CHAPTER 2 ALKANES SOLUTIONS TO TEXT PROBLEMS 2.1 A carbonyl group is C=O. Of the two carbonyl functions in prostaglandin E, one belongs to the ketone family, the other to the carboxylic acids functional OH 2.2 An unbranched alkane(n-alkane)of 28 carbons has 26 methylene(CH,) groups flanked by a methyl (CH3) group at each end. The condensed formula is CH3(CH,)26CH3. 2.3 The alkane represented by the carbon skeleton formula has ll carbons. The general formula for an alkane is CnH,n+2, and thus there are 24 hydrogens. The molecular formula is ClH 4; the condensed structural formula is CH3(CH)CH 2.4 In addition to CH3 (CH2)4CH3 and(CH,)2CHCH,CHa CH3, there are three more isomers. One has a five-carbon chain with a one-carbon(methyl) branch CHa CH, CHCH, CH The remaining two isomers have two methyl branches on a four-carbon chain. H3 CH3 CHCHCH3 CH, CH, CCH3 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
25 CHAPTER 2 ALKANES SOLUTIONS TO TEXT PROBLEMS 2.1 A carbonyl group is C?O. Of the two carbonyl functions in prostaglandin E1 one belongs to the ketone family, the other to the carboxylic acids. 2.2 An unbranched alkane (n-alkane) of 28 carbons has 26 methylene (CH2) groups flanked by a methyl (CH3) group at each end. The condensed formula is CH3(CH2)26CH3. 2.3 The alkane represented by the carbon skeleton formula has 11 carbons. The general formula for an alkane is CnH2n2, and thus there are 24 hydrogens. The molecular formula is C11H24; the condensed structural formula is CH3(CH2)9CH3. 2.4 In addition to CH3(CH2)4CH3 and (CH3)2CHCH2CH2CH3, there are three more isomers. One has a five-carbon chain with a one-carbon (methyl) branch: The remaining two isomers have two methyl branches on a four-carbon chain. CH or or 3CHCHCH3 CH3 CH3 CH3CH2CCH3 CH3 CH3 CH3CH2CHCH2CH3 CH3 or Ketone OH functional group Carboxylic acid functional group O O HO OH Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
26 ALKANES 2.5 (b) Octacosane is not listed in Table 2.4, but its structure can be deduced from its systematic name. The suffix-cosane pertains to alkanes that contain 20-29 carbons in their longest con- nuous chain. The prefix octa- means"eight. Octacosane is therefore the unbranched alkane having 28 carbon atoms. It is CHi(CH))2 CH (c) The alkane has an unbranched chain of ll carbon atoms and is named undecane 2.6 The ending -hexadecane reveals that the longest continuous carbon chain has 16 carbon atoms There are four methyl groups(represented by tetramethyl-), and they are located at carbons 2, 6, 10, and 14 2, 6, 10, 14-Tetramethylhexadecane (phytane) 2.7 (b) The systematic name of the unbranched CsHiz isomer is pentane(Table 2. 4) CHaCH,CH,CH,CH A second isomer, (CH3)2 CHCH,CH3, has four carbons in the longest continuous chain and so is named as a derivative of butane. Since it has a methyl group at C-2, it is 2-methylbutane IUPAC name: 2-methylbutane Common name: isopentane methyl group at C-2 The remaining isomer, (CH3)4C, has three carbons in its longest continuous chain and so is named as a derivative of propane. There are two methyl groups at C-2, and so it is a 2, 2-dimethyl derivative of propane CH CCH IUPAC name: 2, 2-dimethylpropane Common name: neopenta (c) First write out the structure in more detail, and identify the longest continuous carbon chain CH CH3C—CH2-CCH3 There are five carbon atoms in the longest chain, and so the compound is named as a deriva tive of pentane. This five-carbon chain has three methyl substituents attached to it, making it Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
