Leaving group effect

Partly due to their availability, most work has featured variations in reactivity in halogenoarenes, where the order of reactivity most commonly fonnd is F Cl Br l [1, 9, 32]. This is shown by the results in Table 6.2 for reaction of l-halogeno-4-nitrobenzenes with methoxide ions in methanol. The high reactivity of aryl flnorides is readily explained by the two-step S Ar mechanism with the first step, nucleophilic attack, being rate limiting. The strong polarization of the bond 

TABLE 6.2 Relative Reactivities of Methoxide and Thiophenoxide Ions Toward l-Halogeno-4-nitrobenzenes in Methanol 

Adapted with permission from Bartoli and Todesco [52], (1977) American Chemical Society. 

The high nucleofugality of the nitro group is noteworthy, due in part to its ability to stabilize the cyclohexadienyl intermediate (Table 6.1). Its high polarizability makes it partieularly suseeptible to replacement by polarizable nucleophiles, such as thiolate ions. Positively charged substituents, NMCj and SMe, are also readily replaced [53], as are protonated heteroarenes such as imidazo- 

It is worth noting that within a closely related series, good linear free energy relationships have been observed. Thus, in the reactions of ring-substituted l-phenoxy-2,4-dinitrobenzenes and 2-phenoxy-5-nitropyridines with methoxide and hydroxide ions, linear Hammett plots of log k versus the pAT value of the departing substituted phenoxide ions have been observed [1, 54]. 

---

Leaving-group effects Because aryl fluorides have the strongest carbon-halogen... 

Figure 11.7 Energy diagrams showing the effects of (a) substrate, (b) nucleophile, (c) leaving group, and (d) solvent on Sn2 reaction rates. Substrate and leaving group effects are felt primarily in the transition state. Nucleophile and solvent effects are felt primarily in the reactant ground state.

Leaving group effects on the rate of reaction (174) at 250C°... 

Leaving group effects A good leaving group must be a weak base, and it should be stable after it has left with the bonding electrons. Thus, the weaker... 

Table 15 Leaving Group Effect on Pyridine Substitution Rates in PtX(dien)+...

---