Boiling Point and Volatility in Organic Homologous Series

Learning Objectives

• Explain the trends (melting point, boiling point, volatility, and solubility in water and organic solvents) within and between homologous series (alkanes, alkenes, alcohols, carboxylic acids) in terms of intermolecular and intramolecular bonding.

• Specifically consider intermolecular forces such as dispersion forces, dipole-dipole interactions, and hydrogen bonds.

• Analyse data to determine the physical properties of an homologous series.

• Determine trends in melting point, boiling point, volatility, and the solubility of alkanes, alkenes, alcohols, and carboxylic acids.

Overview of Physical Properties and Intermolecular Forces

• Physical properties of organic molecules include melting point, boiling point, solubility, and volatility.

• Molecules in the liquid and solid states are held together by intermolecular forces.

• A homologous series containing the same functional group will demonstrate steady, predictable changes in physical properties.

Boiling Point Fundamentals

• Boiling occurs when molecules in the liquid state possess enough energy to completely overcome the intermolecular forces holding them together.

• There is a direct relationship between the strength of intermolecular forces and boiling point:

  • Stronger intermolecular forces require more energy to break.

  • Higher energy requirements result in higher boiling points.

Detailed Analysis of Intermolecular Forces

Dispersion Forces

• Description: Dispersion forces occur between all molecules, regardless of whether they are polar or non-polar. They are caused by temporary dipoles in the molecules resulting from the random movement of the electrons surrounding the molecules.

• Relative Strength: These are the weakest intermolecular forces when comparing molecules of the same molecular mass.

Dipole-Dipole Forces

• Description: These forces occur between polar molecules. They result from the attraction between the positive and negative ends of polar molecules.

• Relative Strength: They are stronger than dispersion forces but significantly weaker than hydrogen bonding.

Hydrogen Bonding

• Description: This is a particularly strong form of dipole-dipole force. It only occurs between highly polar molecules in which a hydrogen atom is covalently bonded to an oxygen, a nitrogen, or a fluorine atom ($O$, $N$, or $F$).

• Examples provided in diagrams:

  • Hydrogen bonding between methanol molecules.

  • Hydrogen bonding between water and the oxygen atom of propanone (acetone).

• Relative Strength: These are the strongest of the intermolecular forces.

Boiling Points in Hydrocarbons (Alkanes, Alkenes, Alkynes)

• Non-polar hydrocarbons possess only weak intermolecular forces (dispersion forces).

• Consequently, they generally have lower boiling points compared to molecules of similar mass that have polar functional groups.

• Specific examples of molecules with weak dispersion forces include:

  • Butane

  • But-1-ene

  • But-1-yne

• Chloroethane is an exception among simple hydrocarbons as it also possesses dipole-dipole interactions due to its polarity.

Boiling Point Trends in Homologous Series

• Chain Length: Boiling points within a homologous series generally increase as the carbon chain length increases.

• Reasoning: As the chain gets longer, the contact surface area between the molecules increases. This larger surface area results in stronger dispersion forces.

• Non-polar Tail: In oxygenated or nitrogenated series, as the non-polar hydrocarbon (tail) end increases in size, the contact surface area and dispersion forces increase, leading to an increase in boiling point.

Influence of Molecular Branching

• Molecular shape significantly influences the strength of dispersion forces.

• Straight-chain alkanes: These molecules can fit closely together, allowing for more surface area in contact. This leads to higher dispersion forces.

• Branched-chain alkanes: These molecules are more compact. This leads to less surface area in contact.

• Comparison: Melting point ($MP$) and boiling point ($BP$) of straight-chain alkanes > $MP$ and $BP$ of branched-chain alkanes.

Boiling Points of Polar Functional Groups

Aldehydes, Ketones, and Esters

• These molecules contain a carbonyl group ($C=O$), which is polar.

• Intermolecular forces include dispersion forces and dipole-dipole interactions.

• These molecules have higher boiling points than hydrocarbons of similar mass, but lower boiling points than alcohols because they cannot form hydrogen bonds with themselves.

Alcohols

• Alcohols form hydrogen bonds with neighbouring molecules because oxygen is more electronegative than hydrogen, making the $-OH$ bond highly polar.

• They have significantly higher boiling points compared to alkanes of similar molecular mass.

Amines and Amides

• Amines: Presence of nitrogen and hydrogen ($N-H$) allows for highly polar bonds and hydrogen bonding.

• Amides: Presence of hydrogen and oxygen allows for hydrogen bonding.

• Both result in higher boiling points due to these strong intermolecular attractions.

Boiling Points in Carboxylic Acids

• Carboxylic acids have the capability to form hydrogen bonds between oxygen and hydrogen twice with each molecule.

• Dimers: Two molecules of carboxylic acids can associate to form a stable, large unit known as a dimer.

• Dimers are stable and have a larger effective mass and surface area, leading to a stronger combination of dispersion forces and hydrogen bonding.

• Carboxylic acids therefore have higher boiling points compared to other molecules of similar molecular masses.

Effects of Branching in Alcohols

• Within a series, boiling points are highest in primary alcohols and decrease transitioning toward tertiary alcohols.

• As the hydroxyl group ($-OH$) becomes increasingly "crowded" by alkyl groups in secondary and tertiary configurations, the molecule's ability to form hydrogen bonds is restricted.

Melting Points in Homologous Series

• Molecular Shape: Shape is crucial for melting points because the closer the molecules can pack together in a solid lattice, the higher the melting point ($MP$) is likely to be.

• Trends: There is a general increase in melting point within a homologous series as chain length increases, similar to boiling point trends.

• Regularity: The increase is usually not as regular as boiling point trends; small changes occur frequently.

• Carbon Count: There is an irregular trend of differences between molecules with even versus odd numbers of carbon atoms (e.g., comparing 2, 4, and 6 carbons).

Volatility and Vapour Pressure

• Definition: Volatility refers to how easily a compound evaporates (a liquid forming a vapour).

• Dependence: Volatility depends on the strength of intermolecular forces in the liquid state.

• Most Volatile: Compounds with weak intermolecular forces are the most volatile because the forces are easily overcome at lower temperatures.

• Boiling Point vs. Evaporation:

  • Boiling occurs at a specific temperature.

  • Evaporation can occur at a range of temperatures.

  • There is a link: compounds with lower boiling points typically have lower intermolecular forces and are more volatile (possessing higher vapour pressure).

Volatility Trends Across Series

• Hydrocarbons: Non-polar with only dispersion forces. They have weak intermolecular forces, which lead to low boiling points and high volatility. As chain length increases, they become less volatile.

• Haloalkanes, Aldehydes, and Ketones: These are polar and held by dispersion and dipole-dipole forces. They are generally in the liquid state at room temperature, having higher boiling points and lower volatility than hydrocarbons.

• Alcohols and Carboxylic Acids: These generally have higher boiling points and are less volatile than hydrocarbons due to the presence of strong hydrogen bonds in the liquid state.

Questions & Discussion

Item 1: Predicting Highest Boiling Points

Question A: In the set of butan-1-ol, hexan-1-ol, and pentan-1-ol, which has the highest boiling point?

• Answer: Hexan-1-ol has the highest boiling point because it has the longest carbon chain, which increases the strength of the dispersion forces.

Question B: In the set of pentan-1-ol, 2-methylbutan-1-ol, and 2-methylbutan-2-ol, which has the highest boiling point?

• Answer: Pentan-1-ol has the highest boiling point.

• Reasoning: It is a straight-chain isomer, meaning its molecules can fit together closely and form strong hydrogen bonds. The other two compounds are branched isomers; their alkyl groups crowd the hydroxyl ($-OH$) functional groups, restricting the formation of hydrogen bonds between neighbouring molecules.