26 ALKANES 2.5 (b) Octacosane is not listed in Table 2.4, but its structure can be deduced from its systematic name. The suffix -cosane pertains to alkanes that contain 20–29 carbons in their longest continuous chain. The prefix octa- means “eight.” Octacosane is therefore the unbranched alkane having 28 carbon atoms. It is CH3(CH2)26CH3. (c) The alkane has an unbranched chain of 11 carbon atoms and is named undecane. 2.6 The ending -hexadecane reveals that the longest continuous carbon chain has 16 carbon atoms. There are four methyl groups (represented by tetramethyl-), and they are located at carbons 2, 6, 10, and 14. 2.7 (b) The systematic name of the unbranched C5H12 isomer is pentane (Table 2.4). A second isomer, (CH3)2CHCH2CH3, has four carbons in the longest continuous chain and so is named as a derivative of butane. Since it has a methyl group at C-2, it is 2-methylbutane. The remaining isomer, (CH3)4C, has three carbons in its longest continuous chain and so is named as a derivative of propane. There are two methyl groups at C-2, and so it is a 2,2-dimethyl derivative of propane. (c) First write out the structure in more detail, and identify the longest continuous carbon chain. There are five carbon atoms in the longest chain, and so the compound is named as a derivative of pentane. This five-carbon chain has three methyl substituents attached to it, making it CH3 C CH3 CH3 CH2 C H CH3 CH3 IUPAC name: 2,2-dimethylpropane Common name: neopentane CH3CCH3 CH3 CH3 IUPAC name: 2-methylbutane Common name: isopentane methyl group at C-2 CH3CHCH2CH3 CH3 IUPAC name: pentane Common name: n-pentane CH3CH2CH2CH2CH3 2,6,10,14-Tetramethylhexadecane (phytane) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ALKANES a trimethyl derivative of pentane. Number the chain in the direction that gives the lowest numbers to the substituents at the first point of difference C H CH H H - C-CH CH3C-CH2-C—CH3 2, 2, 4-Trimethylpentane(correct) 2.4, 4-Trimethylpentane(incorrect) (d) The longest continuous chain in(CH3)CC(CH3)3 contains four carbon atoms. CHCH CH -CH The compound is named as a tetramethyl derivative of butane; it is 2, 2, 3, 3-tetramethylbutane. 2.8 There are three CsHu alkyl groups with unbranched carbon chains. One is primary, and two are sec- ondary. The IUPAC name of each group is given beneath the structure. Remember to number the alkyl groups from the point of attachment. CH3,,,CH CHaCH,CHLCHCH CH-CH, CHCH, CH3 Pentyl group(primary) 1-Methylbutyl group(secondary) 1-Ethylpropyl group(secondary) Four alkyl groups are derived from(CH3)2CHCH, CH. Two are primary, one is secondary, and one IS CH H.CH CHCh,CH 3-Methylbutyl group(primary) 2-Methylbutyl group(prim CHa CCH2CH CH, CHCHCH 1, 1-Dimethylpropyl group(tert 1. 2-Dimethylpropyl group(secondary) 2.9(b) Begin by writing the structure in more detail, showing each of the groups written in parenthe ses. The compound is named as a derivative of hexane, because it has six carbons in its longest continuous chain CH, CH, CHCH,CHCH CH3CH2 CH The chain is numbered so as to give the lowest number to the substituent that appears closest to the end of the chain in this case it is numbered so that the substituents are located at C-2 and C-4 rather than at C-3 and C-5. In alphabetical order the groups are ethyl and methyl; they are listed in alphabetical order in the name. The compound is 4-ethyl-2-methy lhexane. Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
