postulate The H is quite general and can be better understood eration of Fig. 6.
ID: 227726 • Letter: P
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postulate The H is quite general and can be better understood eration of Fig. 6.10. One way that the postulate can be stated is to say that of a transition state resembles the stable species that is nearest it in free energy. grucrain a highly endergonic step (blue curve) the transition state lies close to exampi in free energy, and we assume, therefore, that it resembles the products of he P highly energonic step (red in structure. Conversely, in a that step in steP te to the reactants in free energy, and we assume it resembles the reactants bhat ljies close well. The great value of the Hammond-Leffler postulate is that ir gives us y exergonic step (red curve) the transition sa structurt way of visualizing those important, but fleeting, species that we call transition 0 jntuitiw hali make use of it in many future discussions. FIGURE 6.10 The trar exergonic step (red cu resembles the reactar an endergonic step( resembles the produ with permission of the Pryor,W. Free Radical state Highly Reactants state exergonic- step Highly Reactants Products step Reaction coordinate the relative rates of ethanolysis (solvolysis in ethanol) of four primary alkyl halides are as follows: CH30%Br, 1.0; CH3CH20%Br, 0.28; (CHal2CHCH2Br, 0.030, (CHalaCCH2Br, 0.00000042. a) Is each of these reactions likely to be Syl or Sy22 b) Provide an explanation for the relative reactivities that are observed. 6.13B The Effect of the Concentration and Strength of the Nucleophile The rate of an Syl reaction is unaffected by either the concentration or entity of the nucleophile, because the nucleophile does not participate in the ra The rate of an Sy2 reaction depends on borth the concentration and the identi 5 how increasing the concentration of the nucleophile increase determining step of an Sy1 reaction. the attacking nucleophile We saw in Section 6.5 increas rate of an Sy2 We 2 reaction. We can now examine how the rate of an Sy2 reaction derExplanation / Answer
a) Primary alkyl halides have very less steric hindrance and hence the electrophilic carbon atom is fully accessible for nucleophilic attack, so each of these reactions likely to be SN2.
Primary alkyl halides from unstable carbocation and hence unlikely to undergo SN1.
b) In SN2 reaction large groups present on electrophile hinders the approach of the nucleophile and hence decrease in the relative reactivities observed as the steric hindrance increases.
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