An alkene 'A' contains three C–C, eight C–H σ bonds and one C–C π bond. 'A' on ozonolysis gives two moles of a

Exercises · 9.6

Q. An alkene 'A' contains three C-C, eight C-H σ bonds and one C-C π bond. 'A' on ozonolysis gives two moles of an aldehyde of molar mass 44 u. Write IUPAC name of 'A'.

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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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Ozonolysis cleaves the $\ce{C=C}$ double bond to give two aldehyde fragments; working backward from the product (ethanal, $44~\text{u}$ ) and the bond count reveals that A is but-2-ene.

The problem hands us structural clues—bond counts and an ozonolysis product—and asks us to reconstruct the alkene. Ozonolysis is the key: it cuts a $\ce{C=C}$ double bond and converts each carbon of that double bond into a carbonyl group. When both fragments are aldehydes, the original double bond must have been between two $\ce{-CH=}$ carbons (each bearing one hydrogen).

Start by identifying the aldehyde product. We're told it has molar mass $44~\text{u}$ . The simplest aldehyde is methanal ( $\ce{HCHO}$ , $30~\text{u}$ ), the next is ethanal ( $\ce{CH3CHO}$ , $44~\text{u}$ ). So the aldehyde is ethanal, $\ce{CH3CHO}$ .

Now reconstruct the alkene:

Ozonolysis gives two moles of the same aldehyde.

That means the alkene is symmetrical about the double bond. Each half, when oxidised, becomes $\ce{CH3CHO}$ .
Reverse the ozonolysis.

Ethanal has the carbonyl carbon bonded to a methyl group: $\ce{CH3-CHO}$ . In the original alkene, that carbonyl carbon was part of the $\ce{C=C}$ double bond. Remove the oxygen and join the two aldehyde fragments at their carbonyl carbons:

\ce{CH3-CH=O} + \ce{O=CH-CH3} \quad \xrightarrow{\text{reverse}} \quad \ce{CH3-CH=CH-CH3}

This is but-2-ene.

Verify the bond counts. …

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Q.An alkene 'A' contains three C–C, eight C–H σ bonds and one C–C π bond. 'A' on ozonolysis gives two moles of an aldehyde of molar mass 44 u. Write IUPAC name of 'A'.

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