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Unformatted text preview: 29 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 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
Pentane (CH3)2CHCH2CH3 Neopentane (b) CH3CH2CH2CH2CH3 Isopentane 2.15 CH3(CH2)4CH3 (CH3)4C In the reaction
CH2 (c) Heat of combustion 4163 kJ/mol
Heat of combustion 3527 kJ/mol
Heat of combustion 3529 kJ/mol
Heat of combustion 3514 kJ/mol
(839.9 kcal/mol) CH2 Br2 BrCH2CH2Br carbon becomes bonded to an atom (Br) that is more electronegative than itself. Carbon is
In the reaction
6CH2 CH2 B2H6 2(CH3CH2)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 C7H16 to be written, begin with the unbranched isomer heptane.
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. CH3CH2CHCH2CH2CH3 (CH3)2CHCH2CH2CH2CH3 CH3
2-Methylhexane 3-Methylhexane Now consider all the isomers that have two methyl groups as substituents on a ﬁve-carbon continuous chain.
3,3-Dimethylpentane (CH3)2CHCHCH2CH3 (CH3)2CHCH2CH(CH3)2 CH3
2,3-Dimethylpentane Back Forward Main Menu TOC 2,4-Dimethylpentane Study Guide TOC Student OLC MHHE Website ...
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