Which are the correct IUPAC names of the following compound? CH3-CH2-CH2-CH2-CH(CH2C(CH3)3)-CH2-CH2-CH2-CH2-CH

NCERT Exemplar · Q14

Q. Which are the correct IUPAC names of the following compound? CH3-CH2-CH2-CH2-CH(CH2C(CH3)3)-CH2-CH2-CH2-CH2-CH3 (Note: more than one of the given options may be correct.)

(i) 5 - (2', 2'-Dimethylpropyl)-decane

(ii) 4 - Butyl - 2,2- dimethylnonane

(iii) 2,2- Dimethyl - 4- pentyloctane

(iv) 5 - neo-Pentyldecane

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Concept understanding — Chain Isomerism

Chain Isomerism: The Skeleton of a Molecule

Imagine you have a set of 5 identical Lego bricks. You can snap them together in a straight line — one after the other — to make a long, thin rod. Or, you could take the same 5 bricks and make a "T" shape: a straight chain of 4 bricks, with the 5th brick attached to the middle one. Both structures use exactly the same number and type of bricks, but their shapes are different.

That difference in shape — specifically, in the carbon skeleton — is the heart of chain isomerism.

The Intuition

Carbon atoms love to bond with each other to form chains. But they don't always have to form a single, straight line. They can branch off. When two molecules have the same molecular formula (same number of each atom) but differ in how their carbon atoms are connected — one is a straight chain, another is branched — they are chain isomers.

Think of it like a family tree. The same set of people can be arranged in a direct line (grandparent → parent → child) or with branches (grandparent has two children, each with their own children). The "people" (carbon atoms) are the same, but the "connections" (bonds) form different patterns.

The Precise Statement

Chain isomerism is a type of structural isomerism where two or more compounds have the same molecular formula but differ in the arrangement of the carbon skeleton (the carbon chain). This difference arises because the carbon atoms can be linked in a straight chain or in one or more branched chains.

Important

Chain isomers always have the same molecular formula but different structural formulas due to different carbon skeletons. They are distinct compounds with different physical and chemical properties.

A Concrete Example: Pentane ( $C_5H_{12}$ )

The molecular formula $C_5H_{12}$ can represent three different chain isomers. Let's draw them:

n-Pentane (normal pentane): A straight chain of 5 carbons.

CH_3-CH_2-CH_2-CH_2-CH_3

Isopentane (2-methylbutane): A chain of 4 carbons with a 1-carbon branch (a methyl group, $-CH_3$ ) attached to the second carbon.

CH_3-CH(CH_3)-CH_2-CH_3

Neopentane (2,2-dimethylpropane): A central carbon bonded to four other carbons — a highly branched structure.

C(CH_3)_4

Isomer Name	Common Name	Structure (Condensed)	Boiling Point (°C)
Pentane	n-Pentane	$CH_3(CH_2)_3CH_3$	36.1
2-Methylbutane	Isopentane	$(CH_3)_2CHCH_2CH_3$	27.7
2,2-Dimethylpropane	Neopentane	$C(CH_3)_4$	9.5

Notice how the boiling point drops as the molecule becomes more branched. This is because branching reduces the surface area available for intermolecular forces (London dispersion forces), making the molecule easier to vaporize.

Watch out

A common mistake is to think that any difference in structure is chain isomerism. It is only about the carbon skeleton. If two molecules have the same skeleton but different positions of a double bond or a functional group, that is a different type of isomerism (position isomerism), not chain isomerism.

When Does Chain Isomerism Occur?

Chain isomerism is possible only when the carbon chain is long enough to allow branching. For alkanes: …

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The compound is a long alkane with a branched substituent. The correct IUPAC names are those that correctly identify the longest carbon chain and number it to give the lowest locants to the substituents. The correct options are (i) and (iv).

The key to naming any alkane is to find the longest continuous carbon chain — that gives the parent name. Then you number the chain so that the substituents get the smallest possible locants. Here, the compound looks like a decane backbone with a bulky substituent attached at position 5. Let’s break it down.

Draw the structure in condensed form

The given formula is:

$\text{CH}_3\text{-CH}_2\text{-CH}_2\text{-CH}_2\text{-CH}(\text{CH}_2\text{C}(\text{CH}_3)_3)\text{-CH}_2\text{-CH}_2\text{-CH}_2\text{-CH}_2\text{-CH}_3$

The part in parentheses is the substituent on the fifth carbon of the main chain. That substituent is $\text{-CH}_2\text{-C}(\text{CH}_3)_3$ , which is a neopentyl group (2,2-dimethylpropyl).
Identify the longest chain

Count the carbons in the main chain: there are 10 carbons in a straight line (from the leftmost CH₃ to the rightmost CH₃). That makes the parent decane. The substituent is attached at carbon 5.
Name the substituent

The group $\text{-CH}_2\text{-C}(\text{CH}_3)_3$ is:
- The longest chain in the substituent is 3 carbons (propyl).
- It has two methyl groups on the second carbon of that propyl chain.
- So the substituent is 2,2-dimethylpropyl, commonly called neopentyl.
Write the IUPAC name

The systematic name is: 5-(2,2-dimethylpropyl)decane.

This matches option (i) exactly. Option (iv) says 5-neo-pentyldecane — “neopentyl” is the common name for 2,2-dimethylpropyl, so that is also correct.

Tip

Common names like “neopentyl” are accepted by IUPAC as long as they are unambiguous. So both the systematic and the common name are valid.

Check the other options …

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Q.Which are the correct IUPAC names of the following compound? CH3-CH2-CH2-CH2-CH(CH2C(CH3)3)-CH2-CH2-CH2-CH2-CH3 (Note: more than one of the given options may be correct.)

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Q.Which are the correct IUPAC names of the following compound?
CH3-CH2-CH2-CH2-CH(CH2C(CH3)3)-CH2-CH2-CH2-CH2-CH3 (Note: more than one of the given options may be correct.)

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