a trimethyl derivative of pentane. Number the chain in the direction that gives the lowest numbers to the substituents at the first point of difference. (d) The longest continuous chain in (CH3)3CC(CH3)3 contains four carbon atoms. The compound is named as a tetramethyl derivative of butane; it is 2,2,3,3-tetramethylbutane. 2.8 There are three C5H11 alkyl groups with unbranched carbon chains. One is primary, and two are secondary. The IUPAC name of each group is given beneath the structure. Remember to number the alkyl groups from the point of attachment. Four alkyl groups are derived from (CH3)2CHCH2CH3. Two are primary, one is secondary, and one is tertiary. 2.9 (b) Begin by writing the structure in more detail, showing each of the groups written in parentheses. The compound is named as a derivative of hexane, because it has six carbons in its longest continuous chain. The chain is numbered so as to give the lowest number to the substituent that appears closest to the end of the chain. In this case it is numbered so that the substituents are located at C-2 and C-4 rather than at C-3 and C-5. In alphabetical order the groups are ethyl and methyl; they are listed in alphabetical order in the name. The compound is 4-ethyl-2-methylhexane. CH3CH2CHCH2CHCH3 CH3CH2 CH3 6 5 43 21 2-Methylbutyl group (primary) 1,2-Dimethylpropyl group (secondary) 3-Methylbutyl group (primary) CH3CHCH2CH2 4 3 2 1 CH3 CH2CHCH2CH3 1 2 3 4 CH3 CH3CHCHCH3 3 1 2 CH3 1,1-Dimethylpropyl group (tertiary) CH3CCH2CH3 1 2 3 CH3 CH3CH2CH2CH2CH2 Pentyl group (primary) CH3CH2CH2CHCH3 4 3 2 1 1-Methylbutyl group (secondary) CH3CH2CHCH2CH3 3 2 1 1-Ethylpropyl group (secondary) CH3 C CH3 CH3 CH3 C CH3 CH3 CH C not 3 CH3 CH3 CH2 C H CH3 CH3 1 2 45 3 2,2,4-Trimethylpentane (correct) CH3C CH3 CH3 CH2 C H CH3 CH3 54 2 1 3 2,4,4-Trimethylpentane (incorrect) ALKANES 27 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
28 ALKANES (c) The longest continuous chain is shown in the structure; it contains ten carbon atoms. The structure also shows the numbering scheme that gives the lowest number to the substituent at the first point of difference CH CH, CH, CHCH, CHCH, CHCHCH CH,CH3 CH,CHCH In alphabetical order, the substituents are ethyl(at C-8), isopropyl at(C-4), and two methyl groups(at C-2 and C-6). The alkane is 8-ethyl-4-isopropyl-2, 6-dimethyldecane. The system- tic name for the isopropyl group(1-methylethyl) may also be used, and the name becomes 8-ethy l-2, 6-dimethyl-4-(1-methylethyl)decane 2.10 (b) There are ten carbon atoms in the ring in this cycloalkane, thus it is named as a derivative of cyclodecane HC、CH3 (CH3),CH Cvclodecane 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 al phabetical order, the correct name is 4-isopropyl-1, l-dimethylcyclodecane. Alternatively, the ystematic name for isopropyl(1-methylethyl) could be used, and the name would become (c) When two cycloalkyl groups are attached by a single bond, the compound is named as a cycloalkyl-substituted cycloalkane. This compound is cyclohexylcyclohexane 2.11 The alkane that has the most carbons(nonane) has the highest boiling point (151C). 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.). The remaining kane, 2-methy heptane, boils at 116.C 2.12 All hydrocarbons burn in air to give carbon dioxide and water. To balance the equation for the com- oustion of cyclohexane( CHv), first balance the carbons and the hydrogens on the right side. Then balance the oxygens on the left side +90 6CO t 6H,O Cyclohexane Oxygen Carbon dioxide Water 2. 13 (b) Icosane(Table 2. 4)is Cz Ha2. It has four more methylene( CH, ) groups than hexadecane, the last Unbranched alkane in Table 2.5. Its calculated heat of combustion is therefore (4 X 653 kJ/mol) Heat of combustion of icosane= heat of combustion of hexadecane+ 4 x653 kJ/mol = 10.701 kJ/mol 2612 kJ/mol 13. 313 kJ/mol Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(c) The longest continuous chain is shown in the structure; it contains ten carbon atoms. The structure also shows the numbering scheme that gives the lowest number to the substituent at the first point of difference. In alphabetical order, the substituents are ethyl (at C-8), isopropyl at (C-4), and two methyl groups (at C-2 and C-6). The alkane is 8-ethyl-4-isopropyl-2,6-dimethyldecane. The systematic name for the isopropyl group (1-methylethyl) may also be used, and the name becomes 8-ethyl-2,6-dimethyl-4-(1-methylethyl)decane. 