Q.(a) [3 marks] Hydrocarbon n-butane on treatment with a limited amount of bromine undergoes monobromination only to produce compound "X". Compound "X" on treatment with alcoholic potash gives a new compound "Y" which can show geometrical isomerism. Identify the compound "X" and the two geometrical isomers of "Y".

Geometric Isomerism: The "Locked in Place" Isomers

Imagine you have two magnets. You can arrange them in two ways: north pole facing north (they repel) or north pole facing south (they attract). The magnets themselves are identical — same size, same material — but the spatial arrangement of their poles is different. That difference in arrangement, when the magnets can't rotate freely, is the core idea behind geometric isomerism.

In organic chemistry, molecules are three-dimensional. Atoms are connected by bonds, and some bonds — specifically double bonds — are rigid. They don't allow free rotation like a single bond does. This rigidity locks certain groups of atoms into fixed positions relative to each other. When you have two identical groups attached to the two ends of a double bond, they can end up on the same side or on opposite sides. These are two different molecules, with different properties, even though they have the same atoms connected in the same order.

That's geometric isomerism: same connectivity, different spatial arrangement due to restricted rotation.

The Precise Statement

Geometric isomerism (also called cis-trans isomerism) occurs when:

1. There is a rigid structural feature in the molecule that prevents free rotation. The most common cause is a carbon-carbon double bond ($\ce{C=C}$). Other causes include cyclic structures (rings) where atoms can't rotate past each other.

2. Each of the two carbon atoms in the double bond must have two different groups attached to it. If either carbon has two identical groups, the two possible arrangements become identical — they are the same molecule.

When these conditions are met, the two isomers are named:

• cis (from Latin cis, meaning "on this side"): the two identical (or similar) groups are on the same side of the double bond.
• trans (from Latin trans, meaning "across"): the two identical (or similar) groups are on opposite sides of the double bond.

A Concrete Example: 2-Butene

Consider the molecule 2-butene: $\ce{CH3-CH=CH-CH3}$.

The double bond is between the second and third carbons. Each of these carbons has a hydrogen ($\ce{H}$) and a methyl group ($\ce{CH3}$) attached. Since $\ce{H} \neq \ce{CH3}$ on each carbon, geometric isomers exist.

The two methyl groups in cis are close together, causing slight repulsion (steric strain), which makes the molecule slightly less stable and gives it a higher boiling point. In trans, the methyl groups are far apart, so the molecule is more stable and packs differently in the liquid state.

Why Does This Matter?

Geometric isomerism is not a textbook curiosity. It has real-world consequences:

• Vision: The molecule retinal in your eye has a cis form that, when hit by light, converts to trans. This shape change triggers a nerve signal — that's how you see.
• Fats: Natural unsaturated fats (like olive oil) are mostly cis. Artificial trans fats (from partial hydrogenation) have a different shape and are linked to heart disease. …