56 CONFORMATIONS OF ALKANES AND CYCLOALKANES (c) These hydrocarbons all have different molecular formulas. Their heats of combustion de- crease with decreasing number of carbons, and comparisons of relative stability cannot be Cyclopropylcyclopropane Heat of combustion (C6H10) 3886 kJ/mol (928.8 kcal/mol) (C5H3) 3296 kJ/mol (87. 8 kcal/mol) Bicyclo[l1.O butane Heat of combustion (C4H6) 2648 kJ/mol (633.0 kcal/mol) (d) Bicyclo[3.3. 0]octane and bicyclo[5.1.0]octane are isomers, and their heats of combustion can be compared on the basis of their relative stabilities. The three-membered ring in bicy clo[5.1.0Joctane imparts a significant amount of angle strain to this isomer, making it less stable than bicyclo[3.3. 0]octane. The third hydrocarbon, bicyclo[4.3. 0]nonane, has a greater number of carbons than either of the others and has the largest heat of combustion cyclo[4.3.0]nonane Heat of combustion 5652 kJr (CH16) (1350.9 kcal/mol) Bicyclo[5.1.0]octane Heat of combustion 5089 kJ/mol (CH14 (1216.3 kcal/mol) cyclo[3.3.0]octane Heat of combustion 5016 kJ/mol (C&H14 (1198.9kca/mol) 3.28 (a) The structural formula of 2, 2, 5,5-tetramethy lhexane is(CH3)3CCH,CH,C(CH3)3. The stituents at C-3 are two hydrogens and a tert-butyl group. The substituents at C-4 are the as those at C-3. The most stable conformation has the large tert-butyl groups anti to each othe H 2, 2,5, 5-tetramethy lexar (b) The zigzag conformation of 2, 2, 5,5-tetramethy hexane is the same co on implied in the Newman pr Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website(c) These hydrocarbons all have different molecular formulas. Their heats of combustion de￾crease with decreasing number of carbons, and comparisons of relative stability cannot be made. (d) Bicyclo[3.3.0]octane and bicyclo[5.1.0]octane are isomers, and their heats of combustion can be compared on the basis of their relative stabilities. The three-membered ring in bicy￾clo[5.1.0]octane imparts a significant amount of angle strain to this isomer, making it less stable than bicyclo[3.3.0]octane. The third hydrocarbon, bicyclo[4.3.0]nonane, has a greater number of carbons than either of the others and has the largest heat of combustion. 3.28 (a) The structural formula of 2,2,5,5-tetramethylhexane is (CH3)3CCH2CH2C(CH3)3. The sub￾stituents at C-3 are two hydrogens and a tert-butyl group. The substituents at C-4 are the same as those at C-3. The most stable conformation has the large tert-butyl groups anti to each other. (b) The zigzag conformation of 2,2,5,5-tetramethylhexane is an alternative way of expressing the same conformation implied in the Newman projection of part (a). It is more complete, Anti conformation of 2,2,5,5-tetramethylhexane C(CH3)3 C(CH3)3 H H H H H H H H H H Bicyclo[4.3.0]nonane (C9H16) Bicyclo[5.1.0]octane (C8H14) Bicyclo[3.3.0]octane (C8H14) Heat of combustion 5652 kJ/mol (1350.9 kcal/mol) Heat of combustion 5089 kJ/mol (1216.3 kcal/mol) Heat of combustion 5016 kJ/mol (1198.9 kcal/mol) Cyclopropylcyclopropane (C6H10) Heat of combustion 3886 kJ/mol (928.8 kcal/mol) Spiropentane (C5H8) Heat of combustion 3296 kJ/mol (787.8 kcal/mol) Bicyclo[1.1.0]butane (C4H6) Heat of combustion 2648 kJ/mol (633.0 kcal/mol) 56 CONFORMATIONS OF ALKANES AND CYCLOALKANES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website