Nomenclature of Organic Compounds: chemistry

IUPAC Nomenclature of Alkanes:Part-1

The alkyl group

The alkyl group is formed by removing one hydrogen atom from an alkane.

The general formula of the alkyl group is C_nH_2n+1 and the alkyl group is generally represented by R–.

Alkane – ane + yl = Alkyl

Examples of Alkyl group

(a) Methyl group

The structural formula of methyl group: CH₃–

(b) Ethyl group

The structural formula of methyl group: CH₃–CH₂–

(c) Propyl group

The structural formula of methyl group: CH₃–CH₂– CH₂–

And so on such as butyl group, pentyl group, hexyl group, heptyl group, etc.

Example (1)

The IUPAC name of the organic compound H₃C.CH(CH₃)CH₃

The structural formula of the given organic compound H₃C.CH(CH₃).CH₃ is

There is a methyl group (CH₃–) attached to the given organic compound.

After numbering the carbon atoms in the longest chain in the given organic compound

Numbering from left to right.

Numbering from right to left.

Since there are three carbon atoms present in the given organic compound (alkane), thus the parent name of the given alkane is Propane

By numbering from either side, we see that the methyl group (CH₃–) falls at 2_nd carbon atom.

Thus, the IUPAC name of the given compound is 2–methylpropane

Other styles of writing the structural formula of 2–methylpropane

The structure formula of the given organic compound 2–methylpropane is written as follows also.

The common name of 2-methylpropane is Isobutane.

Example (2)

Write the IUPAC name of the given compound.

Step (1) Selecting the longest chain.

To select the longest carbon-carbon chain, we need to number it.

After numbering from left to right

After numbering from right to left

Step (2) The numbering of carbon-carbon chain is done in such a fashion that the functional group attached to the longest carbon-carbon chain must fall at lowest number.

We see that after numbering from left to right the carbon-carbon chain to the given organic compound, the methyl group attached to it falls at 2_nd carbon atom.

While by numbering from right to left the methyl group attached to the longest carbon-carbon chain of the given organic compound falls at 4_th carbon atom.

Hence, numbering from left to right is considered as in this way the attached methyl group to the given longest carbon-carbon chain organic falls at 2_nd number of carbon atom, which is lowest.

Step (3) Since there are five carbon atoms present in the longest chain, thus the parent name of the given organic compound is Pentane.

Hence, the IUPAC name of the given organic compound is 2-methylpentane.

The structure of the given compound can be written in following ways also:

H₃C . CH(CH₃) . CH₂ . CH₂ . CH₃

Example (3)

Write the IUPAC name of the given organic compound.

After numbering the carbon-carbon atoms to the structure of given organic compound from left to right

And after numbering the carbon-carbon atoms to the structure of given organic compound from right to left

In either way of numbering, the methyl group attached to the carbon-carbon atoms fall at 3_rd carbon atom. So, numbering can be done in either way.

There are five carbon atoms present in the given organic compound while numbering the longest carbon-carbon chain.

Thus, the parent name of the given organic compound is Pentane

And, hence, the IUPAC name of the given organic compound is 3Ómethylpentane

The structural formula of the given organic compound can be written in different ways, which are

Reference¹ This image has been taken from https://wikipedia.org.

Example (4)

Write the IUPAC name of the given organic compound.

To write the IUPAC name of an organic compound, first of all to assign the longest carbon-carbon chain, number of carbon atoms is done.

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

After number from right to left to the carbon atoms in the given structure of organic compound

The numbering is done in such way that the functional group attached to the longest carbon chain should fall at the lowest number of carbon atom.

We see that by numbering from right to left the methyl group attached to the given organic compound falls at 5_th position.

While numbering from left to right the methyl group attached to the given organic compound falls at 2_nd position.

Since, by numbering from left to right the methyl group attached to the given organic compound falls at the lowest number, i.e. at the 2_nd number, thus, this numbering style is selected to IUPAC name.

Since, there are six carbon atoms present in the longest chain of the given organic compound and a methyl group attached at 2_nd carbon atom.

Thus, IUPAC name of the given organic compound is 2–methylhexane.

If the numbering from right to left is considered to assign the IUPAC name of the given organic compound, then the methyl group attached to the given organic compound will fall at 5_th carbon atom. And the IUPAC name will become 5–methylpentane. But, this name 5–methylpentane will become incorrect.

So, the correct IUPAC name of the given organic compound is 2–methylhexane.

The structure formula of 2–methylhexane can be written in the following ways also.

Reference² This figure, the 3D structure of 2–methylhexane has been taken from https://Wikipedia.org

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Reference:

Reference-1 By Jynto (talk) - This image was created with Discovery Studio Visualizer., CC0, Link

Reference-2 By Ben Mills and Jynto - Derived from File:2,5-Dimethylhexane-3D-balls.png., Public Domain, Link