1. A. E2 shows a Primary (Deuterium) Isotope Effect. What is a Primary (Deuteriu
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1. A. E2 shows a Primary (Deuterium) Isotope Effect. What is a Primary (Deuterium) Isotope Effect? B. According to this is hydrogen lost at a fast or slow step? 2. A. E2 shows a (Halogen) Element Effect. What is an Element Effect? B. According to this is the halogen lost at a fast or slow step? 3. Why do the Element Effect plus the Isotope Effect suggest that the E2 is a concerted reaction?Explanation / Answer
1.a)The Hydrogen Effect is an effect that alters the Rate of product formation as a result of a carbon hydrogen bond being broken in the rate determining step. Since the E2 path involves only one step which is the rate determining step, the carbon hydrogen bond is broken in the rate determining step. If we begin with an alkyl halide whose beta hydrogens have been replaced with DEUTERIUM atoms then the breaking of the Carbon deuterium bond will be more difficult since the dissociation energy of a carbon deuterium bond is greater that that of a Carbon-Hydrogen bond. This is termed as Primary Isotope effect. 1.b) In E-2 reactions , hydrogen is lost in the slow step which is rate determining step. 2.a) The reactions also show a substantial element effect. The relative reactivity dpends on halogen R-F> R-CL>R-BR>R-I ( R IS AN ALKYL GROUP) An element effect of this kind is again in keeping with a transition state involving carbon-halogen bond cleavage. Thus, 2-phenylethyl iodide reacts fastest because the carbon-iodine bond is weakest; 2-phenylethyl fluoride reacts slowest as the carbonfluorine bond is strongest. 2.B)HALOGEN IS LOST IN SLOW STEP SO A GOOD LEAVING GROUP IS REQUIRED. 3) E2 reactions are typically seen with secondary and tertiary alkyl halides, but a hindered base is necessary with a primary halide. In reactions like E2 elimination (it is a concerted reaction, the two substituents are eliminatedin the same step) several bonds are made and broken simultaneously.
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