2.10 (b) There are ten carbon atoms in the ring in this cycloalkane, thus it is named as a derivative of cyclodecane. 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-1,1-dimethylcyclodecane. Alternatively, the systematic name for isopropyl (1-methylethyl) could be used, and the name would become 1,1-dimethyl-4-(1-methylethyl)cyclodecane. (c) When two cycloalkyl groups are attached by a single bond, the compound is named as a cycloalkyl-substituted cycloalkane. This compound is cyclohexylcyclohexane. 2.11 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. 2.12 All hydrocarbons burn in air to give carbon dioxide and water. To balance the equation for the combustion of cyclohexane (C6H12), first balance the carbons and the hydrogens on the right side. Then balance the oxygens on the left side. 2.13 (b) Icosane (Table 2.4) is C20H42. It has four more methylene (CH2) groups than hexadecane, the last unbranched alkane in Table 2.5. Its calculated heat of combustion is therefore (4 653 kJ/mol) higher. Heat of combustion of icosane heat of combustion of hexadecane 4 653 kJ/mol 10,701 kJ/mol 2612 kJ/mol 13,313 kJ/mol 9O2 6CO2 6H2O Cyclohexane Oxygen Carbon dioxide Water Cyclodecane (CH3)2CH H3C CH3 3 6 7 2 1 8 5 4 9 10 CH3CH2CHCH2CHCH2CHCHCH3 CH2CH3 CH2CHCH3 CH3 CH3 CH3 3 2 1 10 9 87 6 4 5 28 ALKANES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
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 Website
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 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 continuous 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
30 ALKANES There is one isomer characterized by an ethyl substituent on a five-carbon chain: H3,)3CH 3-Ethylpentane The remaining isomer has three methyl substituents attached to a four-carbon chain. (CH3)3 CCH(CH3)2 2, 2, 3-Trimethylbutane 2.17 In the course of doing this problem, you will write and name the 17 alkanes that, in addition to oc- tane, CH;(CH2)CH3, comprise the 18 constitutional isomers of C&H (a) The easiest way to attack this part of the exercise is to draw a bond-line depiction of heptane and add a methyl branch to the various positions 2-Methy heptane 3-Methy heptane 4-Methylheptane Other structures bearing a continuous chain of seven carbons would be duplicates of the isomers rather than unique isomers. 5-Methy heptane, " for example, is an incorrect name methy heptane, and"6-methy heptane"is an incorrect name for 2-methy heptane (b Six of the isomers named as derivatives of hexane contain two methyl branches on a continu- ous chain of six carbons Dimethy lhexane 2, 3-Dimethy lhexane 2, 4-Dimethy lhexane ,5-Dimethy lhexane 3, 3-Dimethylhexane 3. 4-Dimethylhexane One isomer bears an ethyl substituent (c) Four isomers are trimethyl-substituted derivatives of pentane 义人 .3, 3-Trimethylpentane 2. 2. 4-Trimethylpentane Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
There is one isomer characterized by an ethyl substituent on a five-carbon chain: The remaining isomer has three methyl substituents attached to a four-carbon chain. 2.17 In the course of doing this problem, you will write and name the 17 alkanes that, in addition to octane, CH3(CH2)6CH3, comprise the 18 constitutional isomers of C8H18. (a) The easiest way to attack this part of the exercise is to draw a bond-line depiction of heptane and add a methyl branch to the various positions. Other structures bearing a continuous chain of seven carbons would be duplicates of these isomers rather than unique isomers. “5-Methylheptane,” for example, is an incorrect name for 3-methylheptane, and “6-methylheptane” is an incorrect name for 2-methylheptane. (b) Six of the isomers