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 carbon¢carbo 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 t¦e pi bond i nto a sigma bond§ For example¥ catalytic Hydrogenation conver¤s ¤he C = C pi bond and tHe ¨ ©¨ sigma bond i nto two C ª ¨ sig«a bonds (Sectio n 7-7). The reaction is exothermic ( .0 ¡ about ¡80 to ¢120 K¬Imol or about ¢20 to ¢30 kcaL jmol­, sHowi ng that the product ®s more stable tHan the reactants¯ ' / C=C + ¨ª¨ catalyst I i ªC°C° + 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, eLim² nation, and substitutio n³ ´ n an addition, two moLecules combi ne to form o ne product molecule³ 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 moµecule splits i nto two fragment moLecuLes. ´ n a substitution¢ one fragment repLaces anotHer fragment i n a moLecuLe. AdditIon ¡ / C¶C + X-Y / ' Elimination i i ªC · 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 Carbon¡Carbon 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 nUc£eopHIle¥ A Strong e£ectro pHI£e attractS ¡He e£ect¦OnS 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 e§ectro nS¨ Fro M tHe e©ectro n-rIcH pI bo nd ¡o ¡He elec ¡ronª poor e£ec ¡ro ph ¢£e. ADDition «¬ the ­o¬® co­­o n reactio n of alke ne¬¯ a nD ° n th«¬ chaP®er We con¬iD± er aDDi®«on¬ ®o alkenes « n De®a«²¥ A W«De Varie®Y of f³nctiona² Gro³P¬ can Be for­eD BY aDD« nG ¬³«®aBle reaGen®¬ to ®he Do³Ble Bo nDs of a²ke ne¬¥ ´n Princ«Ple¯ ­anY D«µerent reaGen®¬ co³lD aDD ®o a Do³B²e ¶o nD to for­ ­ore ¬®aB²e ProD³c®s; tha® is · ®he reac®«o ns are e nerGe®«ca²lY faVoraBle. Not a²² of these react«o ns haVe conve n«e nt ra®es¯ hoWever¥ ¸or exa­Ple¹ the react«on of e®hY²e ne With hYDroGen º®o Gi ve e®hane» i¬ ¬®ro nGlY exother­«c¯ B³t the rate «¬ VerY ¬loW¥ A ­ix®³re of ethYlene anD hYDroGen can re­ai n for Year¬ W«tho³t aPPreciaB²e reac®io n¥ ADD« nG a catalYst ...
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