ALCOHOLS AND ALKYL HALIDES 4.18 Writing the structural formula for ethyl chloride reveals that there are two nonequivalent sets of hydrogen atoms, in either of which a hydrogen is capable of being replaced by chlorine CHCHCI CH, CICH, CH,CI Ethyl chloride L. 1-Dichloroethane The two dichlorides are 1.1-dichloroethane and 1. 2-dichloroethane. 4.19 Propane has six primary hydrogens and two secondary. In the chlorination of propane, the relative proportions of hydrogen atom removal are given by the product of the statistical distribution and the relative rate per hydrogen. Given that a secondary hydrogen is abstracted 3.9 times faster than a pri mary one, we write the expression for the amount of chlorination at the primary relative to that at the Number of primary hydrogens X rate of abstraction of primary hydrog 6×10.77 Number of secondary hydrogens X rate of abstraction of a secondary hydrogen 2 X 3.9 1.00 Thus, the percentage of propyl chloride formed is 0.77/1.77, or 43%, and that of isopropyl chloride is 57%. (The amounts actually observed are propyl 45%, isopropyl 55%0.) 4.20(b) In contrast with free-radical chlorination, alkane bromination is a highly selective process The major organic product will be the alkyl bromide formed by substitution of a tertiary hydrogen with a b omine Br CH(CH3)2 C(CH3), Tertiary Br 1-lsopropy l-1 1-(1-Bromo-1-methylethyl)- (c) As in part(b), bromination results in substitution of a tertiary hydrogen CH CCH, CHCH CHCCHCCH 2, 2, 4-Trimethylpentane 4.21 (a) Cyclobutanol has a hydroxyl group attached to a four-membered ring (b) sec-Butyl alcohol is the functional class name for 2-butanol CH3 CHCH2 CH3 OH sec-Butyl alcohol (c) The hydroxyl group is at C-3 of an unbranched seven-carbon chain in 3-heptanol CH3 CH, CHCH, CHL CH2CH3 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
4.18 Writing the structural formula for ethyl chloride reveals that there are two nonequivalent sets of hydrogen atoms, in either of which a hydrogen is capable of being replaced by chlorine. The two dichlorides are 1,1-dichloroethane and 1,2-dichloroethane. 4.19 Propane has six primary hydrogens and two secondary. In the chlorination of propane, the relative proportions of hydrogen atom removal are given by the product of the statistical distribution and the relative rate per hydrogen. Given that a secondary hydrogen is abstracted 3.9 times faster than a primary one, we write the expression for the amount of chlorination at the primary relative to that at the secondary position as: Thus, the percentage of propyl chloride formed is 0.771.77, or 43%, and that of isopropyl chloride is 57%. (The amounts actually observed are propyl 45%, isopropyl 55%.) 4.20 (b) In contrast with free-radical chlorination, alkane bromination is a highly selective process. The major organic product will be the alkyl bromide formed by substitution of a tertiary hydrogen with a bromine. (c) As in part (b), bromination results in substitution of a tertiary hydrogen. 4.21 (a) Cyclobutanol has a hydroxyl group attached to a four-membered ring. (b) sec-Butyl alcohol is the functional class name for 2-butanol. (c) The hydroxyl group is at C-3 of an unbranched seven-carbon chain in 3-heptanol. OH CH3CH2CHCH2CH2CH2CH3 3-Heptanol OH CH3CHCH2CH3 sec-Butyl alcohol OH Cyclobutanol CH3CCH2CHCH3 2,2,4-Trimethylpentane 2-Bromo-2,4,4-trimethylpentane light Br2 CH3 CH3 CH3 CH3CCH2CCH3 CH3 CH3 CH3 Br CH(CH3)2 CH3 C(CH3)2 CH3 Br Tertiary hydrogen Br2 light 1-(1-Bromo-1-methylethyl)- 1-methylcyclopentane 1-Isopropyl-1- methylcyclopentane 0.77 1.00 6 1 2 3.9 Number of primary hydrogens rate of abstraction of primary hydrogen Number of secondary hydrogens rate of abstraction of a secondary hydrogen CH3CH2Cl Ethyl chloride CH3CHCl2 1,1-Dichloroethane 1,2-Dichloroethane ClCH2CH2Cl light or heat Cl2 72 ALCOHOLS AND ALKYL HALIDES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
ALCOHOLS AND ALKYL HALIDES (d) a chlorine at C-2 is on the opposite side of the ring from the C-l hydroxyl group in trans 2-chlorocyclopentanol. Note that it is not necessary to assign a number to the carbon that bears th requires the hydroxyl group to be located at C-1 (e) This compound is an alcohol in which the longest continuous chain that incorporates the hydroxyl function has eight carbons. It bears chlorine substituents at C-2 and C-6 and methyl and hydroxyl groups at C-4 CH3 CH,CHCH,CCH,CHCH, CH3 OH CI (f) The hydroxyl group is at C-1 in trans-4-tert-butylcyclohexanol; the tert-butyl group is at C-4. The structures of the compound can be represented as shown at the left; the structure at the right depicts it in its most stable conformation 3)C (CH)3C trans-4-fert-Butylcyclohexanol (g) The cyclopropyl group is on the same carbon as the hydroxyl group in 1-cyclopropylethanol ch) The cyclopropyl group and the hydroxyl group are on adjacent carbons in 2-cyclopro- CH..OH 4.22(a) This compound has a five-carbon chain that bears a methyl substituent and a bromine. The numbering scheme that gives the lower number to the substituent closest to the end of the chain is chosen. Bromine is therefore at C-l, and methyl is a substituent at C-4 CH