Alkyl halides LUMO energies

From Table 5.3, it can be seen that for the problem of C- versus O-alkylation by alkyl halides, AMI, 3-21G and even PM3 calculations give better results than STO-3G calculations, which put the LUMO of bromides at higher energies than the LUMO of chlorides ab initio calculations are not necessarily better than semi-empirical calculations, from the point of view of FO theory. 

When steric hindrance in substrates is increased, and when the leaving anion group in substrates is iodide, SET reaction is much induced (Cl < Br < I). This reason comes from the fact that steric hindrance retards the direct nucleophilic reduction of substrates by a hydride species, and the a energy level of C-I bond in substrates is lower than that of C-Br or C-Cl bond. Therefore, metal hydride reduction of alkyl chlorides, bromides, and tosylates generally proceeds mainly via a polar pathway, i.e. SN2. Since LUMO energy level in aromatic halides is lower than that of aliphatic halides, SET reaction in aromatic halides is induced not only in aromatic iodides but also in aromatic bromides. Eq. 9.2 shows reductive cyclization of o-bromophenyl allyl ether (4) via an sp2 carbon-centered radical with LiAlH4. 

The hydrocarbon framework of organic molecules is unreactive. Functional groups such as NH2 and OH are nucleophilic because they have nonbonding lone pairs. Carbonyl compounds and alkyl halides are electrophilic functional groups because they have low-energy LUMOs (it for 0=0 and G for C-X, respectively). 

However, the energy of the HOMO in a range of a-nucleophiles does not actually correlate well enough with their nucleophilicity for this explanation of the effect to be satisfactorily settled. The problem raised by the small a-effect in reactions with alkyl halides, where the LUMO energy is not conspicuously low, and yet they are soft electrophiles, raises the question of what happens when both charge and frontier orbital terms are small, and what happens when they are both large Prediction is not simple in this situation, but Hudson has suggested the two rules in the box, which are usually but not invariably observed. 

By similar reasoning, we can further conclude that oxidation reactions will be most favorable for the lesser-halogenated isomers. This concept is illustrated in Figure 5(b) where relative rates of oxidation and reduction are plotted as a function of level of chlorination. In practice, the literature has revealed that there may be a (chemical-dependent) zone where oxidation and reduction reactions are equally likely. This zone captures aryl halides with four to six chlorines, and alkyl halides with one to three chlorines per molecule. Figure 5(c) shows a trend between the Gibbs free energy for reductive dechlorination and the HOMO-LUMO gap (HOMO = highest occupied... 

HOMO and LUMO of the alkyl halide will be low-lying, and the HOMO and LUMO of the alkene will be raised in energy by the X-substituent. As a result the a orbital is low in energy for a a bond and n is high in energy for a n bond, and the major interaction lowering the energy of the transition structure is between these two orbitals (Fig. 8.12). 

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