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182 Organic Chemistry Solutions Manual Purpose of the problem Revision of electrophilic substitution and a problem in nucleophilic aromatic substitution. Suggested solution The first reaction is standard nitration of benzene with an ortho, para-directing group (Chapter 22). The first nitration can go either 2- or 4- to the chlorine and the second goes meta to the first. CI CI CI repeat ortho to CI NO₂ meta to NO₂ NO₂ H NO₂ NO₂ NO₂ The reaction with hydrazine is a nucleophilic substitution activated by two nitro groups. 1 CI NH CI 0 NH N + H NO₂ 0 NO₂ NO₂ NO₂ Problem 9 Pyridine is a six-electron aromatic system like benzene. You have not yet been taught anything systematic about pyridine but see if you can work out why 2- and 4-chloropyridines react with nucleophiles but 3-chloropyridine does not. CI CI RNH₂ R N CI N N N CI N N H 2-chloropyridine 3-chloropyridine 4-chloropyridine 2-chloropyridine Purpose of the problem Extension of the ideas on nucleophilic aromatic substitution into new compounds. Suggested solution The main need is to find somewhere to park the negative charge in the intermediate. The only place we have in pyridine is the nitrogen atom. We can put the negative charge on the nitrogen when attack 2- or 4-chloropyridine, but not when we do the same thing with 3-chloropyridine. Try for yourself. CI X X X N CI N N N N N The amine formation given in the question is just one important example of this reaction. CI R R N CI N No N H 2-chloropyridine H H H H