Hyperconjugation

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In

β position can have this sort of direct stabilizing effect — donating from a sigma bond on an atom to an orbital in another atom directly attached to it. However, extended versions of hyperconjugation (such as double hyperconjugation[5]) can be important as well. The Baker–Nathan effect, sometimes used synonymously for hyperconjugation,[6] is a specific application of it to certain chemical reactions or types of structures.[7]

Hyperconjugation: orbital overlap between a σ orbital and π* orbital stabilizes alkyl-substituted alkenes. The σ orbital (solid color) is filled, while the π* orbital (grayed) is an unpopulated antibonding orbital. Ref. Clayden, Greeves, Warren

Applications

Hyperconjugation can be used to rationalize a variety of chemical phenomena, including the

Zaitsev's rule for alkene stability. More controversially, hyperconjugation is proposed by quantum mechanical modeling to be a better explanation for the preference of the staggered conformation rather than the old textbook notion of steric hindrance.[8][9]

Effect on chemical properties

Hyperconjugation affects several properties.[6][10]

  1. hydrocarbons
    . For butadiene, this can be explained as normal conjugation of the two alkenyl parts. But for propyne, it is generally accepted that this is due to hyperconjugation between the alkyl and alkynyl parts.
  2. Dipole moments: The large increase in dipole moment of 1,1,1-trichloroethane as compared with chloroform
    can be attributed to hyperconjugated structures.
  3. 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
    .
  4. 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.[11] The more adjacent methyl groups there are, the larger hyperconjugation stabilization is because of the increased number of adjacent C–H bonds.

Hyperconjugation in unsaturated compounds

Hyperconjugation was suggested as the reason for the increased stability of carbon-carbon double bonds as the degree of substitution increases. Early studies in hyperconjugation were performed by in the research group of

Hofmann's rule
for cases where the kinetic product is the less substituted one.)

One set of experiments by Kistiakowsky involved collected heats of

alkyl-substituted alkenes, they found any alkyl group noticeably increased the stability, but that the choice of different specific alkyl groups had little to no effect.[13]

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.[13]

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.

Stabilization of 1,3-butadiyne and 1,3-butadiene

The

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.[14]

Another group led by Houk

1-butene
.

Deleting the hyperconjugative interactions gives virtual states that have energies that are 4.9 and 2.4 kcal/mol higher than those of

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.[16]" 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.[16]” 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.[16]

A group whose ΔEpi values were very thoroughly analyzed were a group of

enones that varied in substituent.

Fernández and Frenking reported that the

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.[16] 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.".[16][17]

Rotational barrier of ethane

An instance where hyperconjugation may be overlooked as a possible chemical explanation is in rationalizing the

steric interactions
between hydrogen atoms.

Newman's Projections:Staggered (left) and eclipsed (right)

In their 2001 paper, however, Pophristic and Goodman

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.[8]

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.[8]

Calculated torsional angle of ethane with deleted hyperconjugative effects
Deleted interaction Torsional angle Corresponding conformer
None 60° Staggered
All hyperconjugation 0° Eclipsed
Vicinal hyperconjugation 0° Eclipsed
Geminal hyperconjugation 60° Staggered

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.[8] Thanks to this work, the following model of the stabilization of the staggered conformation of ethane is now more accepted:

Based on a figure in Schreiner (2002)
Based on a figure in Schreiner (2002)

Hyperconjugation can also explain several other phenomena whose explanations may also not be as intuitive as that for the rotational barrier of ethane.[18]

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.[19] A valence bond theory study also emphasizes the importance of steric effects.[20]

See also

References

  1. doi:10.1351/goldbook.H02924
  2. ISBN 0-534-07968-7
  3. S2CID 222197582
    .
  4. ^ The mixed orbital of antibonding character is, in fact, raised in energy compared to the original antibonding orbital. However, since the antibonding orbital remains unpopulated in most cases, this does not usually affect the energy of the system.
  5. doi:10.1002/9781118906378.ch8
  6. ^
    doi:10.1021/cr60114a001
    .
  7. ISBN 9789332901070
    .
  8. ^
    S2CID 205017635
    .
  9. S2CID 9812878
    .
  10. doi:10.1016/0040-4020(59)80102-2
    .
  11. doi:10.1002/poc.3382
    .
  12. PMID 19562814
    .
  13. ^
    doi:10.1021/cr60066a002
    .
  14. PMID 12841733
    .
  15. PMID 15547994
    .
  16. ^
    PMID 16502455
    .
  17. ^ Refer to Reference 12 for the graph and its full analysis
  18. ^
    PMID 12370897
    .
  19. PMID 14502731
    .
  20. PMID 15065281
    .

External links
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