In organic chemistry , HYPERCONJUGATION is the interaction of the electrons in a sigma bond (usually C–H or C–C) with an adjacent empty (or partially filled) non-bonding p-orbital , antibonding σ or π orbital , or filled π orbital, to give an extended molecular orbital that 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 Applications
* 1.1 Effect on chemical properties
* 2 See also * 3 References * 4 External links
EFFECT ON CHEMICAL PROPERTIES
HYPERCONJUGATION IN UNSATURATED COMPOUNDS
Early studies in hyperconjugation were performed by in the research
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 effect.
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.
STABILIZATION OF 1,3-BUTADIYNE AND 1,3-BUTADIENE
The conjugation of
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 1-butyne and 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-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.
Schleyer's model has several marked differences from Gronert's. He uses a new isodesmic additivity design that in his view faithfully reproduces heats of formation for many alkanes, alkenes, alkynes, and alkyl radicals. All 1,3 interactions are stabilizing so they support branching and hyperconjugation. All adjustable parameters originate from assumption that the magnitude of stabilizations effects at a specific carbon are eased when more than one substituent contributes: ∆Hf = base – 2.15n(CH2) – 1,3CCC branching attraction – hyperconjugation
Schleyer notes several advantages of his approach in comparison to Gronert's:
* Gronert's derivation method arbitrarily set some parameters and adjusted the others as best-fit averages of experimental hydrocarbon heats of formation. * Gronert's derived C–C and C–H bond energy values are higher than those accepted in the literature. * Gronert uses 7 adjustable parameters, whereas Schleyer uses only 4. Four is the minimum chemically plausible number of parameters, and the added flexibility of additional terms is not necessarily an improvement of general theory. * Schleyer's single attractive geminal term is sufficient to reproduce data satisfactorily. * Well-established theories of branching, hyperconjugation and attenuation. * Schleyer's method depends only on energetic relationships between the simplest hydrocarbon molecules.
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 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. Thanks to this work, the following model of the stabilization of the staggered conformation of ethane is now more accepted:
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 effects.
* ^ John McMurry. Organic chemistry, 2nd edition. ISBN 0-534-07968-7 * ^ IUPAC , Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "hyperconjugation". * ^ Alabugin, I.V.; Gilmore, K.; Peterson, P. (2011). "Hyperconjugation". WIREs Comput Mol Sci. 1: 109–141. doi :10.1002/wcms.6 . * ^ Alabugin, I. V. (2016) Remote Stereoelectronic Effects, in Stereoelectronic Effects: A Bridge Between Structure and Reactivity, John Wiley & Sons, Ltd, Chichester, UK. doi :10.1002/9781118906378.ch8