Question

The highest boiling point is expected for: (a) iso-octane (b) n-octane (c) \(2,2,3,3\) -tetramethyl butane (d) \(\mathrm{n}\) -butane

Short answer

n-octane has the highest boiling point.

Step-by-step solution

Understand the Relationship Between Structure and Boiling Point

Boiling point is influenced by the structure of a hydrocarbon. Linear alkanes tend to have higher boiling points than their branched isomers due to stronger Van der Waals forces, as linear structures allow molecules to pack closer together.

Compare the Structures of the Given Compounds

Let's consider the structures: (a) iso-octane is a branched alkane, (b) n-octane is a straight-chain alkane, (c) \(2,2,3,3\) -tetramethyl butane is heavily branched, and (d) n-butane is a smaller straight-chain alkane.

Determine Boiling Points Based on Structures

n-Octane, being a straight-chain alkane with eight carbon atoms, has a higher boiling point than iso-octane and \(2,2,3,3\) -tetramethyl butane, due to its linear structure. n-Butane, with only four carbon atoms, has a lower boiling point than octane variants.

Conclude the Comparison

Based on structural analysis, n-octane should have the highest boiling point among the choices due to its linear structure and longer chain compared to others, which are branched or shorter.

Key concepts

Structure of Hydrocarbons

Hydrocarbons are organic compounds made up entirely of hydrogen and carbon atoms. The way these atoms are arranged can vary greatly, leading to different structures such as straight chains or branched forms.
The structure of hydrocarbons significantly influences their physical and chemical properties. For instance:

• Straight-chain hydrocarbons, or linear alkanes, allow molecules to come closer to each other, increasing the effectiveness of intermolecular forces like van der Waals forces.
• Branched hydrocarbons have a more compact and complicated structure, preventing them from packing closely, which often results in lower boiling points compared to their linear counterparts.

These structural differences make it important to understand how hydrocarbons are arranged when predicting their boiling points and other characteristics.

n-Octane

n-Octane is a type of hydrocarbon known as an alkane, specifically a straight-chain (or normal) alkane. Its molecular formula is C\(_{8}\)H\(_{18}\). The 'n' in n-octane stands for 'normal,' indicating its linear, unbranched structure.
A few key points about n-octane include:

• It consists of a continuous chain of eight carbon atoms.
• This linear arrangement allows the molecules to align closely, resulting in stronger van der Waals forces compared to branched isomers.
• As a result, n-octane typically has a relatively high boiling point compared to other isomers with the same number of carbon atoms but more branching.

Because of its straight-chain structure, it often serves as a reference point in studies discussing the properties of hydrocarbons.

Van der Waals Forces

Van der Waals forces are weak intermolecular forces that play a crucial role in determining the boiling points of hydrocarbons.
There are several types of van der Waals forces, but the most relevant to hydrocarbons are London Dispersion Forces. These are weak attractions that occur due to the temporary dipoles formed when the electrons in two adjacent atoms occupy positions that make the atoms form temporary dipoles. Key points include:

• Linear structures such as n-octane allow for more contact points between molecules, leading to stronger van der Waals forces and thus higher boiling points.
• Branched structures disrupt the ability of the molecules to tightly pack together, reducing the contact points and, consequently, the effectiveness of these forces.

Understanding van der Waals forces helps explain why some hydrocarbons have higher boiling points than others with an identical molecular formula.

Branching in Alkanes

Branching in alkanes refers to the presence of one or more carbon atoms that connect to more than two other carbon atoms, creating a network different from a straight chain. Branching has significant effects on the properties of hydrocarbons.
When comparing branched alkanes to their straight-chain counterparts:

• Branched alkanes generally have lower boiling points. This is because the branched shapes prevent the compact packing of the molecules, reducing the effectiveness of van der Waals forces.
• Examples include iso-octane and 2,2,3,3-tetramethylbutane, which have lower boiling points than n-octane despite having the same molecular formula.

The density and melting points of alkanes can also be influenced by the degree of branching, making it a critical factor in understanding and predicting the behavior of organic compounds.