Why do you use acetanilide rather than aniline in the synthesis of 4-acetamidobenzenesulfonyl chloride?
Sulfanilamide has two groups on the benzene ring in para-
positions to each other. The amino group is a highly activating
ortho-directing group and the sulfonamide group is a meta-directing
group.
Consequently, the amino group must be introduced first followed
by the introduction of the sulfonamide group. To introduce the
sulfonamide group one must first use chlorosulfonic acid then
ammonium hydroxide.
Unfortunately, amino groups react with chlorosulfonic acid
leading to undesired products and the direct chlorosulfonation of
aniline would not lead to the desired intermediate product.
However, if the amino group is converted to an amide, the undesired
side reaction with chlorosulfonic acid is avoided. The acetyl group
in this
case serves as a "protecting group" for the amine that allows a
smooth chlorosulfonation of the benzene ring.
The amide nitrogen does not activate the ring towards
substitution as much as the free amino group, but it can still
exert its ortho/para- directing influence. In this case the steric
hindrance provided by the acetyl group results in the substitution
occurring predominantly at the para- substitution. The protecting
acetyl group is removed later to give the desired "free" amino
group. The protection of groups that would interfere with a given
step in a synthesis is not uncommon, but I it is necessary for the
"protecting group" to be introduced and removed readily by
reactions that will not adversely affect other groups in the
molecule.
REFERENCE:
http://www.uwf.edu/chemistry/faculty/Gurst/SULFANIL.pdf
Sulfanilamide has two groups on the benzene ring in para- positions
to each other. The amino group is a highly activating
ortho-directing group and the sulfonamide group is a meta-directing
group.
Consequently, the amino group must be introduced first followed
by the introduction of the sulfonamide group. To introduce the
sulfonamide group one must first use chlorosulfonic acid then
ammonium hydroxide.
Unfortunately, amino groups react with chlorosulfonic acid
leading to undesired products and the direct chlorosulfonation of
aniline would not lead to the desired intermediate product.
However, if the amino group is converted to an amide, the undesired
side reaction with chlorosulfonic acid is avoided. The acetyl group
in this
case serves as a "protecting group" for the amine that allows a
smooth chlorosulfonation of the benzene ring.
The amide nitrogen does not activate the ring towards
substitution as much as the free amino group, but it can still
exert its ortho/para- directing influence. In this case the steric
hindrance provided by the acetyl group results in the substitution
occurring predominantly at the para- substitution. The protecting
acetyl group is removed later to give the desired "free" amino
group. The protection of groups that would interfere with a given
step in a synthesis is not uncommon, but I it is necessary for the
"protecting group" to be introduced and removed readily by
reactions that will not adversely affect other groups in the
molecule.
REFERENCE:
http://www.uwf.edu/chemistry/faculty/Gurst/SULFANIL.pdf
Sulfanilamide has two groups on the benzene ring in para- positions
to each other. The amino
group is a highly activating ortho-directing group and the
sulfonamide group is a meta-directing group.
Consequently, the amino group must be introduced first followed
by the introduction of the sulfonamide
group. To introduce the sulfonamide group one must first use
chlorosulfonic acid then ammonium hydroxide.
Unfortunately, amino groups react with chlorosulfonic acid
leading to undesired products and the direct
chlorosulfonation of aniline would not lead to the desired
intermediate product. However, if the amino group is
converted to an amide, the undesired side reaction with
chlorosulfonic acid is avoided. The acetyl group in this
case serves as a "protecting group" for the amine that allows a
smooth chlorosulfonation of the benzene ring.
The amide nitrogen does not activate the ring towards
substitution as much as the free amino group, but it can
still exert its ortho/para- directing influence. In this case
the steric hindrance provided by the acetyl group
results in the substitution occurring predominantly at the para-
substitution. The protecting acetyl group is
removed later to give the desired "free" amino group. The
protection of groups that would interfere with a
given step in a synthesis is not uncommon, but I it is necessary
for the "protecting group" to be introduced and
removed readily by reactions that will not adversely affect
other groups in the molecule.
From:
http://www.uwf.edu/chemistry/faculty/Gurst/SULFANIL.pdf
