S3.2.4 Physical trends in homologous series

Melting and Boiling Points, Volatility, and Solubility of Organic Compounds

Picture this: you're boiling water on the stove.
As the water heats up, its molecules gain enough energy to break free from the attractive forces holding them together, transforming into steam.
Now imagine trying to boil honey instead.
It would take significantly more heat to reach its boiling point. Why?

The answer lies in the forces acting between the molecules, which determine properties like melting and boiling points, volatility, and solubility.

Melting and Boiling Points: The Role of Molecular Size and Intermolecular Forces

How Molecular Size Affects Boiling Points

Boiling occurs when molecules in a liquid gain enough energy to overcome the forces holding them together.
For organic compounds, these forces are primarily intermolecular forces, such as London dispersion forces, dipole-dipole interactions, and hydrogen bonding.
As molecular size increases, such as in a homologous series like the alkanes, boiling points also rise.
Why? Larger molecules have more electrons, which increases the strength of Van der Waals forces.

These forces result from temporary dipoles created by the random movement of electrons.

Example

Consider the boiling points of the first few alkanes:

Methane (CH₄): Boiling point = -161°C
Ethane (C₂H₆): Boiling point = -89°C
Propane (C₃H₈): Boiling point = -42°C

As the carbon chain length increases, the boiling point rises because the larger molecules experience stronger dispersion forces.

The table showing boiling points of alkanes.

Effect of Branching on Physical Trends in Homologous Series

Branching significantly influences the physical properties of compounds in a homologous series:

Boiling Point: Branching reduces the surface area available for intermolecular forces, weakening Van der Waals forces and lowering boiling points compared to straight-chain isomers.
Melting Point: Branching disrupts regular crystal packing, often lowering melting points. However, highly symmetrical branching can enhance packing efficiency, leading to higher melting points.

The Effect of Functional Groups on Boiling Points

Functional groups can significantly impact boiling points by introducing polar interactions or hydrogen bonding, both of which strengthen intermolecular forces.

Hydroxyl Groups (-OH):
- Alcohols, which contain hydroxyl groups, exhibit hydrogen bonding—a particularly strong type of dipole-dipole interaction.
- This raises their boiling points compared to alkanes of similar molecular size.

Example

Ethanol ( $C_{2} H_{O} H$ ) has a boiling point of 78°C, while ethane ( $C_{2} H_{6}$ ) boils at -89°C.

Carbonyl Groups (C=O):
- Aldehydes and ketones, with their polar carbonyl groups, experience dipole-dipole interactions.
- These are weaker than hydrogen bonds but still stronger than dispersion forces.

Note

Hydrogen bonding occurs when a hydrogen atom is directly bonded to a highly electronegative atom (N, O, or F) and interacts with a lone pair on another electronegative atom.

Common Mistake

Students sometimes confuse hydrogen bonding with dipole-dipole interactions. Remember, hydrogen bonding is a specific, stronger type of dipole-dipole interaction.

Volatility and Solubility: Smaller Molecules and Polar Functional Groups

Volatility

Definition

Volatility

Volatility describes how easily a substance evaporates.

Substances with lower boiling points are more volatile because their molecules require less energy to escape into the gas phase.

Smaller Molecules: Smaller organic molecules generally have weaker intermolecular forces, making them more volatile.

Example

Methanol ( $C H_{3} O H$ ) is more volatile than ethanol ( $C_{2} H_{5} O H$ ) because it has fewer dispersion forces.

Functional Groups and Polarity: Polar functional groups, such as hydroxyl (-OH) or carbonyl (C=O), can reduce volatility by increasing intermolecular attractions.

Example

Propanone (acetone, $C H_{3} C O C H_{3}$ ) is less volatile than propane ( $C_{3} H_{8}$ ) because of its polar carbonyl group.

Tip

Volatility decreases as the strength of intermolecular forces increases. Substances with hydrogen bonding are generally less volatile than those with only dispersion forces.

Schematic drawing the trend in increasing boiling point for different organic compounds.

Solubility in Water

Water, a highly polar solvent, dissolves polar and hydrogen-bonding substances well. This principle can be summarized as: "like dissolves like."

Polar Functional Groups: Molecules with polar functional groups (e.g., hydroxyl, carbonyl, or amino groups) can form hydrogen bonds with water, making them soluble.

Example

Ethanol ( $C_{2} H_{5} O H$ ) is highly soluble in water because its -OH group forms hydrogen bonds with water molecules.

Nonpolar Molecules: Nonpolar molecules, such as hydrocarbons, are generally insoluble in water because they cannot form significant interactions with water molecules.

Example

Hexane ( $C_{6} H_{14}$ ) is immiscible in water.

Effect of Chain Length: As the carbon chain length increases, the molecule's nonpolar character dominates, reducing its solubility in water.

Example

Methanol ( $C H_{3} O H$ ) is completely miscible in water, while hexanol ( $C_{6} H_{13} O H$ ) is only sparingly soluble.

Analogy

Think of water molecules as a tightly knit group of dancers holding hands (hydrogen bonding). Polar molecules can join the dance because they, too, can "hold hands" with water. Nonpolar molecules, however, are like people who can't connect—they're left out of the group.

Self review

What happens to the solubility of an alcohol as its carbon chain length increases? Why?

Summary of Trends

Here’s a summary of how molecular size, functional groups, and intermolecular forces influence the properties of organic compounds:

Property	Key Factors
Boiling Point	Increases with molecular size due to stronger London dispersion forces. Functional groups like -OH and C=O introduce hydrogen bonding or polarity, further raising boiling points.
Volatility	Decreases with stronger intermolecular forces, such as hydrogen bonding.
Solubility in Water	Favored by polar functional groups (e.g., -OH, C=O), but decreases with increasing chain length as the non-polar hydrocarbon portion dominates.

Applications and Implications

Understanding these trends has real-world significance across various fields:

Pharmaceuticals: Drug design often requires balancing polarity to ensure drugs are soluble in bodily fluids but can still cross nonpolar cell membranes.
Environmental Science: Volatility determines how pollutants spread. For instance, volatile organic compounds (VOCs) evaporate easily, contributing to air pollution.

Theory of Knowledge

How does understanding the balance between solubility and volatility help chemists design safer and more effective drugs or industrial chemicals?

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S3.2.4 Physical trends in homologous series

Melting and Boiling Points, Volatility, and Solubility of Organic Compounds

Melting and Boiling Points: The Role of Molecular Size and Intermolecular Forces

How Molecular Size Affects Boiling Points

Effect of Branching on Physical Trends in Homologous Series

The Effect of Functional Groups on Boiling Points

Volatility and Solubility: Smaller Molecules and Polar Functional Groups

Volatility

Solubility in Water

Summary of Trends

Applications and Implications

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Introduction to Physical Trends in Homologous Series