Sn2 reaction transition state for

Electrostatic potential map for transition state for Sn2 reaction of trimethylamine and methyl iodide shows negatively-charged regions (in red) and positively-charged regions (in blue). 

In Section 3 (p. 148) we saw that both early and late transition states for SN2 reactions of methyl derivatives are likely to have progressed about 50% in terms of their charge development. This proposal is quite contrary to the generally accepted view and in this section we wish to discuss how general this disparity between charge and geometric progression is likely to be. 

To determine the extent to which a nucleophile is involved in the transition state for Sn2 reactions of i and ii, Swain-Scott plots were generated (see right). The slopes of the plots (s values) for / and ii are only 0.15 and 0.18, respectively, meaning that the rate of the reaction does not depend very much upon the reactivity of the nucleophile. 

Figure 6 8 Transition states for Sn2 reactions of hydroxide ion with...

Figure 6-10 Hashed-wedged line and space-filling renditions of the transition states for Sn2 reactions of hydroxide ion with (A) 1-chloropropane, (B) 1-chloro-2-methylpropane, and (C) 1-chloro-2,2-dimethylpropane. Increasing steric hindrance from a second gauche interaction reduces the rate of reaction in (B). Sn2 reactivity in (C) is eliminated almost entirely because a methyl group prevents backside attack by the nucleophile in all accessible conformations of the substrate. (See also Figures 6-8 and 6-9.)...

Fig. 13 The relative structures for the SN2 transition states for the reactions of p-substituted benzyl chlorides with cyanide ion. Reproduced, with permission, from...

TABLE 10.11 Transition states for SnI reactions of charged and uncharged substrates, and for Sn2 reactions of the four charge types366... 

The high reactivity of allylic halides in SN2 reactions indicates some special stabilization of the transition state ascribable to resonance involving the adjacent tt bond. We can express this in terms of the valence-bond structures, 2a-2c, for the transition state of the reaction of iodide ion with 3-chloropropene (Section 8-7A). The extra stabilization over the corresponding transition state for the reaction of iodide with a saturated chloride (e.g., CH3CH2CH2C1 + 1 ... 

Consider the free energy versus reaction progress diagram for the SN2 reaction shown in Figure 8.1. Does the transition state for this reaction have the C—Cl bond less than half broken, approximately half broken, or more than half broken ... 

Here you see a typical Sn2 reaction of allyl bromide. We have drawn the transition state for this reaction. This is not because we want to encourage you to do this for all Sn2 reactions but so that we can explain the role of the allyl system. Allyl compounds react rapidly by the 8 2 mechanism because the double bond can stabilize the transition state by conjugation. 

TABLE 10.10. Transition States for SnI Reactions of Charged and Uncharged Substrates, and for Sn2 Reactions of the Eour Charge Types ... 

The idealized Sn2 transition state has a linear arrangement of Nu- C- -X. Use Spar-tanView to examine Su2 transition states for the reaction of Cl" with CHaBr, CHrjCH Br, (CH3)2CHBr, (CH. liCBr, and (CH3)sCCH2Br. Which transition. state is most ideal Identify significantly distorted bond angles in the other transition states. 

Inversion of configuration always accompanies SN2 reactions the transition state is shown schematically in Figure 7.1. An everyday analogy of this inversion is shown by an umbrella that blows inside out on a windy day of course, the umbrella has more spokes than a carbon atom has valencies. This analogy has some further merit in that the nucleophile, the central carbon and the leaving group are collinear, and remain so in the transition state for the reaction. 

To understand how the solvent affects nucleophilic aliphatic substitution reactions, we must first examine the possibilities for the creation or destruction of charge in Sn2 and SnI reactions. Table 11.2 shows the possibilities, considering charges on reactants and transition states for Sn2 and SnI reactions. 

In the transition state for the Sn2 reaction of the allyl halide, there is overlap between the 2p orbital developed on the carbon at which displacement occurs and the 2/> orbitals of the double bond (Fig. 12.49). Therefore, this transition state contains an allylic system. It will benefit energetically from the delocalization of electrons, and will lie at lower energy than the transition state for Sn2 displacement in the 1-propyl system, in which there is no delocalization. 

Benzyl halides are also especially reactive in the Sn2 reaction. The reason is the same as that for the enhanced reactivity of allyl chloride (p. 543).The transition state for Sn2 displacement is delocalized, and therefore especially stable. If the transition state for a reaction is at relatively low energy, the activation energy for the reaction will also be relatively low and the reaction will be fast (Fig. 13.73). 

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