Directing Electron-Donating Effects of Alkyl Groups

DIRECTING ELECTRON-DONATING EFFECTS OF ALKYL GROUPS 

We are now ready to tackle the question of the regioselectivity (orientation) of electrophilic aromatic substitution of substituted benzenes. What controls the position along the benzene ring at which an electrophile will attack We begin with the electrophilic substitution 

Groups that donate electrons by induction and hyperconjugation are activating and direct ortho and para 

Electrophilic bromination of methylbenzene (toluene) is considerably faster than the bromi-nation of benzene itself. The reaction is also regioselective It results mainly in para (60%) and ortho (40%) substitutions, with virtually no meta product 

Is bromination a special case The answer is no nitration, sulfonation, and Friedel-Crafts reactions of the alkylbenzene give similar results— mainly ortho and para substitutions (see also Table 16-2 in Section 16-3). Evidently, the nature of the attacking electrophile has hole influence on the observed orientation it is the alkyl group that matters. Because there is virtually no meta product, we say that the activating methyl substituent is ortho and para directing. 

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Directing Electron-Donating Effects of Alkyl Groups CHAPTER 16... 

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