Unformatted text preview: 658 ARYL HALIDES Elimination of ﬂuoride ion from the cyclohexadienyl anion intermediate restores the aromaticity of
the ring and completes the reaction.
F F F
F F F OCH3
F F OCH3
F Cyclohexadienyl anion
intermediate 23.6 F
F F 2,3,4,5,6-Pentafluoroanisole Fluoride
ion 4-Chloropyridine is more reactive toward nucleophiles than 3-chloropyridine because the anionic
intermediate formed by reaction of 4-chloropyridine has its charge on nitrogen. Because nitrogen is
more electronegative than carbon, the intermediate is more stable.
Cl Y Cl
Y N N 4-Chloropyridine Anionic
(more stable) Cl
N N 3-Chloropyridine 23.7 Anionic
(less stable) The aryl halide is incapable of elimination and so cannot form the benzyne intermediate necessary
for substitution by the elimination–addition pathway.
(No protons ortho to bromine;
elimination is impossible.) CH3
2-Bromo-1,3dimethylbenzene 23.8 The aryne intermediate from p-iodotoluene can undergo addition of hydroxide ion at the position
meta to the methyl group or para to it. The two isomeric phenols are m- and p-methylphenol.
NaOH, H2O NaOH, H2O (elimination phase) (addition phase) I
p-Iodotoluene CH3 CH3 OH
m-Methylphenol Back Forward Main Menu TOC p-Methylphenol Study Guide TOC Student OLC MHHE Website ...
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