Systems—Hyperconjugation

One of the key assumptions of the Hiickel approximation is the noninteraction of the TT-orbital system with the a--molecular framework. This is a good approxima- 

The molecular orbital picture of such interactions flows from the idea that individual orbitals encompass the entire molecule. Thus, while the MO description of ethylene involved no interaction between the C orbitals and the H Is orbitals (see p. 35 to recall this discussion), this strict separation would not exist in propene since the hydrogens of the methyl group are not in the nodal plane of the tt bond. The origin of interactions of these hydrogens with the tt orbital can be indicated as in Fig. 1.32, which shows propene in a geometry in which two of the hydrogen l5 atomic orbitals are in a position to interact with the 2p orbital of carbon-2. 

An ab initio calculation using a STO-3G basis set was carried out on propene in two distinct geometries, eclipsed and staggered. 

The interaction of the lone pair electrons on an amine nitrogen with adjacent C H bonds is another example of a hyperconjugative effect which can be described in MO language. The lone pair electrons, when properly aligned with the C—H bond, lead to a donation of electron density from the lone pair orbital to the 

In acyclic structures, such effects are averaged by rotation but in cyclic structures differences in C—H bond strengths, based on the different alignments, can be recognized. The C—H bonds that are in an anti orientation to the lone pair are weaker than the C—H bonds in other orientations. 

The calculations of the optimum geometry show a slight lenghthening of the C—H bonds because of the electron release to the tc system. These calculations also reveal a barrier to rotation of the methyl group of about 1.5-2.0 kcal/mol. Interaction between the hydrogens and the n system favors the eclipsed conformation to fliis extent. Let us examine the 

One of the frmdamental structural facets of organic chemistry, which has been explained most satisfactorily in MO terms, is the existence of a small barrier to rotation about single bonds. In ethane, for example, it is known that the staggered conformation is about 3kcal/mol more stable than the ecl sed conformation so that the eclipsed conformation represents a transition state for transformation of one staggered conformation into another by rotation. 

Valence bond theory offers no immediare qualitative explanation since the a bond that is involved is cylindrically symmetrical. A steric argument based on repulsions between hydrogens also fails because on detailed examination of this hypothesis, it is found that the 

One of the key assumptions of the Huckel approximation is the noninteraction of the TT-orbital system with the cr-molecular framework. This, as was mentioned, is a good approximation for completely planar molecules where the a framework is in the nodal plane of the tt system. For other molecules, as for example when an sp carbon is added as a substituent group, this approximation is no longer entirely valid. Qualitative application of molecular orbital theory can be enlightening in describing interactions between the tt system and substituent groups. In valence 

Ingold, Structure and Mechanism in Organic Chemistry, Second Edition, Cornell University Press, Ithaca, New York, 1969. 

Ferguson, Organic Molecular Structure, Willard Grant Press, Boston, 1975. 

Streitwieser, Jr., Molecular Orbital Theory for Organic Chemists, John Wiley and Sons, New York, 1961. 

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In toluene (LXX), like in a number of analogs, the preferred conformation shows eclipsing between a hydrogen atom and the w-system, as represented in diagram LXX. This conformation minimizes a- and w-electron loss to the 7r-system (hyperconjugation) and therefore allows maximal one-electron interaction [80]. Toluene is characterized by an extremely low rotation barrier (a few cal/mol) due to the fact that this barrier is sixfold, and the latter is always found to be extremely small. Indeed, an equivalent conformation is found after every 60° rotation, and no real relief of conformational strain is obtained after a rotation of only 30°. 

C—C) bonds with the 7t-system (hyperconjugation, see p. 80) to supply electrons to the conjugated system.39 The effect is like that of the lone pairs it is usually much smaller, but quite noticeable. We shall be using this classification again in Chapter 4, on pericyclic reactions, the subject into which it was first introduced by Houk.40... 

