238 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS (b) Write the carbon skeleton corresponding to 2, 2,6, 6-tetramethylheptane Compounds of molecular formula CuH2o have two double bonds or one triple bond. The only compounds with the proper carbon skeleton are the alkyne and the allene shown. (CH,)3CC=CCH, C(CH3)3(CH3)3CCH=C==CHC(CH3)3 10.20 The dienes that give 2, 4-dimethylpentane on catalytic hydrogenation must have the same carbon skeleton as that alkane 2. 4-Dimethyl- 2. 4-Dimethylpentane 1. 3-pentadiene 2,3-pentadiene conjugated diene isolated diene cumulated diene 10.21 The important piece of information that allows us to complete the structure properly is that the ant repellent is an allenic substance. The allenic unit cannot be incorporated into the ring, because the three carbons must be collinear. The only possible constitution is therefore CH CH HO C=CHCCH 10.22 (a) Allylic halogenation of propene with N-bromosuccinimide gives allyl bromide M-bromosuccinimid H,C=CHCH3 CCI heat H,C=CHCH, Br Allyl bromide (b) Electrophilic addition of bromine to the double bond of propene gives 1, 2-dibromopropane HC=CHCH BrChcHCh Propene 1, 2-Dibromopropane (c) 1, 3-Dibromopropane is made from allyl bromide from part(a) by free-radical addition of HC=CHCHBr des BrCH2CH2CH2BI Allyl bromid Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website(b) Write the carbon skeleton corresponding to 2,2,6,6-tetramethylheptane. Compounds of molecular formula C11H20 have two double bonds or one triple bond. The only compounds with the proper carbon skeleton are the alkyne and the allene shown. 10.20 The dienes that give 2,4-dimethylpentane on catalytic hydrogenation must have the same carbon skeleton as that alkane. 10.21 The important piece of information that allows us to complete the structure properly is that the ant repellent is an allenic substance. The allenic unit cannot be incorporated into the ring, because the three carbons must be collinear. The only possible constitution is therefore 10.22 (a) Allylic halogenation of propene with N-bromosuccinimide gives allyl bromide. (b) Electrophilic addition of bromine to the double bond of propene gives 1,2-dibromopropane. (c) 1,3-Dibromopropane is made from allyl bromide from part (a) by free-radical addition of hydrogen bromide. H2C CHCH2Br Allyl bromide 1,3-Dibromopropane BrCH2CH2CH2Br HBr peroxides H2C CHCH3 Propene 1,2-Dibromopropane BrCH2CHCH3 Br2 Br H2C CHCH3 Propene Allyl bromide H2C CHCH2Br CCl4, heat N-bromosuccinimide HO CHCCH C 3 O CH3 CH3 HO CH3 or or 2,4-Dimethyl- 1,3-pentadiene conjugated diene (a) 2,4-Dimethyl- 1,4-pentadiene isolated diene 2,4-Dimethyl- 2,3-pentadiene cumulated diene (c) 2,4-Dimethylpentane (b) C H2 Pt (CH3)3CC CCH2C(CH3)3 (CH3)3CCH CHC(CH3) C 3 2,2,6,6-Tetramethyl-3-heptyne 2,2,6,6-Tetramethyl-3,4-heptadiene 238 CONJUGATION IN ALKADIENES AND ALLYLIC SYSTEMS Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website