Localized electron model alkanes

The simplest member of the saturated hydrocarbons, which are also called the alkanes, is methane (CH4). As discussed in Section 14.1, methane has a tetrahedral structure and can be described in terms of a carbon atom using an sp-J hybrid set of orbitals to bond to the four hydrogen atoms (see Fig. 22.1). The next alkane, the one containing two carbon atoms, is ethane (C2H6), as shown in Fig. 22.2. Each carbon in ethane is surrounded by four atoms and thus adopts a tetrahedral arrangement and sp3 hybridization, as predicted by the localized electron model. 

In the previous four sections, several solvent radical ions that cannot be classified as molecular ions ( a charge on a solvent molecule ) were examined. These delocalized, multimer radical ions are intermediate between the molecular ions and cavity electrons, thereby bridging the two extremes of electron (or hole) localization in a molecular liquid. While solvated electrons appear only in negative-EAg liquids, delocalized solvent anions appear both in positive and negative-EAg liquids. Actually, from the structural standpoint, trapped electrons in low-temperature alkane and ether glasses [2] are closer to the multimer anions because their stabilization requires a degree of polarization in the molecules that is incompatible with the premises of one-electron models. 

The propargyl radical is 10.6 kcal mol more stable than the methyl radical in the G3(MP2)B3 model chemistry, presumably because of increased electron delocalization over the whole -CH2C = CH radical as contrasted to restricted localization of the free electron in CH3 . Analogous calculations in the G3(MP2), CBS-QB3 etc. model chemistries give answers that are quite close to the one just shown for the G3(MP2)B3 model chemistry, as they must if we are to believe that calculation of A Hm of alkanes and alkyl radicals gives a good approximation to the true value. 

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