Rank The Following Alkanes In Order Of Increasing Boiling Point

Ranking Alkanes by Increasing Boiling Point: A Comprehensive Guide

Understanding the relationship between alkane structure and boiling point is crucial in organic chemistry. This article delves deep into the factors influencing boiling points of alkanes, providing a clear methodology for ranking them and explaining the underlying principles with numerous examples. We'll move beyond simple memorization and equip you with the knowledge to predict boiling points accurately.

Understanding Boiling Point and Intermolecular Forces

Before we rank alkanes, let's establish a foundation. Boiling point is the temperature at which a substance transitions from a liquid to a gas. This transition requires overcoming the intermolecular forces (IMFs) holding the molecules together in the liquid phase. For alkanes, the primary IMF is the London Dispersion Force (LDF), also known as Van der Waals forces.

London Dispersion Forces (LDFs): The Key Player

LDFs are temporary, weak attractions arising from instantaneous fluctuations in electron distribution around a molecule. These fluctuations create temporary dipoles, which induce dipoles in neighboring molecules, leading to a net attractive force. The strength of LDFs depends on several factors:

• Molecular Size: Larger molecules have more electrons, leading to stronger and more frequent instantaneous dipoles. Therefore, larger alkanes experience stronger LDFs.
• Molecular Shape: A more compact, spherical shape minimizes surface area, resulting in weaker LDFs compared to a longer, more extended shape with increased surface contact. Branching significantly impacts this factor.
• Surface Area: Greater surface area allows for more points of contact between molecules, thus enhancing LDFs.

Factors Affecting Alkane Boiling Points: A Detailed Breakdown

With the understanding of LDFs, let's examine the specific factors that dictate the boiling points of alkanes:

1. Molecular Weight (or Carbon Chain Length): The Dominant Factor

The most significant factor influencing alkane boiling points is the molecular weight, directly correlated with the number of carbon atoms in the chain. As the carbon chain length increases, the number of electrons increases, leading to stronger LDFs and a higher boiling point. This is consistently observed across the alkane homologous series.

Example: Consider methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀). Their boiling points increase steadily due to the increasing molecular weight and corresponding increase in LDF strength.

2. Branching: A Significant Modifier

Branching significantly alters the boiling point. Branched alkanes have lower boiling points than their straight-chain isomers (alkanes with the same molecular formula but different structures). This is because branching reduces the surface area available for intermolecular contact, weakening the LDFs.

Example: Compare n-butane (straight-chain) and isobutane (branched). n-butane has a higher boiling point because its linear structure allows for greater surface contact and stronger LDFs than the more compact isobutane.

3. Shape and Conformation: Subtle but Important

While branching is the most impactful shape-related factor, even subtle differences in conformation (the spatial arrangement of atoms) can affect boiling points to a lesser extent. Extended, linear conformations generally lead to slightly higher boiling points compared to more folded or coiled conformations. However, this effect is less pronounced than the impact of branching.

4. Intermolecular Interactions Beyond LDFs: A Minor Consideration

Although LDFs are the primary IMFs in alkanes, other weak interactions, such as dipole-dipole interactions, are negligible because alkanes are essentially nonpolar molecules. Hydrogen bonding is entirely absent in alkanes.

Ranking Alkanes: A Step-by-Step Approach

Now let's apply this knowledge to rank alkanes based on increasing boiling points. Consider the following set of alkanes:

1. 2-methylbutane
2. Pentane
3. 2,2-dimethylpropane
4. Hexane
5. 2-methylpentane

Step 1: Identify the dominant factor – molecular weight:

Hexane (C₆H₁₄) has the highest molecular weight and will likely have the highest boiling point. Pentane (C₅H₁₂), 2-methylpentane (C₆H₁₄), and 2-methylbutane (C₅H₁₂) are next, followed by 2,2-dimethylpropane (C₅H₁₂).

Step 2: Account for branching:

Within the same molecular weight group (C₅H₁₂), consider branching:

• 2,2-dimethylpropane: This is the most branched isomer, leading to the weakest LDFs and the lowest boiling point within this group.
• 2-methylbutane: This is less branched than 2,2-dimethylpropane, resulting in slightly stronger LDFs and a higher boiling point.
• Pentane: Being a straight-chain alkane, pentane experiences the strongest LDFs amongst this group.

Within the C₆H₁₄ group:

• 2-methylpentane: This branched isomer will have a slightly lower boiling point than hexane.

Step 3: Arrange in increasing boiling point order:

Based on molecular weight and branching, the ranking of the alkanes in order of increasing boiling point is:

1. 2,2-dimethylpropane (most branched, lowest molecular weight)
2. 2-methylbutane (branched, intermediate molecular weight)
3. Pentane (straight-chain, intermediate molecular weight)
4. 2-methylpentane (branched, highest molecular weight)
5. Hexane (straight-chain, highest molecular weight)

Advanced Considerations and Examples

Let's extend this analysis with more complex examples:

Example 1: Rank the following alkanes in order of increasing boiling point:

1. Octane
2. 2,2,4-trimethylpentane
3. Nonane
4. 2-methylheptane

Solution:

Nonane > Octane > 2-methylheptane > 2,2,4-trimethylpentane. Nonane has the highest molecular weight. Octane has a higher molecular weight than the others in this set, followed by 2-methylheptane (branched), and finally 2,2,4-trimethylpentane (highly branched, lowest boiling point among the group).

Example 2: Explain why cyclohexane has a higher boiling point than n-hexane, despite having the same molecular formula (C₆H₁₂).

Solution: While both have the same molecular weight, cyclohexane's ring structure allows for more efficient packing and increased surface contact between molecules, resulting in stronger LDFs and a higher boiling point than the more flexible n-hexane. The ring structure constraints the molecule into a particular shape and reduces the conformational freedom compared to n-hexane.

Conclusion

Predicting the relative boiling points of alkanes requires a thorough understanding of intermolecular forces, primarily London Dispersion Forces. Molecular weight and branching are the most dominant factors. By systematically considering these factors, you can confidently rank alkanes in order of increasing boiling points and grasp the fundamental principles governing their physical properties. Remember, this understanding extends beyond simple memorization, providing a powerful tool for predicting and interpreting the behavior of organic molecules. Consistent practice with diverse examples will solidify your comprehension and expertise in this area.