Hyperconjugation

6 Inductive effects, hyperconjugation and mesomeric effects 1.6.1 Inductive effects 

In a covalent bond between two different atoms, the electrons in the o-bond are not shared equally. The electrons are attracted towards the most electronegative atom. An arrow drawn above the line representing the covalent bond can show this. (Sometimes an arrow is drawn on the line.) Electrons are pulled in the direction of the arrow. 

When the atom (X) is more When the atom (Z) is less 

Pauling electronegativity scale The inductive effect of the atom rapidly 

The overall polarity of a molecule is determined by the individual bond polarities, formal charges and lone pair contributions, and this can be measured by the dipole moment (pi). The higher the dipole moment (measured in debyes (D)), the more polar the compound. 

All of the delocalization discussed so far involves u electrons. Another type, called hyperconjugation, involves t electrons.253 When a carbon attached to at least one hydrogen is attached to an unsaturated atom or one with an unshared orbital, canonical forms such as 95 can be drawn. In such canonical forms there is no bond at all between the carbon and 

Hyperconjugation in the above case may be regarded as an overlap of the a orbital of the C—H bond and the tt orbital of the C—C bond, analogous to the rr-n-orbital overlap previously considered. As might be expected, those who reject the idea of resonance in butadiene (p. 31) believe it even less likely when it involves no-bond structures. 

The concept of hyperconjugation arose from the discovery of apparently anomalous electron-release patterns for alkyl groups. By the field effect alone, the order of electron release for simple alkyl groups connected to an unsaturated system is f-butyl isopropyl ethyl methyl, and this order is observed in many phenomena. Thus, the dipole moments in the gas phase of PhCH3, PhC2H5, PhCH(CH3)2, and PhC(CH3)3 are, respectively, 0.37, 0.58, 0.65 and 0.70 D.254 

However. Baker and Nathan observed that the rates of reaction with pyridine of /j-substituted benzyl bromides (see reaction 0 43) were about opposite that expected from 

This came to be called the Baker-Nathan effect and has since been found in many processes. Baker and Nathan explained it by considering that hyperconjugative forms contribute to the actual structure of toluene  

All the examples of resonance cited in the previous section dealt with conjugation through TT bonds. VB theory also incorporates the concept of hyperconjugation, which is the idea that there can be electronic interactions between ct and a bonds and between ct and tt bonds. In alkenes such as propene or 2-methylpropene, the electronreleasing effect of the methyl substituents can be represented by hyperconjugated 

Description of Molecular Structure Using Valence Bond Concepts 

Hyperconjugation also can describe the electron-releasing effect of alkyl groups on aromatic rings. 

While part of the electron-releasing effect of alkyl groups toward double bonds and aromatic rings can be attributed to the electronegativity difference between sp and sp carbon, the fact that the (3-carbon of alkenes and the ortho and para positions of aromatic rings are selectively affected indicates a resonance component. 

Heteroatoms with unshared electron pairs can also interact with adjacent ct bonds. For example, oxygen and nitrogen substituents substantially weaken an adjacent (geminal) C—H bond. 

A simple example of the effect of an attached saturated group X on a pi bond, 

We now discuss systematic hyperconjugative effects on orbital composition and stabilization, torsion barriers, and spectroscopic properties, for the CH2=CHX species summarized in Table 3.22. 

From Table 3.22 one can see that typical hyperconjugative interactions of vinyl pi bonds with hydride bonds are rather weak (2 1 kcal mol-1), but those with lone pairs are considerably stronger (7-30 kcal mol-1). (Of course, a more polar pi bond 

The relative strengths of hyperconjugative stabilizations could be rationalized with contour plots similar to Fig. 3.49. The most important features of such plots could be predicted from the transferable forms of the NBOs (cf. Figs. 3.16 and 3.25) and expected variations with electronegativity (Sections 3.2.5 and 3.2.8). As shown in Fig. 3.49, the hyperconjugating ax and ax NBOs are typically canted away from the pi NBOs, weakening their interactions compared with ordinary (7r-7r ) con-jugative stabilizations in Table 3.19. However, the dependences on bond/antibond 

X NRT bcca 7t Polarization (% on CO ftcc, rtcc+ NLMOs rx (iix) moiety TTcC- -CX CTX 7tCC  

Resonance theory suggests that the positive charge in 2-methyl-2-propyl cation is dispersed onto the hydrogens. 

Hyperconjugation, as it i termed, implies that the electron pair associated with out-of-plane CH bond is donated into the empty p Orbital at the carbocation center. 

Step through the sequence of structures depicting rotation about the C i - C+ bond in 2-methyl-2-butyl cation. Plot energy (vertical axis) vs. CCCC dihedral angle (horizontal axis). What is the preferred conformation, with the ethyl group in plane or perpendicular to the plane  

Does the C3C4 distance change between these two conformers Explain. Are your data consistent with CC hyperconjugation  

What does your result tell you about the relative importance of CH and CC hyperconjugation Explain. 

The tc-MO patterns for the common cyclic conjugated systems can all be determined in this way. The resulting MOs are given in Chap. 6 and the patterns of the energy levels in the Fig. 3.17. 

In 1939, Mulliken proposed that there was also a similar kind of an effect between a saturated carbon atom and an adjacent unsaturated carbon atom, for example, in propene (Structures 3 and 4).  

In this case, the p component of one or two of the CH a orbitals of the methyl group (depending on the rotational orientation of the methyl group) could overlap strongly with the p orbital of the n system on the adjacent carbon, and this would lead to an effect similar to that of conjugation. 

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MM2 was, according the web site of the authors, released as MM2 87). The various MM2 flavors are superseded by MM3, with significant improvements in the functional form [10]. It was also extended to handle amides, polypeptides, and proteins [11]. The last release of this series was MM3(%). Further improvements followed by starting the MM4 series, which focuses on hydrocarbons [12], on the description of hyperconjugative effects on carbon-carbon bond lengths [13], and on conjugated hydrocarbons [14] with special emphasis on vibrational frequencies [15]. For applications of MM2 and MM3 in inorganic systems, readers are referred to the literature [16-19]. 

Valence bond representation of the hyperconjugation effect which leads to a lengthening of the C—H bond icetaldeyde. 

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

Excluding the phenomenon of hyperconjugation, the only other means by which electronic effects can be transmitted within saturated molecules, or exerted by inductive substituents in aromatic molecules, is by direct electrostatic interaction, the direct field effect. In early discussions of substitution this was usually neglected for qualitative purposes since it would operate in the same direction (though it would be expected to diminish in the order ortho > meta > para) as the cr-inductive effect and assessment of the relative importance of each is difficult however, the field effect was recognised as having quantitative significance. ... 

When the /)-positions are considered it is seen that they follow the sequence of inductive effects, and not of hyperconjugation. In this respect nitration is unusual amongst electrophilic substitutions. ... 

Data for the other compounds in table 9.2 are less complete. The trihalogenomethyl groups are usually regarded as exerting powerful — I effects, but the hyperconjugative properties of considered. ... 

The importance of a primary steric effect in the nitration of alkyl-benzenes has been mentioned ( 9.1.1). The idea was first introduced by Le Fevre to account for the fact that -alkyltoluenes (alkyl = Et, -Pr,68a t-Bu ) are nitrated mainly adjacent to the methyl group. Without the rate data reported for the alkylbenzenes the effect might equally well have been accounted for by hyperconjugation. 

Hammett s equation, and substituent effects, 137-43 heteromolecules, 130 Holleman s product rule, 3 hyperconjugation, in nitration of alkyl-benzenes, 165-7 in nitration of positive poles, 169... 

The ketone is added to a large excess of a strong base at low temperature, usually LDA in THF at -78 °C. The more acidic and less sterically hindered proton is removed in a kineti-cally controlled reaction. The equilibrium with a thermodynamically more stable enolate (generally the one which is more stabilized by substituents) is only reached very slowly (H.O. House, 1977), and the kinetic enolates may be trapped and isolated as silyl enol ethers (J.K. Rasmussen, 1977 H.O. House, 1969). If, on the other hand, a weak acid is added to the solution, e.g. an excess of the non-ionized ketone or a non-nucleophilic alcohol such as cert-butanol, then the tautomeric enolate is preferentially formed (stabilized mostly by hyperconjugation effects). The rate of approach to equilibrium is particularly slow with lithium as the counterion and much faster with potassium or sodium. 

The introduction of a methyl substituent into the empirical calculations may be performed according to two main different models the pseudoheteroatomic model and the hyperconjugated model (161-166). Both approximations have been used in rr-electron methods (HMO, w, PPP). On the other hand, in the all-valence-electrons... 

Only electrons in bonds that are f3 to the positively charged carbon can stabilize a car bocation by hyperconjugation Moreover it doesn t matter whether H or another carbon IS at the far end of the (3 bond stabilization by hyperconjugation will still operate The key point is that electrons m bonds that are (3 to the positively charged carbon are more stabilizing than electrons m an a C—H bond Thus successive replacement of first one... 

For the general case of R = any alkyl group how many bonded pairs of electrons are involved in stabilizing RjC by hyperconjugation How many in RzCH"" In RCNz"" ... 

Hydrophilic (Section 19 5) Literally water loving a term applied to substances that are soluble in water usually be cause of their ability to form hydrogen bonds with water Hydrophobic (Section 19 5) Literally water hating a term applied to substances that are not soluble in water but are soluble in nonpolar hydrocarbon like media Hydroxylation (Section 15 5) Reaction or sequence of reac tions in which an alkene is converted to a vicinal diol Hyperconjugation (Section 4 10) Delocalization of a electrons... 

The Hydrate and Enol Form. In aqueous solutions, acetaldehyde exists in equihbrium with the acetaldehyde hydrate [4433-56-17, (CH2CH(0H)2). The degree of hydration can be computed from an equation derived by BeU and Clunie (31). Hydration, the mean heat of which is —21.34 kJ/mol (—89.29 kcal/mol), has been attributed to hyperconjugation (32). The enol form, vinyl alcohol [557-75-5] (CH2=CHOH) exists in equihbrium with acetaldehyde to the extent of approximately 1 molecule per 30,000. Acetaldehyde enol has been acetylated with ketene [463-51-4] to form vinyl acetate [108-05-4] (33). 

Phenyl norhornane is benzoylated faster than isopropylbenzene or toluene despite the bulkiness of the norbomyl group probably because of hyperconjugation (87). Hyperconjugation of the C—C bond is at least as or more important as that of the C—H bond since 1-phenylnorhornane has no a-hydrogen atom. 

Metal Alibis and Alkoxides. Metal alkyls (eg, aluminum boron, sine alkyls) are fairly active catalysts. Hyperconjugation with the electron-deficient metal atom, however, tends to decrease the electron deficiency. The effect is even stronger in alkoxides which are, therefore, fairly weak Lewis acids. The present discussion does not encompass catalyst systems of the Ziegler-Natta type (such as AIR. -H TiCl, although certain similarities with Friedel-Crafts systems are apparent. 

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