Nomenclature of Organic Compounds: chemistry

IUPAC Nomenclature of Alkanes:Part-2

IUPAC Naming of alkanes when only one alkyl group is present in the organic compound.

Example (1) What is the IUPAC name of the given organic compound?

To assign an IUPAC name of an organic compound,

(a) The longest chain is selected

(b) To select the longest chain the numbering is done in such a way that a functional group, if any, attached to the carbon-carbon chain must falls at the lowest number of carbon atom.

Thus,

After numbering the carbon atoms from left to right in the given organic compound.

After numbering the carbon atoms from right to left in the given organic compound.

We see that methyl group attached to it falls at 3_rd position while numbering from left to right which is the lowest number, thus this numbering will be selected to assign the IUPAC name of the given organic compound.

The longest chain have six carbon atoms, thus the parent name of the given organic compound is Hexane, and a methyl group is attached to the 3_rd carbon atom.

Thus, the IUPAC name of the given organic compound is 3–methylhexane and not the 4–methylhexane.

The structure formula of the 3–methylhexane can be written in the following ways also.

Example (2) What is the IUPAC name of the given organic compound?

After numbering the given organic compound, we find that the longest chain has total 10 (ten) carbon atoms.

Thus, the parent name of this given organic compound is Decane.

And a propyl group is attached to the 4^th carbon atom, thus the IUPAC name of this compound is 4 – propyldecane.

Other structures of this 4 – propyldecane can be written as given below.

Example (3) What is the IUPAC name of the given organic compound?

Solution

To assign an IUPAC name, first of all we need to find the longest carbon-carbon chain.

To find the longest carbon-carbon chain the carbon-carbon chain of the given organic compound is numbered.

Numbering of the carbon-carbon chain in the given organic compound can be done in following ways:

Numbering from left to right to the above chain only.

Numbering from right to left and above to below.

[This is the correct way of numbering the carbon-carbon chain.]

Numbering from right to left and below to above.

[This is not the correct way of numbering the carbon-carbon chain. Because the functional group (ethyl group) falls at higher number, i.e. 6^th carbon atom. The functional group should be at lowest position of the carbon atom.]

After number the carbon atoms in the given organic compound, it becomes clear that the longest chain has 9 (nine) carbon atoms.

Thus, the parent name of the given organic compound is Nonane.

Since, by numbering started from right, the ethyl group present in the given organic compound falls at the 4^th carbon atom while numbering from the down to up, the ethyl group present in the given organic compound falls at the 6^th carbon atom.

Thus, the IUPAC name of the given organic compound is 4 – ethylnonane and not the 6 – ethylnonane

The structure of the 4 – ethylnonane can be written in different ways as given below:

Example (4) What is the IUPAC name of the given organic compound?

To select the longest chain first of all carbon atoms are numbered.

After numbering of the carbon-carbon chain, we find that there are nine (9) carbon atoms present in the longest chain and a propyl group is present as a functional group to the given organic compound.

Numbering from above to below and right to left.

Numbering from below to above and right to left.

Since, the propyl group attached to the given organic compound falls at 5^th position by numbering from either side, thus, numbering of the longest carbon-carbon chain can be done in either way.

Thus, the parent name of this compound is Nonane.

And hence, the IUPAC name of this compound is 5 – propylnonane.

The other types of structure of the 5 – propylnonane can be written as follows.

Example (5) What is the IUPAC name of the given organic compound?

To select the longest chain, the numbering of carbon atoms can be done in various ways.

The first two method of numbering are incorrect because in those orders of numbering the ethyl group present in the given organic compound falls at the 6^th carbon atoms.

While in the last two methods of numbering, the ethyl group present falls at the 3^rd carbon atom.

Thus, numbering in which the functional group present in the organic compound falls at the lower number of carbon atom is considered for assigning the IUPAC name.

Now, since there are eight carbon atoms present in the longest chain, thus parent name of the given organic compound is Octane.

And, since a ethyl group is present at the 3^rd carbon atom, thus the IUPAC name of the given compound is 3 – ethyloctane.

And the IUPAC name is not the 6 – ethyloctane.

Example (6) What is the IUPAC name of the given organic compound?

After numbering the carbon atoms to the given organic compound we find that the longest chain has nine (9) carbon atoms.

Thus, the parent name of the given organic compound is Nonane.

The numbering of carbon atom can be done in another way also.

By numbering from below to above in the given organic compound the methyl group present falls at 4^th carbon atom while by numbering from above to below, the methyl group present in the given organic compound falls at 6^th place.

According IUPAC rule, the functional group present in the organic compound should be at lower position.

Thus, numbering from below to above in which the methyl group present in the compound falls at 4^th position is selected for IUPAC naming.

Thus, the IUPAC name of the given compound is 4 – methylnonane and not the 6 – methylnonane

The structure of the 4 – methylnonane can be written as follows also.

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