named as derivatives of hexane contain two methyl branches on a continuous chain of six carbons. One isomer bears an ethyl substituent: (c) Four isomers are trimethyl-substituted derivatives of pentane: 2,2,3-Trimethylpentane 2,3,3-Trimethylpentane 2,2,4-Trimethylpentane 2,3,4-Trimethylpentane 3-Ethylhexane 2,2-Dimethylhexane 2,3-Dimethylhexane 2,4-Dimethylhexane 2,5-Dimethylhexane 3,3-Dimethylhexane 3,4-Dimethylhexane 2-Methylheptane 3-Methylheptane 4-Methylheptane 2,2,3-Trimethylbutane (CH3)3CCH(CH3)2 3-Ethylpentane (CH3CH2)3CH 30 ALKANES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ALKANES 31 Two bear an ethyl group and a methyl group on a continuous chain of five carbons 3-Ethyl-2-methylpentane 3-Ethyl-3-methylpenta (d) Only one isomer is named as a derivative of butane: 2, 2.3.3-Tetramethylbutan 2.18 (a) The longest continuous chain contains nine carbon atoms. Begin the problem by writing and umbering the carbon skeleton of nonane Now add two methyl groups(one to C-2 and the other to C-3)and an isopropyl group(to C-6) to give a structural formula for 6-isopropyl-2, 3-dimethylnonane CH3 CH(CHsh or CH: CHCHCH,CH,CHCH,CH,CH (b) To the carbon skeleton of heptane(seven carbons) add a tert-butyl group to C-4 and a methyl C-3 to giv C(CH3) CHCH.. CHCH (c) An isobutyl group is-CH, CH(CH3). The structure of 4-isobutyl-l, I-dimethylcyclohexane H3C、 CH, CH(CH3)2 H3C (d) A sec-butyl group is CH_CHCH, CH,. sec-Butylcycloheptane has a sec-butyl group on a even-membered ring (e) A cyclobutyl group is a substituent on a five-membered ring in cyclobutylcycloper Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
Two bear an ethyl group and a methyl group on a continuous chain of five carbons: (d) Only one isomer is named as a derivative of butane: 2.18 (a) The longest continuous chain contains nine carbon atoms. Begin the problem by writing and numbering the carbon skeleton of nonane. Now add two methyl groups (one to C-2 and the other to C-3) and an isopropyl group (to C-6) to give a structural formula for 6-isopropyl-2,3-dimethylnonane. (b) To the carbon skeleton of heptane (seven carbons) add a tert-butyl group to C-4 and a methyl group to C-3 to give 4-tert-butyl-3-methylheptane. (c) An isobutyl group is GCH2CH(CH3)2. The structure of 4-isobutyl-1,1-dimethylcyclohexane is as shown. (d) A sec-butyl group is CH3 = CHCH2CH3. sec-Butylcycloheptane has a sec-butyl group on a seven-membered ring. (e) A cyclobutyl group is a substituent on a five-membered ring in cyclobutylcyclopentane. or CH3CHCH2CH3 1 2 6 4 3 5 H3C H3C CH2CH(CH3)2 or or 1 3 2 4 5 7 6 CH3CH2CHCHCH2CH2CH3 C(CH3)3 CH3 or CH3CHCHCH2CH2CHCH2CH2CH3 CH3 CH(CH3)2 CH3 1 3 2 46 8 57 9 1 2 3 4 5 6 8 7 9 2,2,3,3-Tetramethylbutane 3-Ethyl-2-methylpentane 3-Ethyl-3-methylpentane ALKANES 31 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
32 ALKANES (f) Recall that an alkyl group is numbered from the point of attachment. The structure of (2, 2-dimethylpropyl)cyclohexane is CH (g) The name"pentacosane"contains no numerical locants or suffixes indicating the presence of alkyl groups. It must therefore be an unbranched alkane. Table 2. 4 in the text indicates that the suffix-cosane refers to alkanes with 20-29 carbons. The prefix penta-stands for"five, and so pentacosane must be the unbranched alkane with 25 carbons. Its condensed structural formula is CHa(CH,)23CH3 (h) We need to add a l-methylpentyl group to C-10 of pentacosane. A l-methylpentyl group is CHCH,CH,CH,CH It has five carbons in the longest continuous chain counting from the point of attachment and bears a methyl group at C-1. 10-(1-Methylpentyl)pentacosane is therefore CH;(CH,)CH(CH)CH CH-CHCHCH CHCH 2. 