chapter 2 key - I t C filAPTER A lkanesa nd C...

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CfilAPTER 2: Alkanes and Cycloalkanes: Introduction to Hydrocarbons 37 The numbering pattern of the ring is chosen so as to give the lowest number to the substituent at the first point of difference between them. Thus, the carbon bearing two methyl groups is C-1, and the ring is numbered counterclockwise, placing the isopropyl group on C-4 (numbering clockwise would place the isopropyl on C-8). Listing the substituent groups in alphabetical order, the correct name is 4-isopropyl-l,l-dimethylcyclodecane. Alternatively, the systematic name for isopropyl (1-methylethyl) could be used, and the name would become l,I-dimethyl-4-(l- methy le t hyI ) cy c I ode cane. (c) When two cycloalkyl groups are attached by a single bond, the compound is named as a cycloalkyl- substituted cycloalkane.This compound is cyclohexylcyclohexane. 2.12 The alkane that has the most carbons (nonane)has the highest boiling point (151'C). Among the others, all of which have eight carbons,the unbranched isomer (octane) has the highest boiling point (126"C) and the most branched one (2,2,3,3-tetramethylbutane) the lowest (106'C). The remaining alkane, 2-methylheptane, boils at 116"C. :.13 All hydrocarbons burn in air to give carbon dioxide and water. To balancethe equation for the combustion of cyclohexane (C6H12), first balancethe carbons and the hydrogens on the right side. Then balance the oxygens on the left side. al + eo2 6co2 + 6Hzo \-J Cyclohexane Oxygen Carbondioxide Water l.lJ (b) Icosane (Table 2.2) is CzoHqz. It has four more methylene (CHz) groups than hexadecane, the last unbranched alkane in Table 2.3. Its calculated heat of combustion is'therefore (4x653 kUmol) higher. Heat of combustion of icosane = heat of combustion of hexadecane + (4 x 653 kUmol) = 10,701kJ/mol +2612 kJ/mol = 13,313 kJimol :.15 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, 2-methylbutane, and 2,2-dimethylpropane are all CsHrz; hexane is C6H1a. Hexane has the largest heat of combustion. Branching leads to a lower heat of combustion; 2,2-dimethylpropane is the most branched and has the lowest heat of combustion. Hexane Pentane 2-Methylbutane 2,2-Dimethylpropane CH3(CH2)aCHr cH3cH2cH2CH2CH3 (cH3)2CHCH2CH3 (cH3)4c Heat of combustion4163 kJ/mol (995.0 kcal/mol) Heat of combustion 3536 kJ/mol (845.3kcal/mol) Heat of combustion 3529 kJlmoI (843.4 kcal/mol) Heat of combustion 3514 kJ/mol (839.9 kcal/mol)
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I 38 CHAPTER 2: Alkanes and Cycloalkanes: Introduction to Hydrocarbons 2.16 An oxidation number for carbon can be calculated from the molecular formula of ethanol (C2H6O) by first assigningoxidation numbers to hydrogen (+1) and to oxygen (-2). The sum of the oxidation numbers of the atoms in a neutral compound equals zero, so by this method both carbonshave an oxidation number of -2. As described in Table 2.5,the oxidation numbers of the carbon atoms in ethanol are -3 for the CH3 carbon and -1 for the carbon of the CH2OH group. The difference between the two methods is that a calculation based on the molecular formula gives the average oxidation state for the two carbons.
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