Ch_322b_15.12 - Chapter 15.12 lecture note

Moderating Reactivity through Synthetic Modification
Acetylation of Phenols and Anilines
Both phenols (Ar- OH ) and anilines (Ar- NH
2
) are extremely reactive
in most electrophilic aromatic substitution reactions. Sometimes it is
necessary to reduce this reactivity by synthetic modification of the
substituent groups.
:OH :
phenol
(more reactive)
:NH
2
aniline
(more reactive)
CH
3
COCCH
3
O
=
O
=
acetic anhydride
acetylation
CH
3
COONa
+
CH
3
COCCH
3
O
=
O
=
acetic anhydride
+
acetylation
:OCCH
3
:
O
=
phenyl acetate
(less reactive)
:NHCCH
3
O
=
acetanilide
(less reactive)

An Explanation of the Decreased Reactivity
The decrease in reactivity of phenol and aniline upon acetylation
is explained through resonance theory .
Additional resonance structures may be written that show movement
of the nonbonding electrons towards the carbonyl function and away
from the benzene ring. Accordingly, the nonbonding electron pairs
will be less able to donate electron density into the aromatic ring.
:O-CCH
3
:
O
=
: :
:O=CCH
3
: O
: :
+
-
:NH-CCH
3
O
=
: :
NH=CCH
3
: O
: :
-
+

The Halo Groups: Ortho-Para Directing but Rate Retarding.
An Oxymoron?
The apparent contradiction in behavior of the halogen substituents is