19 (a) This compound is an unbranched alkane with 27 carbons. As noted in part(g) of the preced- ing problem, alkanes with 20-29 carbons have names ending in-cosane. Thus, we add the prefix hepta-("seven")to-cosane to name the alkane CH3(CH)25CH3 as heptacosane. (b) The alkane( CH3),CHCH,(CH,)CH, has 18 carbons in its longest continuous chain. It is named as a derivative of octadecane. There is a single substituent, a methyl group at C-2. The compound is 2-methyloctadecane (c) Write the structure out in more detail to reveal that it is 33, 4-triethy lhexane CH CH CHCH (CH CH2)3CCH(CH, CH3)2 is rewritten as CH3 CH,C--CHCH, CH3 CH- CH (d) Each line of a bond-line formula represents a bond between two carbon atoms. Hydrogens are added so that the number of bonds to each carbon atom totals four is the same as CH3 CH, CHCH C(CH3)3 CH CH The IuPAC name is 4-ethyl-2, 2-dimethylhexand is the same as CH3 CH, CHCH, CHCH2CH3 The IUPAC name is 3, 5-dimethylheptane Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
( f ) Recall that an alkyl group is numbered from the point of attachment. The structure of (2,2-dimethylpropyl)cyclohexane is (g) The name “pentacosane” contains no numerical locants or suffixes indicating the presence of alkyl groups. It must therefore be an unbranched alkane. Table 2.4 in the text indicates that the suffix -cosane refers to alkanes with 20–29 carbons. The prefix penta- stands for “five,” and so pentacosane must be the unbranched alkane with 25 carbons. Its condensed structural formula is CH3(CH2)23CH3. (h) We need to add a 1-methylpentyl group to C-10 of pentacosane. A 1-methylpentyl group is: It has five carbons in the longest continuous chain counting from the point of attachment and bears a methyl group at C-1. 10-(1-Methylpentyl)pentacosane is therefore: 2.19 (a) This compound is an unbranched alkane with 27 carbons. As noted in part (g) of the preceding problem, alkanes with 20–29 carbons have names ending in -cosane. Thus, we add the prefix hepta- (“seven”) to -cosane to name the alkane CH3(CH2)25CH3 as heptacosane. (b) The alkane (CH3)2CHCH2(CH2)14CH3 has 18 carbons in its longest continuous chain. It is named as a derivative of octadecane. There is a single substituent, a methyl group at C-2. The compound is 2-methyloctadecane. (c) Write the structure out in more detail to reveal that it is 3,3,4-triethylhexane. (d) Each line of a bond-line formula represents a bond between two carbon atoms. Hydrogens are added so that the number of bonds to each carbon atom totals four. The IUPAC name is 4-ethyl-2,2-dimethylhexane. (e) The IUPAC name is 3,5-dimethylheptane. is the same as CH3CH2CHCH2CHCH2CH3 CH3 CH3 is the same as CH3CH2CHCH2C(CH3)3 CH2CH3 (CH3CH2)3CCH(CH2CH3)2 is rewritten as CH3CH2C CHCH2CH3 CH3CH2 CH2CH3 CH3CH2 1 2 3 45 6 CH3(CH2)8CH(CH2)14CH3 CH3CHCH2CH2CH2CH3 CHCH2CH2CH2CH3 CH3 12 3 4 5 CH2 CH3 CH3 CH3 C 32 ALKANES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ALKANES H ∠CH is the same CHCH CHCH HC HC—CH The IUPAC name is l-butyl-l-methylcyclooctane (g) Number the chain in the direction shown to give 3-ethyl-4, 5, 6-trimethyloctane. When num- bered in the opposite direction, the locants are also 3, 4, 5, and 6. In the case of ties, however, choose the direction that gives the lower number to the substituent that appears first in the name. Ethyl"precedes"methyl"alphabetically 2.20 (a) The alkane contains 13 carbons. Since all alkanes have the molecular formula CH,n+2, the molecular formula must be C,h (b) The longest continuous chain is indicated and numbered as shown CH,CHCH, CH-CHCHCH In alphabetical order, the substituents are ethyl(at C-5), methyl (at C-2), methyl(at C-6).The (c) Fill in the hydrogens in the alkane to identify the various kinds of groups present. There are five methyl( CH,) groups, five methylene( CH,) groups, and three methine( CH) groups in (d) A primary carbon is attached to one