CHCH, CH, CH, Br Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(d) A chlorine at C-2 is on the opposite side of the ring from the C-1 hydroxyl group in trans- 2-chlorocyclopentanol. Note that it is not necessary to assign a number to the carbon that bears the hydroxyl group; naming the compound as a derivative of cyclopentanol automatically requires the hydroxyl group to be located at C-1. (e) This compound is an alcohol in which the longest continuous chain that incorporates the hydroxyl function has eight carbons. It bears chlorine substituents at C-2 and C-6 and methyl and hydroxyl groups at C-4. ( f ) The hydroxyl group is at C-1 in trans-4-tert-butylcyclohexanol; the tert-butyl group is at C-4. The structures of the compound can be represented as shown at the left; the structure at the right depicts it in its most stable conformation. (g) The cyclopropyl group is on the same carbon as the hydroxyl group in 1-cyclopropylethanol. (h) The cyclopropyl group and the hydroxyl group are on adjacent carbons in 2-cyclopropylethanol. 4.22 (a) This compound has a five-carbon chain that bears a methyl substituent and a bromine. The numbering scheme that gives the lower number to the substituent closest to the end of the chain is chosen. Bromine is therefore at C-1, and methyl is a substituent at C-4. CH3 CH3CHCH2CH2CH2Br 1-Bromo-4-methylpentane CH2CH2OH 2-Cyclopropylethanol CHOH CH3 1-Cyclopropylethanol trans-4-tert-Butylcyclohexanol (CH3)3C OH (CH3)3C H H OH 2,6-Dichloro-4-methyl-4-octanol OH CH3CHCH2CCH2CHCH2CH3 Cl Cl CH3 OH Cl trans-2-Chlorocyclopentanol ALCOHOLS AND ALKYL HALIDES 73 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
74 ALCOHOLS AND ALKYL HALIDES (b) This compound has the same carbon skeleton as the compound in part (a) but bears a hydroxyl group in place of the bromine and so is named as a derivative of l-pentanol CH CHCH,CHCH,OH (c) This molecule is a derivative of ethane and bears three chlorines and one bromine The name 2-bromo-l, 1, I-trichloroethane gives a lower number at the first point of difference than 1-bromo-2. 2.2-trichloroethane. ClCCH, Br 2-Bromo.1.1.1-trichloroethane (d) This compound is a constitutional isomer of the preceding one. Regardless of which carbon the numbering begins at, the substitution pattern is 1, 1, 2, 2. Alphabetical ranking of the halo- gens therefore dictates the direction of numbering. Begin with the carbon that bears bromine ClCHCHBr I-Bromo-1. 2.2-trichloroethane (e) This is a trifluoro derivative of ethanol. The direction of numbering is dictated by the hydroxyl group, which is at C-l in ethanol CF, CHOH 2.2.2-Trifluoroethanol (f) Here the compound is named as a derivative of cyclohexanol, and so numbering begins at the carbon that bears the hydroxyl group cis-3-tert-Butylcyclohexanol (g) This alcohol has its hydroxyl group attached to C-2 of a three-carbon continuous chain; it is named as a derivative of 2-propanol - OH 2-Cyclopentyl-2-propanol (h) The six carbons that form the longest continuous chain have substituents at C-2, C-3, and c-5 when numbering proceeds in the direction that gives the lowest locants to substituents at the first point of difference. The substituents are cited in alphabetical order 5-Bromo-2.3-dimethylhexane Had numbering begun in the opposite direction, the locants would be 2, 4, 5 rather than 2,3,5 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
(b) This compound has the same carbon skeleton as the compound in part (a) but bears a hydroxyl group in place of the bromine and so is named as a derivative of 1-pentanol. (c) This molecule is a derivative of ethane and bears three chlorines and one bromine. The name 2-bromo-1,1,1-trichloroethane gives a lower number at the first point of difference than 1-bromo-2,2,2-trichloroethane. (d) This compound is a constitutional isomer of the preceding one. Regardless of which carbon the numbering begins at, the substitution pattern is 1,1,2,2. Alphabetical ranking of the halogens therefore dictates the direction of numbering. Begin with the carbon that bears bromine. (e) This is a trifluoro derivative of ethanol. The direction of numbering is dictated by the hydroxyl group, which is at C-1 in ethanol. ( f ) Here the compound is named as a derivative of cyclohexanol, and so numbering begins at the carbon that bears the hydroxyl group. (g) This alcohol has its hydroxyl group attached to C-2 of a three-carbon continuous chain; it is named as a derivative of 2-propanol. (h) The six carbons that form the longest continuous chain have substituents at C-2, C-3, and C-5 when numbering proceeds in the direction that gives the lowest locants to substituents at the first point of difference. The substituents are cited in alphabetical order. Had numbering begun in the opposite direction, the locants would be 2,4,5 rather than 2,3,5. 1 2 3 4 5 6 Br 5-Bromo-2,3-dimethylhexane CH3 CH3 OH 2-Cyclopentyl-2-propanol cis-3-tert-Butylcyclohexanol OH 2,2,2-Trifluoroethanol CF3CH2OH 1-Bromo-1,2,2-trichloroethane Cl2CHCHBr Cl Cl3CCH2Br 2-Bromo-1,1,1-trichloroethane CH3 CH3CHCH2CH2CH2OH 4-Methyl-1-pentanol 74 ALCOHOLS AND ALKYL HALIDES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website