The alkene that exhibits geometrical isomerism is
(a) propene
(b) 2-methyl propene
(c) 2-butene
(d)

Q: Why does restricted rotation cause isomerism?

Because the double bond cannot rotate freely, the atoms attached to it are locked in space. If the groups are arranged differently (e.g., both heavy groups on top vs. one on top and one on bottom), you can't just twist the bond to make them look the same. They become permanently distinct molecules, or isomers.

Q: Can an alkene with 3 identical groups show geometrical isomerism?

No. If an alkene has 3 identical groups, at least one of the double-bonded carbons must have two identical groups attached to it. This immediately violates the condition for geometrical isomerism.

Q: What exactly are cis and trans isomers?

They are specific types of geometrical isomers. 'Cis' means the similar or higher-priority groups are on the same side of the double bond. 'Trans' means they are on opposite sides, diagonally across from each other.

Q: Does 1-butene show geometrical isomerism?

No, it does not. The structure of 1-butene is $CH_2=CH-CH_2-CH_3$. The first carbon atom is attached to two identical hydrogen atoms, which disqualifies it.

Q: Why is 2-butene capable of showing geometrical isomerism?

Because both carbon atoms of its double bond are attached to two different groups (one Hydrogen and one Methyl group). This allows for two unique spatial arrangements (cis and trans) that cannot be interconverted without breaking bonds.

Question

The alkene that exhibits geometrical isomerism is

Options:

(A) propene

(B) 2-methyl propene

(C) 2-butene

(D) 2-methyl-2-butene

Sigma Percentile · Accepted Answer

### The Core Principle of Geometrical Isomerism

When we talk about geometrical isomerism in alkenes, we are fundamentally discussing the spatial arrangement of atoms around a rigid structure. The carbon-carbon double bond ($C=C$) is the star of the show here. Unlike a single bond, which allows free rotation like a wheel on an axle, a double bond is locked in place. This restricted rotation means that the groups attached to the double-bonded carbons are fixed in their relative positions.

However, restricted rotation alone is not enough! For an alkene to exhibit geometrical isomerism, it must satisfy a strict symmetry condition: **Each carbon atom of the double bond must be attached to two different groups.** Mathematically, if we have an alkene of the form $abC=Ccd$, it will only show geometrical isomerism if $a 
eq b$ and $c 
eq d$.

### Analyzing the Candidates

Let's put our four options to the test using this golden rule.

**1. Propene ($CH_3-CH=CH_2$)**
If we look at the rightmost carbon atom of the double bond in propene, we immediately spot a problem. It is bonded to two identical hydrogen atoms. Because swapping these two identical hydrogens doesn't create a new spatial arrangement, propene fails the test.

**2. 2-methyl propene ($(CH_3)_2C=CH_2$)**
This molecule fails spectacularly on both sides! The left carbon is attached to two identical methyl ($-CH_3$) groups, and the right carbon is attached to two identical hydrogen atoms. There is absolutely no chance for geometrical isomerism here.

**3. 2-methyl-2-butene ($(CH_3)_2C=CH-CH_3$)**
While the right carbon of this molecule has two different groups (a hydrogen and a methyl group), the left carbon ruins the party. It is bonded to two identical methyl groups. Remember, the rule must be satisfied for *both* carbons independently. Thus, this option is also incorrect.

### The Winning Molecule

**4. 2-butene ($CH_3-CH=CH-CH_3$)**
Finally, we arrive at 2-butene. Let's inspect it carefully. The left carbon is attached to a methyl group and a hydrogen atom—these are different! The right carbon is also attached to a methyl group and a hydrogen atom—different again!

Because 2-butene perfectly satisfies our condition, it can exist in two distinct spatial forms. When the two bulky methyl groups are on the same side of the double bond, we call it the **cis-isomer**. When they are locked on opposite sides, we call it the **trans-isomer**. Therefore, 2-butene is the only alkene among the choices that exhibits geometrical isomerism.

Animated Solution for Chemistry - Organic Chemistry: The alkene that exhibits geometrical isomerism is

Select Answer:

Visualized Solution

$C o n d i t i o n f or G eo m e t r i c a l I so m er i s m$

$A na l y z in g P r o p e n e$

$A na l y z in g 2 - m e t h y l p r o p e n e$

$A na l y z in g 2 - m e t h y l - 2 - b u t e n e$

$A na l y z in g 2 - b u t e n e$

$C o n c l u s i o n$

The Sigma Insight: Nomenclature and Characterisation

The Core Principle of Geometrical Isomerism

Analyzing the Candidates

The Winning Molecule

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