Organic Chemistry : Identifying Electron-Donating Groups
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Example Question #1 : Identifying Electron Donating Groups
Electron donation and withdrawal have important impacts on acidity. What R group would yield the species with the highest pKa?
This question tests your knowledge about electron donation, as well as acidity. The question asks you to identify the species with the highest pKa, which means you need to look for the R group that will be the most donating. Electron donation will destabilize the conjugate base anion, localized partially on the oxygen of the hydroxyl moiety; the result is a less acidic acid, and a higher pKa.
Of the answer choices, three are electron-withdrawing. These include the nitro (-NO2), the ketone (-CCOMe), and the carboxyl (-COOH). The only answer choices that are electron-donating are the methyl (-Me), and the ether (-OMe). As the ether can push its lone pairs into the pi system of the ring and the carboxyl group, it is the stronger electron-donating group. The methyl can only donate electrons through inductive effects, or electronic polarization of sigma bonds, which is not as strong of an electron donation effect. Thus, -OMe is the correct answer.
Example Question #2 : Identifying Electron Donating Groups
Which of the given compounds is not a Lewis acid?
All of these are Lewis acids
A Lewis acid is an electron pair acceptor. A Lewis base is an electron pair donor.
The carbocation is clearly trying to accept electrons due to the positive charge. 
The correct answer has a lone pair on the nitrogen, and thus has electrons to donate and as a Lewis base. 
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