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How to write IUPAC Name of sulfonamides

by egpat 08-07-2017

Sulfonamides are one of the oldest antibacterial agents that were developed for various types of bacterial infections. Since the entry of antibiotics into the medical world, sulfonamides usage was somewhat declined but still these drugs are useful in ophthalmic infections, urinary tract infections and even for treatment of burns and wounds.

In this article, we will discuss how to write IUPAC name of sulfonamides.

Let’s start with simple sulfonamide i.e. sulfanilamide

Working Example 1: Sulfanilamide

Step 1: Identify the principal functional group

Here two functional groups are present. One is sulfonamide and another is amine group. Principal functional group in sulfanilamide According to the priority order of functional groups with respect to IUPAC naming, sulfonamide should be given more preference over amine group. Hence the principle functional group is sulfonamide attached to aromatic ring.

Step 2: Identify the parent chain and give the root name

Since the principal functional group is present on benzene, the later acts a parent chain. Hence the root name is benzene sulfonamide.

Note: It can also be called as phenyl sulfonamide but IUPAC allows it as benzene sulfonamide as the principal functional group is a non-carbon containing group.

Step 3: Give numbering to the structure

Numbering should be done according to the following rule.

Numbering should be done from that direction which gives least possible number to the principal functional group.

Here benzene sulfonamide is the parent chain and numbering should be started from sulfonamide.

Step 4: Identify the side chains and indicate their location

Now let us see side chains. Sulfanilamide has an amine side chain at para position of the benzene ring.

Amino group at para position in sulfanilamide

Hence prefix is 4-Amino

Step 5: Now arrange the side chains in alphabetical order.

Only one side chain is present so no need of arranging in alphabetical order.

Step 6 : Complete the name

Hence the IUPAC name of sulfanilamide is

Working Example 2: Sulfamethoxazole

Step 1: Identify the principal functional group

Here again two functional groups are present viz. sulfonamide and amine.' As similar with the previous example, the principle functional group is sulfonamide attached to benzene.

Step 2: Identify the parent chain and give the root name

Now we can observe two ring systems. One is benzene and another is 1,2-oxazole.

Since sulfonamide is present on benzene, it should be selected as parent chain.

Hence root name of the compound is benzenesulfonamide.

Step 3: Give numbering to the structure

Again numbering should be started from sulfonamide group on benzene.

Step 4: Identify the side chains and indicate their location

Now let’s identify side chains. One side chain is amino group at para position of benzene and another is a heterocyclic ring on amine of the sulfonamide group.

One of the side chain is therefore 4-Amino.

Second side chain is the heterocyclic ring 1,2-oxazole and its numbering starts from oxygen. 5-methylisoxazole ring in sulfamethoxazole

It has methyl side chain at 5^th position therefore ring is

5'-methyl-1',2'-oxazole

It is attached to main chain by 3rd position. Hence the suffix “e” should be replaced by “yl” with its location. Therefore, name of the side chain is now

5'-methyl-1',2'-oxazol-3'-yl

This entire chain is attached to amine group of benzenesulfonamide.

N-[5'-methyl-1',2'-oxazol-3'-yl]

Step 5: Now arrange the side chains in alphabetical order.

Let's arrange all the prefixes in alphabetical order.

4-Amino

N-[5'-methyl-1',2'-oxazol-3'-yl]

Step 6 : Complete the name

Hence the IUPAC name of sulfamethoxazole is

4-Amino-N-[5'-methyl-1',2'-oxazol-3'-yl] benzesulfonamide

Working Example 3: Sulfadiazine

Step: 1,2 and 3

Step 1,2 and 3 are same as in the above example.

Hence root name is benzenesulfonamide

Step 4: Identify the side chains and indicate their location

It has two side chains.

Amine group at para position of benzene ring
4-Amino
Heterocyclic ring on amine group of sulfonamide

Amine group at para position of benzene ring

4-Amino

Heterocyclic ring on amine group of sulfonamide

The ring is pyrimidine attached by 2^nd position.

Hence N-[pyrimidin-2'-yl]

Step 5:Now arrange the side chains in alphabetical order.

While arranging in alphabetical order, the

4-Amino

N-(pyrimidin-2'-yl)

Step 6 : Complete the name

4-Amino-N-(pyrimidin-2'-yl)benzenesulfonamide

Working Example 4: Sulfacetamide

Now let’s take example of sulfacetamide. Even it is pharmacologically similar to other sulfonamides, its IUPAC name varies slightly.

Step 1: Identify the principal functional group

Here two functional groups are present viz. Sulfonamide and acetamide.

sulfonamide and acetamide groups in sulfacetamide

According to IUPAC rules, acetamide should be given more preference and it act as principle functional group here.

Step 2: Identify the parent chain and give the root name

Hence root name is acetamide

Actually, it should be written as ethanamide, but IUPAC allows few of trivial names like acetamide.

Step 3: Give numbering to the structure

Numbering should be started from carbon of amide and parent chain contains two carbons.

Hence it is acetamide

Step 4: Identify the side chains and indicate their location

Side chain is present at 2^nd position of acetamide.

Step 5:Now arrange the side chains in alphabetical order.

Now let’s build the name of the side chain one by one.

“Remember ! Here benzene ring is treated as side chain hence it should not be called as benzene sulfonamide”

The side chain is immediately attached to main chain by sulphonyl (–SO2-) group. Hence it is

1-[sulphonyl]

Now sulfonyl group is attached to phenyl

1-[(phenyl)sulfonyl]

Phenyl group is attached with amino group at 4^th position.

1-[(4-aminophenyl)sulfonyl]

Step 6 : Complete the name

1-[4-aminophenylsulfonyl]acetamide

Note: Even though the preference is given to acetamide instead of sulfonamide according to IUPAC rules, still sulfonamide is responsible for pharmacological activity of the drug.