Understanding Hydrocarbon Boiling Points

The boiling point of a substance is the temperature at which it changes from a liquid to a gas. For hydrocarbons, especially simple alkanes, the boiling point is primarily influenced by the strength of intermolecular forces between the molecules. These forces are mainly London dispersion forces.

London dispersion forces arise from temporary fluctuations in electron distribution, creating temporary dipoles. Larger molecules with more electrons have stronger temporary dipoles and thus stronger London dispersion forces. More energy (higher temperature) is required to overcome these stronger forces and boil the substance.

Therefore, for a series of similar hydrocarbons like alkanes, the boiling point generally increases as the number of carbon atoms (and thus molecular size and mass) increases. Branched hydrocarbons have slightly lower boiling points than their straight-chain isomers due to reduced surface area for interaction, but the primary factor when comparing different hydrocarbons is the total number of carbon atoms.

Analyzing Hydrocarbon Boiling Points in Options

Let's examine the hydrocarbons mentioned in the options and determine their number of carbon atoms. We can use a table to organize this information.

Now, let's look at the order of hydrocarbons in each option and check if the number of carbon atoms is increasing. This indicates the order of increasing boiling points.

Option 1: Methane, Butane, Propane, Heptane

• Methane (C1)
• Butane (C4)
• Propane (C3)
• Heptane (C7)

Carbon count order: 1, 4, 3, 7. This is not an increasing order because 4 is not less than 3.

Option 2: Propane, Butane, Pentane, Octane

• Propane (C3)
• Butane (C4)
• Pentane (C5)
• Octane (C8)

Carbon count order: 3, 4, 5, 8. This is an increasing order (3 < 4 < 5 < 8). Therefore, their boiling points are likely in increasing order.

Option 3: Propane, Butane, Heptane, Methane

• Propane (C3)
• Butane (C4)
• Heptane (C7)
• Methane (C1)

Carbon count order: 3, 4, 7, 1. This is not an increasing order because 7 is not less than 1.

Option 4: Octane, Ethane, Methane, Propane

• Octane (C8)
• Ethane (C2)
• Methane (C1)
• Propane (C3)

Carbon count order: 8, 2, 1, 3. This is not an increasing order because 8 is not less than 2.

Conclusion: Increasing Order of Boiling Points

Based on the analysis of carbon atom numbers and the general trend of boiling points in hydrocarbons, Option 2 presents the hydrocarbons in an increasing order of their boiling points because the number of carbon atoms increases consistently: Propane (3 carbons), Butane (4 carbons), Pentane (5 carbons), Octane (8 carbons).

Revision Table: Hydrocarbon Properties

Note: Boiling points are for n-alkanes (straight-chain isomers).

Additional Information: Factors Affecting Boiling Points

While the number of carbon atoms is the primary factor for boiling points in a homologous series like alkanes, other factors can also play a role:

• Branching: For isomers with the same number of carbon atoms, branched hydrocarbons tend to have lower boiling points than straight-chain ones. This is because branching reduces the surface area available for London dispersion force interactions.
• Polarity: Although hydrocarbons are nonpolar, the presence of polar functional groups (like -OH in alcohols or -COOH in carboxylic acids) introduces stronger intermolecular forces like hydrogen bonding or dipole-dipole interactions, significantly increasing boiling points compared to hydrocarbons of similar size.
• Molecular Shape: Compact, spherical molecules have smaller surface areas and thus lower boiling points compared to elongated molecules of similar size.

Understanding these factors helps predict the relative boiling points of various organic compounds.