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Ch 4 - The Study of Chemical Reactions - 4 p orbital vacant...

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Ch 4 - The Study of Chemical Reactions

Ch 4 - The Study of Chemical Reactions - 4 p orbital vacant...

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4 The Study of Chemical Rea±²Ions The most interesting and useful aspect of organic chemIstry Is the study of ReactIons. We cannot RemembeR thousands of specif±c organic reactIons, but we can oRganIze the Reactions Into LogIcaL groups based on how the ReactIons take pLace and what interme- diates are InvoLved. We begin our study by consIdeRing the halogenation of alkanes² a relatively sImple reaction that takes pLace In the gas phase, wIthout a solvent to com³ plicate the reaction. ´n pRactIce, aLkanes are so unReactive that they aRe RaRely used as starting materials foR most organIc syntheses. We start wIth them because we have aLready studIed theIr stRucture and propeRtIesµ and their reactions aRe RelativeLy uncom¶ plIcated· Once we have used alkanes to IntRoduce the tools for studyIng ReactIons¸ we wIll appLy those tools to a vaRiety of moRe usefuL ReactIons¹ The overalL ReactIonº with the reactants on the Left and the pRoducts on the RIght¸ is only the F»rst step In ouR study of a ReactIon. ´f we truLy want to understand a reactIon² we must also know the mechanism, the step¼by½step pathway fRom Reactants to pRoducts. To know how welL the Reaction goes to products¾ we study ¿ts thermo- dynamics± the energetics of the ReactIon at equILIbrium. The amounts of Reactants and pRoducts pResent at equILibrium depend on theIr reLative stabilItIesÀ Even though the equILIbRium may favoR the formatIon of a product, the reaction may not take pLace at a usefuL rate. To use a Reaction in a ReaListic tIme peRIod (and to keep the reactIon fRom becoming vIolent), we study its kinetics² the vaRiatIon of Reac³ tion Rates with diÁeRent condItIons and concentRatIons of reagents¹ UndeRstanding the ReactIon ' s kinetIcs helps us to pRopose ReactIon mechanIsms that aRe consIstent wIth the propeRties we observe. The reactIon of methane with chLorine produces a mIxtuRe of chlorInated productsµ whose compositIon depends on the amount of chLorIne added and also on the Reaction conditIons¹ EitheR lIght oR heat Is needed foR the ReactIon to take place at a usefuL Rate. When chloRine is added to methane the F»rst reactIon is H H I heat or light i H-C-H + C ] - C l ) H ±C± CL ± H ± ± i H H methane chloRine cHloromethane hydrogen (methyl chloride) chloRIde vacant p oRb±taL \ caRBocAt±on al²yl GRoup 4-1 Introduction 4-2 Ch lori nation of Methane 125
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126 Chapter 4: The Study of Chemical Reactions 4-3 The Free±Radical Chain Reaction Thi± reaction May con²inUe; hea² or Light i± needed For each ±²ep: H C ³ C I I H -C- i H-C- C µ I - I C ] ·C· I H ± H + H C l I H + H C ¹ I ± H Thi± ±eqUence rai±e± ±evera¸ qUe±tion± aboU² ²he chlorina²ion oF Me²hane. Why i± heat or light needed for the reaction ²o go? Why do we get a Mix²Ure of prodUct±? º± there any way to Modify the reaction ²o get jU±t one pUre prodUct? Are the ob±erved prodUc²± ForMed becaU±e ²hey are ²he Mo±t ±²abLe prodUct± po±±ible? Or are they Fa» vored becaU±e ²hey are ForMed Fa±²er ²han any other prodUc²±?
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