In thermal cracking (Sec. 5.8), bonds generally break homolytically.

(a) In the thermal cracking of 2,2,3,3-tetra me thylbutane, which bond would be most likely to break? Explain.

(b) Which compound, 2,2,3,3-tetramethylbutane or ethane, undergoes thermal cracking more rapidly at a given temperature? Explain.

(c) Calculate the ΔH° for the initial carbon–carbon bond breaking for the thermal cracking of both 2,2,3,3-tetramethylbutane and ethane. Use the ΔH°_f values in Table 5.2 as well as the ΔH°_f of 2,2,3,3-tetramethylbutane (2225.9 kJ mol^–1, 253.99 kcal mol^–1) and ethane (284.7 kJ mol^–1, 220.24 kcal mol^–1). Use these calculations to justify your answer to part (b).

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TABLE 5.2 Heats of Formation of Some Free Radicals (25 °C) AH; (kJ mol-¹) 146.6 121.3 100.4 Radical methyl ethyl propyl isopropyl butyl isobutyl sec-butyl tert-butyl Structure CH3 -CH₂CH3 -CH₂CH₂CH₂ CH₂ CHCH -CH₂CH₂CH₂CH₂ -CH₂CH(CH3)₂ CH, CHCH₂CH3 -C(CH3)3 90.0 79.7 70 67.4 51.5 AH; (kcal mol-¹) 35.0 29.0 24.0 21.5 19.0 17 16.1 12.3