Effective charge maps

Effective Charge Maps for More Than Two Bond Changes... 

The displacement of substituted phenoxide ions from aryl monophosphate monoanions by nicotinamide (Equation 38) has a PLg of -0.95. Reaction of substituted pyridines with 2,4-dinitrophenyl-phosphate monoanion has a 3 of 0.56. Using effective charge data from Scheme 1 construct a combined effective charge map for the... 

Scheme Effective charge map for the alkaline hydrolysis of aryl acetate esters. Both Bronsted and Hammett terminologies are illustrated but this makes no difference to the magnitude of either Leffler s a or the effective charges...

Scheme 3 Effective charge map for aryl oxygen in the reactions of fluorenoyl esters. The ketene intermediate reacts rapidly with water and hydroxide ion to give the acid product...

Effective Charge Map for a Putative Stepwise Process. Effective charge maps can also be employed to discount a stepwise process if estimates of the effective charge change from reactant and product to putative intermediates are not consistent with expectation. Consider the Bronsted dependence for reaction of substituted phenolate ions with 4-nitrophenyl acetate (Figure 6). The value of Pnu is approximately 0.80 for attack of substituted phenolate ions on 4-nitrophenyl acetate when the second step of the putative two-step mechanism (decomposition of the putative tetrahedral intermediate) would be rate limiting (pA pA a ) (Scheme 11). 

Scheme 11 Effective charge map for putative stepwise mechanism for phenoxide ion attack on 4-nitrophenyl acetate...

Assuming the conservation of effective charge, construct a full effective charge map for an identity reaction of phenolate ions with... 

The Bronsted in the range of > pA a (7-10) is 0.95. The overall Peq for the reaction is 1.42. Draw an effective charge map for the reaction and apply the conservation of effective charge to show that the Peq for the second step (Keq(2)) is too large to support the stepwise process shown. 

Scheme 1 (Chapter 3) shows that aryl acetate esters and aryl thioacetate esters have effective charges of -1-0.7 and -t-0.4 respectively on the O and S leaving atom. The effective charge map for the equilibrium can thus be written as in Scheme 3. 

The Peq is independent of the nucleophile and reference to (Chapter 3, Scheme 1) shows that the effective charge on the pyridinium nitrogen is +1.6 in 7-acetylpyridinium ions. The following effective charge map (Scheme 6) can be drawn ... 

The effective charge map for the sulfation of phenols with pyridine-A/ -sulfonate esters is illustrated in Schemes 13 and 14 for N-S bond fission and S-O bond formation. Effective charges are defined respectively by the dissociation reactions of substituted pyridinium ions and phenols. The pyridine nitrogen and the phenol oxygen suffer respectively changes in effective charge of - 1.00 and 4-0.23 units from reactant to transition state structure. 

The effective charge on the product Xpy-POjH" is derived by consideration of the map of Scheme 16 and the data yield the combined effective charge map as shown in Scheme 17 ... 

The alkaline hydrolysis of aryl acetates has an effective charge map (Scheme 24) derived from the given data ... 

---