other carbon. There are five primary carbons(the carbons of the five CH, groups). A secondary carbon is attached to two other carbons, and there are five of these(the carbons of the five CH, groups). A tertiary carbon is attached to three other carbon is attached to four other carbons. None of the carbons is a quaternary carbonatemary carbons, and there are three of these( the carbons of the three methine groups).A 2.21 (a) The group CH3(CH,)1oCH2-is an unbranched alkyl group with 12 carbons. It is a dodecyl group. The carbon at the point of attachment is directly attached to only one other carbon. It is a primary alkyl group (b) The longest continuous chain from the point of attachment is six carbons; it is a hexyl group bear- ing an ethyl substituent at C-3. The group is a 3-ethylhexyl group. It is a primary alkyl group CH, CH, CHCH,CH,CH3 CHCH (c) By writing the structural formula of this alkyl group in more detail, we see that the longest con- tinuous chain from the point of attachment contains three carbons. It is a 1, 1-diethylpropyl group. Because the carbon at the point of attachment is directly bonded to three other carbons it is a tertiary alkyl group CHCH L2CH3 CH, CH3 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
( f ) The IUPAC name is 1-butyl-1-methylcyclooctane. (g) Number the chain in the direction shown to give 3-ethyl-4,5,6-trimethyloctane. When numbered in the opposite direction, the locants are also 3, 4, 5, and 6. In the case of ties, however, choose the direction that gives the lower number to the substituent that appears first in the name. “Ethyl” precedes “methyl” alphabetically. 2.20 (a) The alkane contains 13 carbons. Since all alkanes have the molecular formula CnH2n2, the molecular formula must be C13H28. (b) The longest continuous chain is indicated and numbered as shown. In alphabetical order, the substituents are ethyl (at C-5), methyl (at C-2), methyl (at C-6). The IUPAC name is 5-ethyl-2,6-dimethylnonane. (c) Fill in the hydrogens in the alkane to identify the various kinds of groups present. There are five methyl (CH3) groups, five methylene (CH2) groups, and three methine (CH) groups in the molecule. (d) A primary carbon is attached to one other carbon. There are five primary carbons (the carbons of the five CH3 groups). A secondary carbon is attached to two other carbons, and there are five of these (the carbons of the five CH2 groups). A tertiary carbon is attached to three other carbons, and there are three of these (the carbons of the three methine groups). A quaternary carbon is attached to four other carbons. None of the carbons is a quaternary carbon. 2.21 (a) The group CH3(CH2)10CH2G is an unbranched alkyl group with 12 carbons. It is a dodecyl group. The carbon at the point of attachment is directly attached to only one other carbon. It is a primary alkyl group. (b) The longest continuous chain from the point of attachment is six carbons; it is a hexyl group bearing an ethyl substituent at C-3. The group is a 3-ethylhexyl group. It is a primary alkyl group. (c) By writing the structural formula of this alkyl group in more detail, we see that the longest continuous chain from the point of attachment contains three carbons. It is a 1,1-diethylpropyl group. Because the carbon at the point of attachment is directly bonded to three other carbons, it is a tertiary alkyl group. CH2CH3 CH2CH3 CCH2CH3 C(CH is rewritten as 2CH3)3 12 3 1 2 34 5 6 CH2CH2CHCH2CH2CH3 CH2CH3 9 7 8 6 5 4 3 2 1 CH3CHCH2CH2CHCHCH3 CH3 CH2CH2CH3 CH2CH3 8 7 5 3 1 6 4 2 H2C H2C H2 C CH2 CH2 C H2C CH3 CH2CH2CH2CH3 is the same as CH2 ALKANES 33 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
