the heats of hydrogenation per
double bond are less than the heat of hydrogenation of ethylene.
Stability of carbocations:
(CH3)3C+ > (CH3)2CH+ > (CH3)CH2+ > CH3+
The three C–H σ bonds of the methyl group(s) attached to the
carbocation can undergo the stabilization interaction but only one of
them can be aligned perfectly with the empty p-orbital, depending on
the conformation of the carbon–carbon bond. Donation from the two
misaligned C–H bonds is weaker. The more adjacent methyl groups
there are, the larger hyperconjugation stabilization is because of the
increased number of adjacent C–H bonds.
Relative hyperconjugation strength: Hydrogen has greater strength than
its isotope Deuterium and Tritium has least ability to show
hyperconjugation among the three. This is because energy required to
break C-T bond > C-D bond > C-H bond, which makes it easier for
H to hyperconjugate.
Hyperconjugation in unsaturated compounds
Early studies in hyperconjugation were performed by in the research
group of George Kistiakowsky. Their work, first published in 1937, was
intended as a preliminary progress report of thermochemical studies of
energy changes during addition reactions of various unsaturated and
One set of experiments involved collected heats of hydrogenation data
during gas-phase reactions of a range of compounds that contained one
alkene unit. When comparing a range of monoalkyl-substituted alkenes,
they found any alkyl group noticeably increased the stability, but
that the choice of different specific alkyl groups had little to no
A portion of Kistiakowsky’s work involved a comparison of other
unsaturated compounds in the form of CH2=CH(CH2)n-CH=CH2 (n=0,1,2).
These experiments revealed an important result; when n=0, there is an
effect of conjugation to the molecule where the ΔH value is lowered
by 3.5 kcal. This is likened to the addition of two alkyl groups into
ethylene. Kistiakowsky also investigated open chain systems, where the
largest value of heat liberated was found to be during the addition to
a molecule in the 1,4-position. Cyclic molecules proved to be the most
problematic, as it was found that the strain of the molecule would
have to be considered. The strain of five-membered rings increased
with a decrease degree of unsaturation. This was a surprising result
that was further investigated in later work with cyclic acid
anhydrides and lactones. Cyclic molecules like benzene and its
derivatives were also studied, as their behaviors were different from
other unsaturated compounds.
Despite the thoroughness of Kistiakowsky’s work, it was not complete
and needed further evidence to back up his findings. His work was a
crucial first step to the beginnings of the ideas of hyperconjugation
and conjugation effects.
1,3-butadiyne and 1,3-butadiene
The conjugation of
1,3-butadiene was first evaluated by Kistiakowsky,
a conjugative contribution of 3.5 kcal/mol was found based on the
energetic comparison of hydrogenation between conjugated species and
unconjugated analogues. Rogers who used the method first applied
by Kistiakowsky, reported that the conjugation stabilization of
1,3-butadiyne was zero, as the difference of ΔhydH between first and
second hydrogenation was zero. The heats of hydrogenation (ΔhydH)
were obtained by computational G3(MP2) quantum chemistry method.
Another group led by Houk suggested the methods employed by Rogers
and Kistiakowsky was inappropriate, because that comparisons of heats
of hydrogenation evaluate not only conjugation effects but also other
structural and electronic differences. They obtained -70.6 kcal/mol
and -70.4 kcal/mol for the first and second hydrogenation respectively
by ab initio calculation, which confirmed Rogers’ data. However,
they interpreted the data differently by taking into account the
hyperconjugation stabilization. To quantify hyperconjugation effect,
they designed the following isodesmic reactions in
Deleting the hyperconjugative interactions gives virtual states that
have energies that are 4.9 and 2.4 kcal/mol higher than those of
1-butyne and 1-butene, respectively. Employment of these virtual
states results in a 9.6 kcal/mol conjugative stabilization for
1,3-butadiyne and 8.5 kcal/mol for 1,3-butadiene.
Trends in hyperconjugation
A relatively recent work (2006) by Fernández and Frenking (2006)
summarized the trends in hyperconjugation among various groups of
acyclic molecules, using energy decomposition analysis or EDA.
