Chemistry Reference
In-Depth Information
8.22
Br
2
O
CH
3
Br
O
CH
3
(CH
3
)
2
CCH
2
HF
C
(CH
3
)
3
C
(CH
3
)
3
1. Mg, ether
2. H
2
Br
O
CH
3
O
CH
3
Direct Friedel-Crafts alkylation of anisole would give
p
-
t
-butylanisole, the sterically
less hindered product. The bromine is therefore used to block the
para
position, then
is removed when no longer needed. (For an activated substrate like anisole, a FeBr
3
catalyst is not required for step 1.)
8.23
a.
Since both alkyl groups are identical and primary, the dehydration route using
sulfuric acid is preferred because it is least expensive and gives a good yield
(see eq. 8.8).
H
2
SO
4
ether
2
CH
3
CH
2
CH
2
OH
CH
3
CH
2
CH
2
OCH
2
CH
2
CH
3
+
H
2
O
b.
The Williamson method is preferred. The sodium phenoxide can be prepared
using NaOH instead of Na, because of the acidity of phenols:
O
-
Na
+
+
CH
3
CH
2
Br
OCH
2
CH
3
+Na
+
Br
-
c.
The following Williamson ether synthesis is preferred. The alternate
Williamson ether synthesis (the reaction between sodium ethoxide and
tert
-
butyl iodide) would fail because dehydrohalogenation (that is, E2 elimination)
would be faster than substitution.
CH
3
CH
2
I
Na
_
Na
+
(CH
3
)
3
COH
(CH
3
)
3
CO
+
(C H
3
)
3
COCH
2
CH
3
+
NaI
1/2 H
2
Another good method for the preparation of this ether is the acid-catalyzed
addition of ethanol to 2-methylpropene:
H
3
C
H
+
(catalyst)
CH
3
CH
2
OH
C
C
H
2
(CH
3
)
3
C
O
CH
2
CH
3
H
3
C
8.24
The second step fails because S
N
2 displacements cannot be carried out on aryl
halides.
