The Effect of the Leaving Group - Aromatic Substitution, Electrophilic - Introduction - March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

Part II. Introduction

Chapter 11. Aromatic Substitution, Electrophilic

11.E. The Effect of the Leaving Group

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In the vast majority of aromatic electrophilic substitutions, the leaving group is H+ as indicated above, and very little work has been done on the relative electrofugal ability of other leaving groups. However, the following orders of leaving-group ability have been suggested106: (1) for leaving groups that depart without assistance (SN1 process with respect to the leaving group), NO2+107 < iPr+ ~ SO3 < t-Bu+ ~ArN2+ < ArCHOH+ < NO+ < CO2; (2) for leaving groups that depart with assistance from an outside nucleophile (SN2 process), Me+ < Cl+ < Br+ < D+ ~ RCO+ < H+ ~ I+ < Me3Si+. We can use this kind of list to help predict which group, X or Y, will cleave from an arenium ion 30(see 1, where Y = H) once it has been formed, and so obtain an idea of which electrophilic substitutions are feasible. However, a potential leaving group can also affect a reaction in another way: by influencing the rate at which attack of the original electrophile leads to attachment directly at the ipso position. Partial rate factors for electrophilic attack at a position substituted by a group other than hydrogen are called ipso partial rate factors img.57 Such factors for the nitration of p-haloanisoles are 0.18, 0.08, and 0.06, for p-iodo, p-bromo-, and p-chloroanisole, respectively.108 This means, for example, that attack at the electrophile in this case leads to attachment at the 4 position of 4-iodoanisole 0.18 times as fast as a single position of benzene. Note that this is far slower than attachment at the 4 position resulting from attack of anisole itself so that the presence of the iodo group greatly slows the reaction at that position. A similar experiment on p-cresol showed that ipso attack at the methyl position was 6.8 times slower than attack of phenol leading to attachment at the para position.109 Thus, in these cases, both an iodo and a methyl group deactivate the ipso position.110