Quantitative Evaluation of Strain Energy

3a Heats of Formation To recapitulate material from Chapter 3 (p. 115), the heat of formation (A/7 ) of a compound is the enthalpy of its formation by the reaction of its constituent elements in their standard states. The standard state of an element is the most stable state at 25 °C and 1 atm pressure. For an element in its standard state, A/7 is taken as zero. The more negative (or less positive) a compound s Hf, the more stable it is. A negative A/ for a compound means that its formation from its constituent elements would be exothermic— heat would be liberated. By contrast, a positive A/ means that the constituent elements are more stable than the compound and its formation would be endothermic—energy would have to be applied (Fig. 5.18). 

FIGURE 5.18 For a compound more stable than its constituent elements, is negative. For a compound less stable than its constituent elements, A// is positive. 

Columns 1 and 2 of Table 5.1 collect the heats of formation for some cycloalkanes and calculate the per methylene group for these hydrocarbons. 

Molecule Measured A// (kcal/mol) per CH2 (kcal/mol) Calcd A// for Strain-free Molecule (kcal/mol) Strain E (kcal/ mol) Strain E per CH2 (kcal/mol) 

PROBLEM 5.5 How would one get the value of for a CH2 group in an infinite chain, (CHj) Use the following data to estimate the value of A/ of a strain-free methylene group in kilocalories per mole hexane = —39.9, 

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