The fact that in the substituent parameter approaches, especially for coupling constants ( y( C H2), 2j( C2 H2), V(H2H3)), the parent compound allene (11) exhibits a larger deviation from the regression lines may be due to the particular electronic situation of the hydrogen atoms in allenes. In mon-substituted allenes the If AOs of H2 are always part of the a electronic system wheras in 11 this proton may also be involved in the tt system ( hyperconjugation ). The substituent constants in the semiquantitative approaches, however, are derived from compounds with a strict v-a separation, where the Is AOs of hydrogen only take part in the a system. 

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]. 

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]

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. 

How do you interpret the values of p and r in these equations Which system is more sensitive to the aryl substituent How would you explain this difference in sensitivity Sketch the resonance, field and hyperconjugative interactions which you believe would contribute to these substituent effects. What, if any, geometric constraints would these interactions place on the ions ... 

Cations are by no means the only species where the effects of hyperconjugative delocalization reveal themselves in such a striking manner. Similar effects exist in neutral systems or in anions. For instance, the normal propyl anion should tend to be eclipsed (E) since in this manner the molecule would optimize the 4-electron interactions between the ethyl group t orbital and the p orbital which carries the electron pair. In the bisected conformation, where ttchs and ttchs have both been raised in energy, the four-electron, destabilizing (see Section 1.7, rule 2) p ->7r interaction is stronger than in the eclipsed conformation. At the same time the two-electron, stabilizing p ->ir interaction is weaker than in the eclipsed conformation. Both effects favor the eclipsed conformation. 

A mode] study has demonstrated the pathways shown in Scheme 4,17. The first cyclization step gave predominantly five-membered rings, the second a mixture of six- and seven-membered rings.155 Relative rate constants for the individual steps were measured. The first cyclization step was found to be some five-fold faster than for the parent 5-hexenyl system. Although originally put forward as evidence for hyperconjugation in 1,6-dienes, further work showed the rate acceleration to be sterie in origin.113-I3j... 

The calculated isotope effects for the solvolyses of t-butyl chloride-dg (previously estimated by Bartell, 1961a) and acetyl chloride-ds were much weaker than the experimentally observed ones. The authors conclude (Karabatsos et al., 1967) that in ordinary systems where hyperconjugation (from the j8-position) is possible, the effect of non-bonded interactions accounts for only a small part (less than 10%) of the observed isotope effect. 

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 fert-butyl > isopropyl > ethyl > methyl, and this order is observed in many phenomena. Thus, the dipole moments in the gas phase of PhCHa, PhC2Hs, PhCH(CHa)2, and PhC(CHa)a are, respectively, 0.37, 0.58, 0.65 and 0.700. ... 

Since the central carbon of tricoordinated carbocations has only three bonds and no other valence electrons, the bonds are sp and should be planar. Raman, IR, and NMR spectroscopic data on simple alkyl cations show this to be so. In methylcycohexyl cations there are two chair conformations where the carbon bearing the positive charge is planar (9 and 10), and there is evidence that difference is hyperconjugation make 10 more stable. Other evidence is that carbocations are difficult to form at bridgehead atoms in [2.2.1] systems, where they cannot be planar (see p. 397). ° Bridgehead carbocations are known, however, as in [2.1.1]... 

The free t-butyl cation [7" ] in the gas phase is nothing more than a species detectable by the electron impact method (Yeo and Williams, 1970). However, it is not only an observable species by nmr studies in SbFs/FSOsH (Olah et al., 1964), but can be isolated from the solution in the form of its SbF or Sb2Ffi salt (Olah and Lukas, 1967a,b Olah et al., 1973 Yannoni et al., 1989). The crystal structure shows that this ion is planar and its carbon-carbon bonds are shortened to 144.2 pm (Hollenstein and Laube, 1993). Its particular electronic stabilization among aliphatic carbocations is attributed by physical organic chemists to the operation of both inductive and hyperconjugative effects in the cr bond system. 

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