Solutions_Manual_for_Organic_Chemistry_6th_Ed 467

Solutions_Manual_for_Organic_Chemistry_6th_Ed 467 - 19-55...

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Unformatted text preview: 19-55 (a) OZN 'Q F � 000 _ � ooF li I oo N H zR .. :0'" . . 00 N:" 'c . N02 0'" Y 0 + NH2R 0 0 � I �. . - 0 : 0'" 0 N+ 0o -; 1 + ... N.... 0 _ � 1j ... .. ... F + N HzR � I � -0'" N+ 0 II F + NH2R I 00 0 '" + N.... 0 _ :0'" C1 N+ I 0: 0 0- (b) Why is fluoride ion a good leaving group from A but not from F _ HC Nu y � N Oz B (either by SNI or SN2)? I N02 A �� A :NU B C Formation of the anionic sigma complex A is the rate-determining (slow) step in nucleophilic aromatic substitution. The loss of fluoride ion occurs in a subsequent fast step where the nature of the Jeaving group does not affect the overall reaction rate. In the SN 1 or SN2 mechanisms, however, the carbon-fluorine hond is breaking in the rate-determining step, so the poor leaving group ability of fluoride does indeed affect the rate. t >-. lrl� t delocalized , ________________ nucleophilic aromatic substitution (c) Amines can act as nucleophiles as long as the electron pair on the N is available for bonding. The initial reactant, methylamine, CH3NHZ' is a very reactive nucleophile. However, once the N is bonded to the benzene ring, the electron pair is delocalized onto the ring, especially with such strong electron­ withdrawing groups like N02 in the ortho and para positions. The electrons on N are no longer available for bonding so there is no danger of it acting as a nucleophile in another reaction. 462 N02 ...
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This note was uploaded on 02/27/2010 for the course CHEM 140 taught by Professor Wade during the Spring '10 term at Whitman.

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