Relative Stability of Isomeric Alkanes

In order to compare the stability of constitutional isomers, we look at the heat liberated when they each undergo combustion. For an alkane, combustion describes a reaction in which the alkane reacts with oxygen to produce CO2 and water. Consider the following example  

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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... 

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... 

The selective monofluorination of difluorinated alkanes with cobalt(III) fluoride at 25 C to give trifluoroalkanes 10 has been reported. 11 Fluorinations with cobalt(lll) fluoride are considered to proceed via carbocations32 and the selectivity of the fluorination is explained by the relative stabilities of the intermediate carbocations.30 Although treatment of alkanes with chlorine trifluoride results in fluorination, it is of little practical use due to over-fluorination, chlorination and in some cases isomerization of the starting alkane.33... 

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. 

The A77° of the combustion of an alkane is called the heat of combustion, AT/ omb- The heats of combustion of isomeric compounds provide an experimental measure of their relative stabilities. 

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