Substituted benzenes para substitution, definition

Because the rate of substitution varies with position, in a benzene derivative it is more informative and frequently more useful to talk about partial rate factors than about relative rates. A partial rate factor is defined as the rate at one particular position in the benzene derivative relative to the rate of substitution at one position in benzene. Let us, for example, calculate the para and meta partial rate factors (pf and mf, respectively) for bromination of toluene with bromine in aqueous acetic acid. Toluene brominates 605 times faster than benzene under these conditions. The product is 66.8 percent p-, 0.3 percent m-, and 32.9 percent o-bromotoluene. Attack at the para position of toluene occurs 0.668 x 605 times as fast as attack at all six positions of benzene but (0.668 x 605 x 6 = 2420) times as fast as at one position of benzene. Therefore pfCH for bromination of toluene under these conditions is 2420. There are only three times as many total carbons in benzene as meta carbons in toluene. Therefore mfca3 = 0.003 x 605 x 3 = 5.5. The definitions of the partial rate factors for monosubstituted benzenes (—R) are given in Equations 7.78-7.80. 

Aromatic bromides (3, 286). The definitive paper on electrophilic aromatic bromination with bromine and thallium(lll) acetate has been published. The two most oul.standingfeaturc.s are I) monobromination is observed in almost all cases, and 2) exclusive para substitution is observed with almost all monosubstituted benzenes. Electron-withdrawing groups inhibit bromination of monosubstituted benzenes. It... 

Di-substitution Products.—Usually however the di-substitution products are designated by numbers as first indicated. The names ortho, meta and para are also sometimes used exactly as in the benzene products together with other similar names applying to definite pairs of positions. By examining the formula we shall find that ten isomeric di-substitution products of naphthalene are possible in case the two substituents are the same. These ten with their numerical designations and names are as follows ... 

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