Alkanes relative stabilities

Heats of combustion can be used to measure the relative stability of isomeric hydrocarbons They tell us not only which isomer is more stable than another but by how much Consider a group of C His alkanes... 

In this section you have seen how heats of com bustion can be used to determine relative stabilities of isomeric alkanes In later sections we shall expand our scope to include the experimentally determined heats of certain other reactions such as bond dissociation energies (Section 4 16) and heats of hydrogenation (Section 6 2) to see how AH° values from various sources can aid our understanding of structure and reactivity... 

The heat evolved on burning an alkane increases with the number of car bon atoms The relative stability of isomers may be determined by com paring their respective heats of combustion The more stable of two iso mers has the lower heat of combustion... 

We assess the relative stability of alkyl radicals by measuring the enthalpy change (AH°) for the homolytic cleavage of a C—H bond m an alkane... 

Scheme 25 Relative stabilities of branched 3 alkanes (kcal mol" ) X...

Gronert [42] and Schleyer [43] are not aware of our theory [41]. Branched alkanes are stabilized by the C-C bond polarization by two antiperiplanar C-H bonds. The polarization is favored by the orbital phase continuity. We can predict the relative stabilities of alkanes only by counting the number of the vicinal bond trios. Neither the Gronert nor the Schleyer model contains any vicinal interactions. 

R H) is much faster than alkylation, so that alkylation products are also derived from the new alkanes and carbocations formed in the exchange reaction. Furthermore, the carbo-cations present are subject to rearrangement (Chapter 18), giving rise to new carbocations. Products result from all the hydrocarbons and carbocations present in the system. As expected from their relative stabilities, secondary alkyl cations alkylate alkanes more Teadily than tertiary alkyl cations (the r-butyl cation does not alkylate methane or ethane). Stable primary alkyl cations are not available, but alkylation has been achieved with complexes formed between CH3F or C2H5F and SbFs-212 The mechanism of alkylation can be formulated (similar to that shown in hydrogen exchange with super acids, 2-1) as... 

The selective monofluorination of difluorinated alkanes with cobalt(III) fluoride at 25 C to give trifluoroalkanes 10 has been reported. Fluorinations with cobalt(lll) fluoride are considered to proceed via carbocations and the selectivity of the fluorination is explained by the relative stabilities of the intermediate carbocations. Although treatment of alkanes with chlorine trifluoride results in fluorination. it is of little practical use due to over-fluorinalion. chlorination and in some cases isomeri7ation of the starting alkane. ... 

Hydrogenation reactions are exothermic because the bonds in the product are stronger than the bonds in the starting materials, making them similar to other alkene addition reactions. The AH° for hydrogenation, called the heat of hydrogenation, can be used as a measure of the relative stability of two different alkenes that are hydrogenated to the same alkane. 

What we really want as standards for stability of carbonium ions are, of course, the kinds of compounds they are generated from alcohols at this particular point or, later, alkyl halides (Chap. 14). However, the relative stabilities of most ordinary neutral molecules closely parallel the relative stabilities of the alkanes, so that the relative order of stabilities that we have arrived at is certainly valid whatever the source of the carbonium ions. To take an extreme example, the difference in stability between methyl and /m-butyl cations relative to the alkanes, as we have just calculated it, is 71 kcal. Relative to other standards, the difference in stability is alcohols, 57 kcal chlorides, 74 kcal bromides, 78 kcal and iodides, 76 kcal. 

Tungstated zirconia (WZ) catalysts have been proposed as viable candidates for alkane isomerizations, especially those heavier than C4 [1]. In this work, alcohol dehydrations were used to rank the acidity of WZ with respect to that of HY zeolite, and to evaluate the ability of these catalysts to resist coking during a reaction that yields an olefin molecule as the primary product. The results from temperature-programmed reaction and infrared spectroscopy studies allowed us to gain some insight into the relative stability of WZ. 

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