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(a) OZN 'Q
_ � 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 :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.
- Spring '10