Which one of the following has the minimum boiling point?
(a) n-butane
(b) 1-butyne
(c) 1-butene
(d)

Q: Why don't the double or triple bonds in 1-butene and 1-butyne increase the boiling point significantly?

While pi bonds do affect the polarizability of a molecule slightly, the macroscopic shape and branching have a much more dominant effect on the overall surface area and the resulting van der Waals forces. The structural compactness of isobutene easily overrides the minor differences caused by the type of bond.

Q: What exactly are London dispersion forces?

They are weak, temporary attractive forces that occur when the electrons in two adjacent atoms or molecules randomly shift, creating temporary dipoles. These temporary dipoles induce dipoles in neighboring molecules, causing them to attract each other.

Q: Why does a spherical shape reduce the boiling point?

Geometrically, a sphere has the minimum surface area for a given volume. Less surface area means less contact area between adjacent molecules. With fewer points of contact, the London dispersion forces are weaker, making it easier for the molecules to escape into the gas phase.

Q: Does molecular mass play a role in this specific question?

All these molecules have 4 carbon atoms, so their molecular masses are very similar (ranging from 54 to 58 g/mol). When the mass is roughly constant, the physical shape (branching) becomes the deciding factor for the boiling point.

Q: Is isobutene considered a polar molecule?

Isobutene has a very small dipole moment due to the electron-donating nature of the methyl groups, but it is essentially considered non-polar for these types of physical property comparisons. The dominant intermolecular force remains the London dispersion force.

Question

Which one of the following has the minimum boiling point?

Options:

(A) n-butane

(B) 1-butyne

(C) 1-butene

(D) Isobutene

Sigma Percentile · Accepted Answer

### Analyzing the Setup

Imagine you are trying to pull apart a bunch of sticky notes. If they are laid out flat against each other, it takes a lot of effort to separate them. But if you crumple them into little balls, they barely stick together at all. This simple analogy is the secret to understanding the boiling points of hydrocarbons!

In this problem, we are given four $C_4$ hydrocarbons: **n-butane**, **1-butyne**, **1-butene**, and **isobutene**. Our mission is to identify the one with the minimum boiling point. To do this, we need to look at the invisible forces holding these molecules together.

### The Master Equation: Intermolecular Forces

Boiling point is essentially a measure of how much thermal energy is required to overcome the intermolecular forces of attraction in a liquid, turning it into a gas. For non-polar molecules like hydrocarbons, the primary forces at play are **London Dispersion Forces** (a type of van der Waals force).

These forces are temporary and weak, but they depend heavily on one crucial geometric factor: **Surface Area**.

$$ 	ext{London Dispersion Force} \propto 	ext{Surface Area} $$

When a molecule is a long, straight chain, it has a large surface area. This allows it to have more points of contact with neighboring molecules, leading to stronger overall attractive forces.

### The Impact of Branching

Now, what happens when we introduce branching into a molecule?

Branching forces the molecule to become more compact and spherical. A sphere has the minimum possible surface area for a given volume. Therefore, as branching increases, the surface area of the molecule decreases.

Let's evaluate our options:
1. **n-butane**: A straight-chain alkane.
2. **1-butyne**: A straight-chain alkyne.
3. **1-butene**: A straight-chain alkene.
4. **Isobutene (2-methylpropene)**: A branched alkene.

### Final Calculation

Notice that n-butane, 1-butyne, and 1-butene are all unbranched, elongated molecules. They have relatively large surface areas.

**Isobutene**, however, has a methyl group branching off the central carbon. This branching makes isobutene the most compact and spherical molecule among the choices.

Because it has the minimum surface area, isobutene experiences the weakest van der Waals forces. Consequently, it requires the least amount of heat energy to boil.

Therefore, **isobutene** has the minimum boiling point. This is a universal rule in organic chemistry: for a given set of isomers, **boiling point is inversely proportional to branching**.

Animated Solution for Chemistry - Organic Chemistry: Which one of the following has the minimum boiling point?

Select Answer:

Visualized Solution

$Analyzing the Hydrocarbons$

$Boiling Point & Intermolecular Forces$

$Surface Area Dependency$

$Identifying the Branched Isomer$

$Conclusion$

$Generalizing the Rule$

The Sigma Insight: Hydrocarbons

Analyzing the Setup

The Master Equation

The Impact of Branching

Final Calculation

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