Branched chain hydrocarbons, heat

Part A of Table 1.4 shows all the acyclic C4-C6 hydrocarbons and a number of the Cg hydrocarbons. A general trend is discernible in the heats of formation data. Branched-chain hydrocarbons are more stable than straight-chain hydrocarbons. For example, the MTf for n-octane is -49.82 kcal/mol, whereas the most highly branched isomer possible, 2,2,3,3-tetramethylbutane, is the most stable of the octanes, with a AH of -53.99 kcal/mol. Similar trends are observed in the other series. 

HYDROFORMING. The use of hydrogen in the presence uf heat, pressure, and catalysis (usually platinumi to convert olclinic hydrocarbons to branched chain paraffins (isomerizationi io y ield high-oeiane gasoline. Catlorriing and similar terms are often used in the same sense. 

Triisobutylaluminum is a colourless transparent liquid (the boiling point is 138 °C at 6.6 GPa), which dissolves well in hydrocarbons. When heated up to 140-160 °C in vacuum (the residual pressure is 35 GPa), triisobutylaluminum disintegrates into isobutene and diisobutylaluminum hydride. Unlike lower trialkyl derivatives of aluminum with the direct carbon chain (e.g. triethylaluminum), aluminumtrialkyls with a branched chain (triisobutylaluminum is one of them) are monomeric. 

Endothermic Reactions. Another example of the need for interstage heat transfer in a series of reactors can be found when upgrading the octane number of gasoline. The more compact the hydrocarbon molecule for a given number of carbon atoms, the higher the octane rating. Consequently, it is desirable to convert straight-chain hydrocarbons to branched isomers, naphthenes, and aromatics. The reaction sequence is... 

Dimerization is well known to be a side reaction in the Chichibabin reaction (78RCR1042). In fact, dimerization can be the only product with certain alkylpyridines (79USP4177349 81USP4267335). Under normal Chichibabin conditions, heating a heterogeneous mixture of sodium amide in a hydrocarbon at atmospheric pressure with a branched chain alkylpyridine leads to... 

Hydrocaibons with branched chain Heat of formation of isomet ic hydrocarbon kcals Corresponding normal hydrocarbon Heat of formation of notmal hydrocarbon kcals Difference kcals... 

Pearce and Reed61 found that the heat of adsorption increases with an increase in the length of the hydrocarbon chain, and that straight chains give a greater heat of adsorption than branch-chain compounds. Although the heat of adsorption tends to increase with the size of the molecule, it does not do so at a uniform rate. 

Using data from the American Petroleum Institute Research Project 44, and extrapolating to the limit of infinitely long chains, values were obtained for the effects of the branching —CH(CH3)— and —C(CH3)2—, occurring every third, fourth, fifth, or sixth carbon atom. By incorporating heats and entropies of vaporization of long chain hydrocarbons, derived by extrapolation of the known... 

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