Problem 35 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

n-Octane has the highest boiling point due to its straight chain and long carbon chain.

Step 1: Understand the Relationship between Structure and Boiling Point

Boiling point is often influenced by the molecular structure. Straight-chain alkanes generally have higher boiling points compared to their branched-chain isomers due to more surface area, which enhances van der Waals forces. It's important to consider the molecular structure of each compound when predicting boiling points.

Step 2: Examine the Molecular Structures

Analyze the given compounds' structures: - Iso-octane (a) is a highly branched C8H18. - n-Octane (b) is a straight-chain alkane, C8H18. - 2,2,3,3-tetramethylbutane (c) is a highly branched C8H18. - n-Butane (d) is a straight-chain alkane, C4H10. Note the difference in branching and chain length.

Step 3: Evaluate the Effect of Branching

Straight-chain alkanes generally have higher boiling points than branched alkanes of similar molecular weights. Therefore, n-octane, being a straight chain, is likely to have a higher boiling point than its isomer, iso-octane, and other highly branched structures like 2,2,3,3-tetramethylbutane.

Step 4: Consider Molecular Size and Chain Length

A longer carbon chain increases the boiling point due to greater van der Waals forces. n-Butane has a shorter chain length compared to n-octane, making n-octane likely to have a higher boiling point due to a longer molecular size.

Step 5: Conclusion

Based on the analysis, n-octane should have the highest boiling point as it is a straight-chain alkane with a longer carbon chain than n-butane and less branching compared to iso-octane and 2,2,3,3-tetramethylbutane.

Key Concepts

Molecular Structure

Molecular structure plays a crucial role in determining the boiling point of a substance. The arrangement of atoms and the shape of the molecule can significantly influence how these molecules interact with one another.
Straight-chain alkanes, such as n-octane, tend to have higher boiling points than their branched counterparts. This is because straight-chain molecules can pack closely together, maximizing surface contact between molecules. This increased contact strengthens intermolecular forces, specifically van der Waals forces, leading to higher boiling points.
In contrast, branched-chain alkanes like iso-octane and 2,2,3,3-tetramethylbutane cannot pack as tightly. Their branching reduces surface contact and weakens the van der Waals forces, resulting in lower boiling points compared to straight-chain isomers.

Van der Waals Forces

Van der Waals forces are weak forces of attraction that play a significant role in determining the boiling points of nonpolar molecules. These forces arise from momentary fluctuations in electron density within molecules, which create temporary dipoles. When molecules are in close proximity, these dipoles interact, leading to an attraction between molecules.
For alkanes, as the molecular size or length increases, so does the strength of these van der Waals forces. This occurs because larger molecules have more surface area over which these attractions can act, leading to higher boiling points. Thus, a molecule like n-octane, which has a relatively long and straight chain, experiences stronger van der Waals forces than a smaller molecule such as n-butane.

• Longer chains = more surface area = stronger van der Waals forces
• Stronger forces = higher boiling points

Understanding van der Waals forces highlights why the structure and size of alkanes are key to predicting their boiling points.

Straight-Chain Alkanes

Straight-chain alkanes are hydrocarbons with carbon atoms arranged in a continuous, unbranched line. This linear configuration maximizes molecular surface area and allows atoms to align closely together. Consequently, these molecules exhibit significant van der Waals forces, which are critical for their physical properties, like the boiling point.
For example, n-octane, a straight-chain alkane with eight carbon atoms, exhibits a high boiling point compared to other branched alkanes with the same number of carbons. This is because its structure allows for increased interaction between molecules directly through surface contacts.

• The longer the chain, the higher the boiling point
• Straight-chain configurations enable better packing of molecules

Therefore, when predicting which alkane will have a higher boiling point, it's beneficial to consider if the molecule is a straight-chain, as seen with n-octane compared to its isomers or smaller alkanes like n-butane.