Ch 8 - reactions of alkenes

Ch 8 - Reactions of Alkenes
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Unformatted text preview: 8 Reactions of Alkenes A ll alke nes have a commo n feature: a carbo n-carbon double bo nd. The reac tio ns of alke nes arise from tHe reactivity of the carboncarbo n double bo nd. O nce agai n, the co ncept of tHe fu nctio naL group HeLps to organize and sim pLify the study of cHemical reactio ns. By studyi ng the characteristic reactions of the doubLe bo nd, we can predict the reactio ns of aL ke nes we have never seen before. Because si ngle bonds (sigma bonds) are more stabLe han pi bonds tHe most coMon re actio ns of double bonds transform te pi bond i nto a sigma bond For example catalytic Hydrogenation convers he C = C pi bond and tHe sigma bond i nto two C siga bonds (Sectio n 7-7). The reaction is exothermic ( .0 about 80 to 120 KImol or about 20 to 30 kcaL jmol, sHowi ng that the product s more stable tHan the reactants ' / C=C + catalyst I i CC + energy / ' ) I I ydrogenatio n of an alkene is an example of an addition, one of tHe tHree maJor reactio n types we have stUdied addition, eLim nation, and substitutio n n an addition, two moLecules combi ne to form o ne product molecule WHen an aLke ne u ndergoes addition, two groups add to the carbon atoms of tHe doubLe bo nd and the carbo ns become saturated. n many ways additio n is the reverse of elimination i n wHicH one moecule splits i nto two fragment moLecuLes. n a substitution one fragment repLaces anotHer fragment i n a moLecuLe. AdditIon / CC + X-Y / ' Elimination i i C C i i X y SubstitUtion I - C X + y C=C / i I I C C I I X Y + X Y C- y + X 8-1 Reactivity of the CarbonCarbon Double Bond 321 322 Chapter 8: Reactio ns of Alkenes 8-2 Electrophilic Addition to AL kenes . Figure 8-1 THe eLec ronS In tHe pI bond are Spread FartHer From tHe carbon nucle and are More ooSe y He d tHan tHe SI gma electronS. FIguRe 8-2 He pI bo nd aS a nUceopHIle A Strong eectro pHIe attractS He eectOnS oUt of tHe pI bo nd to Form a ne w SI gma bo nd, ge nerating a carbocatIo n. THe (red) c urved arrow SHo wS tHe mo vemen oF eectro nS Fro M tHe eectro n-rIcH pI bo nd o He elec ron poor eec ro ph e. ADDition the o coo n reactio n of alke ne a nD n th chaPer We coniD er aDDion o alkenes n Dea A WDe VarieY of fnctiona GroP can Be foreD BY aDD nG aBle reaGen to he DoBle Bo nDs of ake ne n PrincPle anY Derent reaGen colD aDD o a DoBe o nD to for ore aBe ProDcs; tha is he reaco ns are e nerGecalY faVoraBle. Not a of these reacto ns haVe conve ne nt raes hoWever or exaPle the reacton of ehYe ne With hYDroGen o Gi ve ehane i ro nGlY exotherc Bt the rate VerY loW A ixre of ethYlene anD hYDroGen can reai n for Year Wthot aPPreciaBe reacio n ADD nG a catalYst ...
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