34 ALKANES (d) This group contains four carbons in its longest continuous chain. It is named as a butyl group with a cyclopropyl substituent at C-1. It is a 1-cyclopropylbutyl group and is a △ (e, f) a two-carbon group that bears a cyclohexyl substituent is a cyclohexylethyl gi umber from the point of attachment when assigning a locant to the cyclohexyl group CH,CH,一 CH H3 2-Cyclohexylethyl 1-Cyclohexylethyl 2.22 The IUPAC name for pristane reveals that the longest chain contains 15 carbon atoms(as indicated by pentadecane). The chain is substituted with four methyl groups at the positions indicated in the name Pristane(2,6, 10, 14-tetramethylpentadecane) 2.23(a) An alkane having 100 carbon atoms has 2(100)+2=202 hydrogens. The molecular formula of hectane is C1o0H2o2 and the condensed structural formula is CH,(CH,oCH3. The 100 carbon atoms are connected by 99 o bonds. The total number of o bonds is 301(99C-C bonds+ 202 C-H bonds ). (b) Unique compounds are formed by methyl substitution at carbons 2 through 50 on the 100-carbon chain(C-51 is identical to C-50, and so on). There are 49 x-methy lhectanes (c) Compounds of the type 2- x-dimethylhectane can be formed by substitution at carbons 2 through 99. There are 98 of these compounds. 2.24 Isomers are different compounds that have the same molecular formula. In all these problems the safest approach is to write a structural formula and then count the number of carbons and hydrogens (a) Among this group of compounds, only butane and isobutane have the same molecular CH CH,CH,CH CH3 CHCH3 CH3 CHCH, CH3 Cyclobutane Isobutane 2-Methylbutane CH (b The two compounds that are isomers, that is, those that have the same molecular formula, are 2, 2-dimethylpentane and 2, 2, 3-trimethylbutane CH CH CH CH,CCH,CH,CH3 CH3C--CHCH CH Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(d) This group contains four carbons in its longest continuous chain. It is named as a butyl group with a cyclopropyl substituent at C-1. It is a 1-cyclopropylbutyl group and is a secondary alkyl group. (e, f ) A two-carbon group that bears a cyclohexyl substituent is a cyclohexylethyl group. Number from the point of attachment when assigning a locant to the cyclohexyl group. 2.22 The IUPAC name for pristane reveals that the longest chain contains 15 carbon atoms (as indicated by -pentadecane). The chain is substituted with four methyl groups at the positions indicated in the name. 2.23 (a) An alkane having 100 carbon atoms has 2(100) 2 202 hydrogens. The molecular formula of hectane is C100H202 and the condensed structural formula is CH3(CH2)98CH3. The 100 carbon atoms are connected by 99 bonds. The total number of bonds is 301 (99 CGC bonds 202 CGH bonds). (b) Unique compounds are formed by methyl substitution at carbons 2 through 50 on the 100-carbon chain (C-51 is identical to C-50, and so on). There are 49 x-methylhectanes. (c) Compounds of the type 2,x-dimethylhectane can be formed by substitution at carbons 2 through 99. There are 98 of these compounds. 2.24 Isomers are different compounds that have the same molecular formula. In all these problems the safest approach is to write a structural formula and then count the number of carbons and hydrogens. (a) Among this group of compounds, only butane and isobutane have the same molecular formula; only these two are isomers. (b) The two compounds that are isomers, that is, those that have the same molecular formula, are 2,2-dimethylpentane and 2,2,3-trimethylbutane. 2,2-Dimethylpentane C7H16 CH3CCH2CH2CH3 CH3 CH3 2,2,3-Trimethylbutane C7H16 CH3C CH3 CH3 CHCH3 CH3 CH3CH2CH2CH3 Butane C4H10 Isobutane C4H10 Cyclobutane C4H8 CH3CHCH3 CH3 2-Methylbutane C5H12 CH3CHCH2CH3 CH3 Pristane (2,6,10,14-tetramethylpentadecane) CH2CH2 CH CH3 2 1 1 2 2-Cyclohexylethyl (primary) 1-Cyclohexylethyl (secondary) CHCH2CH2CH3 12 3 4 34 ALKANES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website