Nomenclature of Organic Compounds: chemistry

IUPAC Rules: Nomenclature of Alkanes:Part-2

IUPAC Rule (4) When two or more same alkyl groups are present in and organic compound, then di, tri, tetra, etc. prefixed writing before the alkyl group. These prefixes are separated by using commas.

Example (1) When two methyl groups are present at different positions in the carbon chain.

What is the IUPAC name of the given organic compound?

After numbering from left to right to the longest carbon-carbon chain in the given organic compound.

And after numbering from left to right to the longest chain of the given organic compound.

In both of the cases, one methyl group falls at 3^rd carbon atom and second falls at the 4^th carbon atom.

Thus, the IUPAC name of the given organic compound will be 3, 4 Dimethyl Hexanee or 3, 4 Dimethylhexanee.

Case (1) When numbering of carbon atoms is done from right to left.

Case (2) When numbering of carbon atoms is done from left to right.

Example (2) When two methyl groups are present at different positions in the carbon chain.

For the given organic compound, number of carbon atoms in the longest carbon chain can be done in following two ways.

(a) From left to right.

Position of methyl group = 3 and 5

And from (b) right to left

Position of methyl group = 2 and 4

In the numbering from right to left, the positions of methyl groups are at 2 and 4 positions. While numbering from left to right, the positions of methyl groups are at 3 and 5 positions.

Since, while numbering from right to left, the positions of methyl group fall at lower positions in the carbon-carbon chain, thus, the numbering from right to left is correct.

Thus, the correct name of the given organic compound is 2, 4– Dimethyl Hexane or 2, 4– Dimethylhexane or 2, 4– dimethyl hexane.

IUPAC Rule (5) If different alkyl groups are present in the organic compound, then

(a) Identify the longest chain

(b) Do numbering of the longest chain so that alkyl groups are situated at the lowest number of carbon atoms.

(b) Identify the correct position of the alkyl groups.

(c) Write the name of the alkyl group by using Arabic numbers as prefixes.

(d) Write the names of alkyl groups in alphabetical order ignoring the bi, tri, tetra, etc. before the parent organic compound.

Example (1)

While numbering carbon atoms from right to left, the ethyl group comes in 4^th place, and the methyl group comes in 5^th place.

Numbering of carbon atoms from left to right, the methyl group comes in 2^nd place, and the ethyl group comes in 3^rd place.

Thus, the numbering of carbon atoms from left to right is correct as alkyl groups fall at a lower position in this way.

Thus, the name of the given organic compound will be 3–ethyl 2–methyl hexane

IUPAC Rule (6) In the case of the presence of two equal longest chains in the organic compound

(a) Select the chain in which the maximum number of alkyl groups are present as the parent compound.

(b) Number the carbon atoms in the selected longest chain so that alkyl groups fall at the lowest position of carbon atoms.

(c) Name the alkyl groups by prefixing Arabic numbers, 1, 2, 3, . . . for the position of carbon atoms in their alphabetical order followed by the parent name of an organic compound.

Example

After numbering the carbon atoms from right to left, we see that both of the longest chains have five carbon atoms.

But, in the horizontal straight carbon-carbon chain, more alky groups are present.

At the 2^nd carbon atom one methyl group, at the 3^rd carbon atom one ethyl group, and at the 4^th carbon atom one methyl group is present.

Thus, by considering the straight horizontal chain the name of the given compound will be 3 – ethyl – 2, 4 – dimethylpentane

IUPAC Rule (7) In the case when in branch of the longest chain also have branch chain of carbon atoms.

(a) Select the continuous longest carbon chain

(b) Number the longest carbon chain in such a way that functional groups should be assigned at the lowest number of carbon atoms.

(c) Name the alkyl group having branch chain attached to the parent organic compound according to the order.

Example

What is the IUPAC name of the given compound?

The numbering of the carbon chain can be done in the following ways:

After numbering the carbon atoms present in the longest chain of the given organic compound, we see that one methyl group is present at the 3^{rd position and one butyl group (Iso-butyl group) is present at the 4th position in the chain.}

Thus, the IUPAC name of the given organic is 3– methyl – 4 – (propanyl–2) heptane or 3– methyl – 4 – (propan–2–yl) heptane or 3– methyl – 4 – isopropylheptane.

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