Cyclohexane combustion

Heat of combustion is the heat liberated or absorbed when one gram mole of the substance is completely oxidized to liquid water and CO2 gas at one atmosphere and 20°C or 25°C. (Cj-C hydrocarbons and cyclohexane at 25°C, others at 20°C). The gross heating value in Btu/ft could be calculated as follows ... 

Figure 4.3 Cycloalkane strain energies, calculated by taking the difference between cycloalkane heat of combustion per CH2 and acyclic alkane heat of combustion per CH2, and multiplying by the number of CH2 units in a ring. Small and medium rings are strained, but cyclohexane rings are strain-free.

Although it forms expl mixts with CO, NH3, CS2, and phosphine, it is not used in the expls industry. It also forms combustible mixts with NO and fuels such as butane, benz, cyclohexane and p-xylene (Ref 8)... 

The amount of strain in cycloalkanes is shown in Table 4.6, which lists heats of combustion per CH2 group. As can be seen, cycloalkanes larger than 13 membered are as strain-free as cyclohexane. 

Cyclohexane has the lowest heat of combustion per CH2 group (658.7 kJ mol-1).=> the same as unbranched alkanes (having no ring strain) => cyclohexane has no ring strain. 

In marked contrast to the n-alkanes, the cycloalkanes exhibit thermodynamic properties where such regularities are no longer present. Heats of formation (AH ) for a substantial number of cycloalkanes are available from heats of combustion. With the exception of cyclohexane, AH°f is always more positive than the quantity — 4.926n. The difference between the two quantities leads to a quantitative assessment of the important notion of ring strain. The AH -values and strain energy data listed in Table 1 were taken from Skinner and Pilcher (1963). Other references give different but usually comparable... 

Since cycloalkanes do not have isomers, the heat evolved has been calculated per CH2 group. First we find the heat of combustion of cyclohexane which is most stable and then the heat of combustion per CH2 group. It comes to 157.4 K cals/mole. Now to calculate the ring strain for other cycloalkanes, we multiply 157.4 K cal/mole by n and then subtract it from the heat of combustion of cycloalkane. 

It should be emphasized that mankind has come and still comes into contact with fullerene in everyday life. Pristine fullerene C60 itself can be found in our environment, e.g., in the soot produced by free burning of hydrocarbons like benzene and cyclohexane, as well as in charcoal, though in very small amounts (Shibuya et al., 1999) and in the kitchen (in natural gas combustion streams) (Bang et al., 2004 Murr and Soto, 2005). The impact of these natural sources is rather negligible though with the growth of production of fullerenes it could lead to much more serious environment pollution and be (or could not ) of a hazard to some extent. 

Problem 9.5 (a) Calculate AH of combustion per CHj unit for the first four cycloalkanes, given the following AH s of combustion, in kJ/mol cyclopropane, -2091 cyclobutane, -2744 cyclopentane, -3320 cyclohexane, -3952. (b) Write (i) the thermochemical equation for the combustion of cyclopropane and (ii) the theoretical equation for the combustion of a CH unit of any given ring, (c) How do ring stability and ring size correlate for the first four cycloalkanes ... 

A mixture weighing 7.290 mg contained only cyclohexane, C6HI2 (FM 84.159), and oxirane, C2H40 (FM 44.053). When the mixture was analyzed by combustion analysis, 21.999 mg of C02 (FM 44.010) was produced. Find the weight percent of oxirane in the mixture. 

Cyclopropane has a A//0 of combustion 27.7 kcal mole-1 greater than expected from bond energies, and this clearly is associated with the abnormal C-C-C bond angles in the ring. These matters will be discussed in detail in Chapter 12. For cyclohexane, which has normal bond angles, the heat of combustion is close to the calculated value. 

All hydrocarbons bum in air to give carbon dioxide and water. To balance the equation for the combustion of cyclohexane (C6H12), first balance the carbons and the hydrogens on the right side. Then balance the oxygens on the left side. 

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