In organic chemistry, hyperconjugation (σ-conjugation or no-bond resonance) refers to the delocalization of electrons with the participation of bonds of primarily σ-character. Usually, hyperconjugation involves the interaction of the electrons in a sigma (σ) orbital (e.g. C–H or C–C) with an adjacent unpopulated non-bonding p or antibonding σ* or π* orbitals to give a pair of extended molecular orbitals. However, sometimes, low-lying antibonding σ* orbitals may also interact with filled orbitals of lone pair character (n) in what is termed negative hyperconjugation. Increased electron delocalization associated with hyperconjugation increases the stability of the system. In particular, the new orbital with bonding character is stabilized, resulting in an overall stabilization of the molecule. Only electrons in bonds that are in the 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) 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.
Applications Hyperconjugation can be used to rationalize a variety of 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, and the thermodynamic 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.
Effect on chemical properties Hyperconjugation affects several properties.
Bond length: Hyperconjugation is suggested as a key factor in shortening of sigma bonds (σ bonds). For example, the single C–C bonds in 1,3-butadiene and propyne are approximately 1.46 Å 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 propyne, it is generally accepted that this is due to 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 hyperconjugated structures. 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.
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 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 cyclic compounds. The importance of hyperconjugation in accounting for this effect has received support from quantum chemical calculations. The key interaction is believed to be the donation of electron density from the neighboring C–H σ bond into the π* antibonding orbital of the alkene (σC–H→π*). The effect is almost an order of magnitude weaker than the case of alkyl substitution on carbocations (σC–H→pC), since an unfilled p orbital is lower in energy, and, therefore, better energetically matched to a σ bond. When this effect manifests in the formation of the more substituted product in thermodynamically controlled E1 reactions, it is known as Zaitsev's rule, although in many cases the kinetic product also follows this rule. (See Hofmann's rule for cases where the kinetic product is the less substituted one.) One set of experiments by Kistiakowsky 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.
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