Chapter_13 - P1 OXT/SRB JWDD052-13 P2 xxx...

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P1: OXT/SRB P2: xxx Printer: Hamilton JWDD052-13 JWDD052-Solomons-v3 June 1, 2007 18:33 13 CONJUGATED UNSATURATED SYSTEMS SOLUTIONS TO PROBLEMS 13.1 (a) 14 CH 2 CHCH 2 X CH 2 CH and 14 CH 2 X (b) The reaction proceeds through the resonance-stabilized radical. 14 CH 2 CH CH 2 14 CH 2 CH or CH 2 14 CH 2 CH δδ CH 2 Thus, attack on X 2 can occur by the carbon atom at either end of the chain since these atoms are equivalent. (c) 50:50 because attack at the two ends of the chain is equally probable. 13.2 (a) CH 4 3 CH 2 D + CH 3 1 CH 2 CH or 43 CH 2 E + CH 3 1 CH 2 CH CH 2 F CH 3 1 CH 2 δ + δ + (b) We know that the allylic cation is almost as stable as a tertiary carbocation. Here we Fnd not only the resonance stabilization of an allylic cation but also the additional stabilization that arises from contributor D in which the plus charge is on a secondary carbon atom. (c) CH CH and CH 2 Cl CH 3 CH Cl CH CH 2 CH 3 , because the Cl will attack the chain at the two positive centers shown in structure F . 13.3 (a) (b) C CH 2 CH 3 CH 2 C CH 2 CH 3 CH 2 + + + 247
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P1: OXT/SRB P2: xxx Printer: Hamilton JWDD052-13 JWDD052-Solomons-v3 June 1, 2007 18:33 248 CONJUGATED UNSATURATED SYSTEMS (f) (g) CH Br CH 2 CH Br CH 2 (c) + ++ (d) CH 2 + CH 2 + CH 2 + + O H O H (e) + + + + CH 2 CH 2 + O H (j) (h) (i) O C CH 2 CH 3 CH 2 O C CH 3 S CH 3 CH 2 + + S CH 3 CH 2 (minor) CH 3 N 2 + CH 3 + N O O N CH 3 + O O O O 13.4 (a) + because the positive charge is on a tertiary carbon atom rather than a primary one (rule 8). (b) + because the positive charge is on a secondary carbon atom rather than a primary one (rule 8).
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P1: OXT/SRB P2: xxx Printer: Hamilton JWDD052-13 JWDD052-Solomons-v3 June 1, 2007 18:33 CONJUGATED UNSATURATED SYSTEMS 249 (c) CH 2 N(CH 3 ) 2 + because all atoms have a complete octet (rule 8b), and there are more covalent bonds (rule 8a). (d) CH 3 O H O C because it has no charge separation (rule 8c). (e) because the radical is on a secondary carbon atom rather than a primary one (rule 8). (f) NH 2 CN because it has no charge separation (rule 8c). 13.5 In resonance structures, the positions of the nuclei must remain the same for all structures (rule 2). The keto and enol forms shown differ not only in the positions of their electrons, but also in the position of one of the hydrogen atoms. In the enol form, it is attached to an oxygen atom; in the keto form, it has been moved so that it is attached to a carbon atom. 13.6 (a) cis -1,3-Pentadiene, trans,trans -2,4-hexadiene, cis,trans -2,4-hexadiene, and 1,3- cyclohexadiene are conjugated dienes. (b) 1,4-Cyclohexadiene and 1,4-pentadiene are isolated dienes. (c) Pent-1-en-4-yne (1-penten-4-yne) is an isolated enyne. 13.7 The formula, C 6 H 8 , tells us that A and B have six hydrogen atoms less than an alkane. This unsaturation may be due to three double bonds, one triple bond and one double bond, or combinations of two double bonds and a ring, or one triple bond and a ring. Since both A and B react with 2 mol of H 2 to yield cyclohexane, they are either cyclohexyne or cyclohexa- dienes. The absorption maximum of 256 nm for A tells us that it is conjugated. Compound B, with no absorption maximum beyond 200 nm, prossesses isolated double bonds. We can
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Chapter_13 - P1 OXT/SRB JWDD052-13 P2 xxx...

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