ALKANES 2.14 Two factors that influence the heats of combustion of alkanes are, in order of decreasing importance, (1)the number of carbon atoms and(2) the extent of chain branching. Pentane, isopentane, and neopentane are all CsH12; hexane is CHia. Hexane has the largest heat of combustion. Branching leads to a lower heat of combustion; neopentane is the most branched and has the lowest heat of Hexane CHa(CH))4CH3 Heat of combustion 4163 kJ/mol (995.0 kcal/mol) Pentane CH3 CH,CH,CH,CH3 Heat of combustion 3527 kJ/mol (845.3 kcal/mol) (CH3),CHCH,CH Heat of combustion 3529 kJ/mol (843. 4 kcal/mol) Neopentane (CH,)4C Heat of combustion 3514 kJ/mol (839.9 kcal/mol 2.15(b)In the reaction CH,=CH,+ Br2 BrCH,CH, Bi carbon becomes bonded to an atom(Br) that is more electronegative than itself. Carbon is (c) In the reaction 6CH,=CH,+ B,H6 2(CHCH,)3B one carbon becomes bonded to hydrogen and is, therefore, reduced. The other carbon is also reduced, because it becomes bonded to boron, which is less electronegative than carbon. 2.16 It is best to approach problems of this type systematically. Since the problem requires all the isomers of ChI to be written, begin with the unbranched isomer heptane CHCH,CH,CH,CH,CH,CH Two isomers have six carbons in their longest continuous chain. One bears a methyl substituent at C-2, the other a methyl substituent at C-3 (CH3) CHCH, CH- CH,CH3 CH CH CHCH,- CH3 H3 2-Methylhexane 3-Methylhexane Now consider all the isomers that have two methyl groups as substituents on a five-carbon continu- (CH3)3CCH, CHCH3 (CH;CH,)2 C(CH3) 3, 3-Dimethylpentane CH3)CHCHCH,CH (CH3)2 CHCH,CH(CH3h2 4-Dimethylpentane Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website2.14 Two factors that influence the heats of combustion of alkanes are, in order of decreasing importance, (1) the number of carbon atoms and (2) the extent of chain branching. Pentane, isopentane, and neopentane are all C5H12; hexane is C6H14. Hexane has the largest heat of combustion. Branching leads to a lower heat of combustion; neopentane is the most branched and has the lowest heat of combustion. Hexane CH3(CH2)4CH3 Heat of combustion 4163 kJ/mol (995.0 kcal/mol) Pentane CH3CH2CH2CH2CH3 Heat of combustion 3527 kJ/mol (845.3 kcal/mol) Isopentane (CH3)2CHCH2CH3 Heat of combustion 3529 kJ/mol (843.4 kcal/mol) Neopentane (CH3)4C Heat of combustion 3514 kJ/mol (839.9 kcal/mol) 2.15 (b) In the reaction carbon becomes bonded to an atom (Br) that is more electronegative than itself. Carbon is oxidized. (c) In the reaction one carbon becomes bonded to hydrogen and is, therefore, reduced. The other carbon is also reduced, because it becomes bonded to boron, which is less electronegative than carbon. 2.16 It is best to approach problems of this type systematically. Since the problem requires all the isomers of C7H16 to be written, begin with the unbranched isomer heptane. Two isomers have six carbons in their longest continuous chain. One bears a methyl substituent at C-2, the other a methyl substituent at C-3. Now consider all the isomers that have two methyl groups as substituents on a five-carbon continu￾ous chain. 2,3-Dimethylpentane (CH3)2CHCHCH2CH3 CH3 2,4-Dimethylpentane (CH3)2CHCH2CH(CH3)2 2,2-Dimethylpentane (CH3)3CCH2CH2CH3 3,3-Dimethylpentane (CH3CH2)2C(CH3)2 2-Methylhexane (CH3)2CHCH2CH2CH2CH3 3-Methylhexane CH3CH2CHCH2CH2CH3 CH3 Heptane CH3CH2CH2CH2CH2CH2CH3 B2H6 2(CH3CH2) 6CH2 CH2 3B Br BrCH2CH2Br CH2 CH2 2 ALKANES 29 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website