Nomenclature of Organic Compounds: chemistry

IUPAC Rules: Nomenclature of Alkanes

IUPAC Rules for the Nomenclature of Alkanes: Part-1

IUPAC Rule (1) The longest carbon chain is selected. And the parent name of organic compound is assigned on the basis number of carbon atoms present in the longest chain.

Example (1)

In this example the longest chain has seven (7) carbon atoms.

Example (2)

Here in the straight chain there are 8 (eight) carbon atoms.

By starting from right (indicated in red numbering) and by going down from the third carbon atoms, there are seven (7) carbon atoms.

By starting from the branched chain, there are ten (10) carbon atoms. (as indicated in blue colour numbering)

Here the longest chain has ten (10) carbon atoms, so this longest chain is selected.

Thus, the parent name of this organic compound will be Decane.

IUPAC Rule (2) The numbering of carbon atoms in the longest chain using Arabic numbers 1,2,3,4, . . . . . is done in such a way that the functional group, here it is alkyl group, must be at the lowest number.

Figure (1) Correct

Figure (2) Wrong

In the figure (1) the alkyl group is situated at 5^th carbon atom.

In the figure (2) the alkyl group is situated at 6^th carbon atom.

Thus, numbering system in figure (1) to name an organic compound as per IUPAC rules is correct.

IUPAC Rule (3) The alkyl group present in the organic compound is named by prefixing the number of carbon atom at which it is present followed by the parent name of the organic compound that has been considered for the longest carbon-carbon chain.

Example (1)

Write the IUPAC name of the given organic compound.

The common name of this organic compound is Iso-hexane.

Because there are six carbon atoms present in this organic compound among which one methyl group is attached with the second carbon atom.

To write the IUPAC name, first of all carbon atoms are numbered from left to right so that the methyl group attached to it will falls at the lowest position.

Here, the longest chain has total five carbon atoms. Thus the parent name of the compound will be Pentane.

And a functional group, i.e. methyl group is attached to the second (2^nd) carbon atom, so the IUPAC name of this compound will be 2–Methyl Pentane or 2 – Methylpentane.

If the numbering of carbon atoms is done from right to left.

Then the name of this organic compound will become 4 – Methyl Pentane, but this name is wrong. Because the functional group, which is methyl in this case, should be considered at lowest position of carbon atom.

Example (2)

What will be the IUPAC name of the given organic compound?

By numbering the carbon atoms from either right to left or left to right present in this given organic compound, we see that the methyl group (functional group) is present at 3^rd carbon atom in both of the conditions.

By number from right to left

By number from left to right

So, the name of this given organic compound will be 3 –Methylpentane or 3– Methyl Pentane in both the cases.

Back to General Chemistry home

---

Reference: