Ch 4 - The Study of Chemical Reactions - 4 The Study of...

Ch 4 - The Study of Chemical Reactions
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Unformatted text preview: 4 The Study of Chemical ReaIons The most interesting and useful aspect of organic chemIstry I s the study of ReactIons. We cannot RemembeR thousands of specifc 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 alkanes a relatively sImple reaction that takes pLace In the gas phase, wIthout a solvent to com 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 propeRtIes 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 ReactIons The overalL ReactIon with the reactants on the Left and the pRoducts on the RIght is only the Frst step In ouR study of a ReactIon. f we truLy want to understand a reactIon we must also know the mechanism, the stepbystep pathway fRom Reactants to pRoducts. To know how welL the Reaction goes to products 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 stabilItIes 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 kinetics the vaRiatIon of Reac tion Rates with dieRent condItIons and concentRatIons of reagents 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 products whose compositIon depends on the amount of chLorIne added and also on the Reaction conditIons EitheR lIght oR heat Is needed foR the ReactIon to take place at a usefuL Rate. When chloRine is added to methane the Frst reactIon is H H I heat or light i H-C-H + C ]- C l ) H C CL H C i H H methane chloRine cHloromethane hydrogen (methyl chloride) chloRIde vacant p oRbtaL \ caRBocAton alyl GRoup 4-1 I ntroduction 4-2 Ch lori nation of Methane 125 1 26 Chapter 4: The Study of Chemical Reactions 4-3 The FreeRadical Chai n Reaction Thi reaction May coninUe; hea or Light i needed For each ep: H C C I C I H-C- C i H-C- C I C- C C I C ] C C I H H + H C l I H + H C I C H C Thi eqUence raie evera qUetion aboU he chlorinaion oF Mehane. Why i heat or light needed for the reaction o go? Why do we get a MixUre of prodUct? there any way to Modify the reaction o get jUt one pUre prodUct? Are the oberved prodUc ForMed becaUe hey are he Mot abLe prodUct poible? Or are they FaprodUc ForMed becaUe hey are he Mot abLe prodUct poible?...
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