Hyperconjugation effects, alkanes

Table 4.6 shows the effects of a substituent on linear and branched alkanes. The effect on the a-carbon parallels the electronegativity of the substituent except for bromine and iodine.t The effect at the /3-carbon seems fairly constant for all the substituents except for the carbonyl, cyano, and nitro groups. The shift to the right at the y carbon results (as above) from steric compression of a gauche interaction. For Y = N, O, and F, there is also a shift to the right with Y in the anti conformation, attributed to hyperconjugation. 

A physical interpretation has been ascribed to each of the three terms in the MM2 torsional expansion from an analysis of ab initio calculations on simple fluorinated hydrocarbons. The first, onefold term corresponds to interactions between bond dipoles, which are due to differences in electronegativity between bonded atoms. The twofold term is due to the effects of hyperconjugation (in alkanes) and conjugation effects (in alkenes), which provide double bond character to the bond. The threefold term corresponds to steric interactions between the 1,4 atoms. It was found that the additional terms in the torsional potential were especially important for systems containing heteroatoms, such as the halogenated hydrocarbons and molecules containing CCOC and CCNC fragments. 

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