Fernández and Frenking define this type of analysis as "...a method
that uses only the pi orbitals of the interacting fragments in the
geometry of the molecule for estimating pi interactions." For this
type of analysis, the formation of bonds between various molecular
moieties is a combination of three component terms. ΔEelstat
represents what Fernández and Frenking call a molecule’s
“quasiclassical electrostatic attractions.” The second term,
ΔEPauli, represents the molecule’s Pauli repulsion. ΔEorb, the
third term, represents stabilizing interactions between orbitals, and
is defined as the sum of ΔEpi and ΔEsigma. The total energy of
interaction, ΔEint, is the result of the sum of the 3 terms.
A group whose ΔEpi values were very thoroughly analyzed were a group
of enones that varied in substituent.
Fernández and Frenking reported that the methyl, hydroxyl, and amino
substituents resulted in a decrease in ΔEpi from the parent
2-propenal. Conversely, halide substituents of increasing atomic mass
resulted in increasing ΔEpi. Because both the enone study and Hammett
analysis study substituent effects (although in different species),
Fernández and Frenking felt that comparing the two to investigate
possible trends might yield significant insight into their own
results. They observed a linear relationship between the ΔEpi values
for the substituted enones and the corresponding Hammett constants.
The slope of the graph was found to be -51.67, with a correlation
coefficient of -0.97 and a standard deviation of 0.54. Fernández
and Frenking conclude from this data that ..."the electronic effects
of the substituents R on pi conjugation in homo- and heteroconjugated
systems is similar and thus appears to be rather independent of the
nature of the conjugating system.".
Rotational barrier of ethane
An instance where hyperconjugation may be overlooked as a possible
chemical explanation is in rationalizing the rotational barrier of
ethane. It had been accepted as early as the 1930s that the staggered
conformations of ethane were more stable than the eclipsed
conformation. Wilson had proven that the energy barrier between any
pair of eclipsed and staggered conformations is approximately 3
kcal/mol, and the generally accepted rationale for this was the
unfavorable steric interactions between hydrogen atoms.
Newman's Projections:Staggered (left) and Eclipsed (right)
In their 2001 paper, however, Pophristic and Goodman revealed that
this explanation may be too simplistic. Goodman focused on three
principal physical factors: hyperconjugative interactions, exchange
repulsion defined by the Pauli exclusion principle, and electrostatic
interactions (Coulomb interactions). By comparing a traditional ethane
molecule and a hypothetical ethane molecule with all exchange
repulsions removed, potential curves were prepared by plotting
torsional angle versus energy for each molecule. The analysis of the
curves determined that the staggered conformation had no connection to
the amount of electrostatic repulsions within the molecule. These
results demonstrate that Coulombic forces do not explain the favored
staggered conformations, despite the fact that central bond stretching
decreases electrostatic interactions.
Goodman also conducted studies to determine the contribution of
vicinal (between two methyl groups) vs. geminal (between the atoms in
a single methyl group) interactions to hyperconjugation. In separate
experiments, the geminal and vicinal interactions were removed, and
the most stable conformer for each interaction was deduced.
Calculated torsional angle of ethane with deleted hyperconjugative
From these experiments, it can be concluded that hyperconjugative
effects delocalize charge and stabilize the molecule. Further, it is
the vicinal hyperconjugative effects that keep the molecule in the
staggered conformation. Thanks to this work, the following model of
the stabilization of the staggered conformation of ethane is now more
Hyperconjugation can also explain several other phenomena whose
explanations may also not be as intuitive as that for the rotational
barrier of ethane. One such example is the explanations for
certain Lewis structures. The
Lewis structure for an ammonium ion
indicates a positive charge on the nitrogen atom. In reality, however,
the hydrogens are more electropositive than is nitrogen, and thus are
the actual carriers of the positive charge. We know this intuitively
because bases remove the protons as opposed to the nitrogen atom.
It should be noted that the matter of the rotational barrier of ethane
is not settled within the scientific community. An analysis within
quantitative molecular orbital theory shows that 2-orbital-4-electron
(steric) repulsions are dominant over hyperconjugation. A valence
bond theory study also emphasizes the importance of steric
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van der Waals