Question

Which of the following has the minimum boiling point? (1) \(\mathrm{n}\) -butane (2) 1 -butyne (3) 1 -butene (4) Isobutene

Short answer

Isobutene

Step-by-step solution

Understand Boiling Point Trends

Boiling points of compounds depend on intermolecular forces. For hydrocarbons, the van der Waals forces (or London dispersion forces) play a major role. The more branched the molecule, the lower the boiling point, because branched molecules have less surface area for these forces to act.

Compare Structures

Compare the structures of - (1) n-butane: a straight-chain alkane - (2) 1-butyne: an alkyne with a triple bond - (3) 1-butene: an alkene with a double bond - (4) Isobutene: a branched alkene.

Evaluate Boiling Points

Among these, n-butane is a straight chain, while isobutene is branched. The branching in isobutene reduces the boiling point more significantly compared to the straight-chain structures of 1-butyne and 1-butene.

Conclusion

Based on structure and branching, isobutene is expected to have the lowest boiling point due to its reduced surface area and consequently weaker van der Waals forces.

Key concepts

hydrocarbons

Hydrocarbons are organic compounds made up of only hydrogen and carbon atoms. They form the basis of many organic substances and fall into various categories such as alkanes, alkenes, and alkynes. Each category has unique bonding and structural characteristics.

Alkanes are hydrocarbons with single bonds between carbon atoms, making them saturated hydrocarbons, like n-butane. Alkenes contain at least one double bond, as seen in 1-butene and isobutene, while alkynes like 1-butyne have at least one triple bond.

The physical properties of hydrocarbons vary based on their structure. For example, the boiling points generally increase with molecular weight but can be significantly influenced by molecular shape and branching, impacting their intermolecular forces.

intermolecular forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles—atoms, molecules, or ions. These forces are weaker than the intramolecular forces, such as covalent or ionic bonds, holding a molecule together.

For hydrocarbons, the main type of intermolecular force is the van der Waals forces (also known as London dispersion forces). These forces arise due to the temporary dipoles created when the electrons in an atom or molecule are momentarily unevenly distributed.

The strength of van der Waals forces depends on several factors including molecular size and surface area. Straight-chain hydrocarbons like n-butane have more surface area for these forces to act upon, leading to higher boiling points compared to branched molecules like isobutene, where the surface area is reduced, making the intermolecular forces weaker and boiling points lower.

branching effects

Branching significantly affects the boiling point and other physical properties of molecules. When a molecule is straight-chained, it has a larger surface area in contact with neighboring molecules, leading to stronger van der Waals forces and a higher boiling point.

In contrast, branched molecules, like isobutene, have a more compact structure and less surface area exposed to neighboring molecules. This results in weaker intermolecular forces (van der Waals forces) and thus a lower boiling point.

Therefore, when comparing different hydrocarbons, branched isomers tend to have lower boiling points than their straight-chain counterparts. This is why isobutene has the lowest boiling point among the given hydrocarbons in the exercise.