Boiling Points Of Functional Groups

1.0What is a Functional Group?

A functional group is a specific group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule. Examples include the hydroxyl group (-OH) in alcohols and the carbonyl group (>C=O) in aldehydes and ketones. Functional groups are the centers of chemical reactivity and largely determine a molecule's properties.

2.0What is Boiling Point?

The boiling point of a substance is the temperature at which it changes its state from liquid to gas at a given pressure. For a liquid to boil, its molecules must have enough kinetic energy to overcome the intermolecular forces (IMFs) that hold them together. Therefore, the strength of these forces is the primary determinant of a compound's boiling point. The stronger the IMFs, the higher the boiling point.

3.0Factors Affecting Boiling Points

Several factors influence the boiling points of organic compounds:

Intermolecular Forces

• Van der Waals Forces: Present in all molecules; strength increases with molecular size.
• Dipole-Dipole Interactions: Occur in polar molecules with permanent dipoles.
• Hydrogen Bonding: A strong type of dipole-dipole interaction occurring when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine.

Molecular Weight

Generally, an increase in molecular weight leads to higher boiling points due to enhanced Van der Waals forces.

Molecular Structure and Branching

• Linear Molecules: Tend to have higher boiling points due to greater surface area, leading to stronger Van der Waals interactions.
• Branched Molecules: Have lower boiling points as branching reduces surface area, weakening intermolecular forces.

4.0Boiling Points of Common Functional Groups

The boiling point (BP) of a compound is the temperature at which it changes from a liquid to a gas. In organic chemistry, the boiling points of functional groups depend primarily on the intermolecular forces present in the molecules, including:

1. London Dispersion Forces – Weak forces in non-polar molecules, stronger in larger molecules.
2. Dipole-Dipole Interactions – Present in polar molecules, increasing BP.
3. Hydrogen Bonding – Strong intermolecular force in –OH, –NH₂, –COOH groups, greatly increasing BP.
4. Molecular Mass and Shape – Higher molecular mass and linear shapes increase BP, while branching lowers BP.

Alkanes

General Formula: CₙH₂ₙ₊₂

Characteristics:

• Non-polar molecules.
• Boiling points increase with molecular weight.
• Branched alkanes have lower boiling points than their linear counterparts.

Example:

• Methane (CH₄): Boiling point ≈ -161.5°C
• Butane (C₄H₁₀): Boiling point ≈ -0.5°C

Alkenes

General Formula: CₙH₂ₙ

Characteristics:

• Non-polar molecules.
• Boiling points are slightly lower than those of alkanes of similar molecular weight.
• Increase in boiling point with molecular weight.

Example:

• Ethene (C₂H₄): Boiling point ≈ -103.7°C
• But-1-ene (C₄H₈): Boiling point ≈ -6.3°C

Alkynes

General Formula: CₙH₂ₙ₋₂

Characteristics:

• Non-polar molecules.
• Boiling points are higher than alkenes but lower than alkanes of similar molecular weight.

Example:

• Ethyne (C₂H₂): Boiling point ≈ -84°C
• But-1-yne (C₄H₆): Boiling point ≈ 8.1°C

Alcohols

Functional Group: Hydroxyl (-OH)

Characteristics:

• Polar molecules capable of hydrogen bonding.
• Significantly higher boiling points than hydrocarbons of similar molecular weight.

Example:

• Methanol (CH₃OH): Boiling point ≈ 64.7°C
• Ethanol (C₂H₅OH): Boiling point ≈ 78.4°C

Aldehydes

• Structure: Carbonyl group (C=O) bonded to at least one hydrogen atom.
• Forces: Dipole-dipole interactions; no hydrogen bonding between molecules.
• Trend: Boiling points are higher than alkanes/alkenes but lower than alcohols.

Examples:

• Methanal (Formaldehyde, HCHO): -19°C
• Ethanal (Acetaldehyde, CH₃CHO): 20.2°C

Ketones

• Structure: Carbonyl group (C=O) bonded to two carbon atoms.
• Forces: Dipole-dipole interactions, similar to aldehydes.
• Trend: Boiling points similar to those of aldehydes with comparable molecular weight.

Examples:

• Propanone (Acetone, CH₃COCH₃): 56.1°C
• Butanone (C₂H₅COCH₃): 79.6°C

Carboxylic Acids

Functional Group: Carboxyl (-COOH)

Characteristics:

• Strong hydrogen bonding leads to dimer formation.
• Higher boiling points than alcohols of similar molecular weight.

Example:

• Methanoic Acid (Formic Acid, HCOOH): Boiling point ≈ 100.8°C
• Ethanoic Acid (Acetic Acid, CH₃COOH): Boiling point ≈ 118.1°C

Ethers

Functional Group: -O-

Characteristics:

• Polar molecules with weak dipole-dipole interactions.
• Boiling points comparable to alkanes of similar molecular weight but lower than alcohols.

Example:

• Dimethyl Ether (CH₃OCH₃): Boiling point ≈ -24.8°C
• Diethyl Ether (C₂H₅OC₂H₅): Boiling point ≈ 34.6°C

Esters

Functional Group: -COO-

Characteristics:

• Polar molecules with dipole-dipole interactions.
• Boiling points are higher than ethers but lower than alcohols and carboxylic acids of similar molecular weight.

Example:

• Methyl Ethanoate (CH₃COOCH₃): Boiling point ≈ 57.1°C
• Ethyl Ethanoate (CH₃COOC₂H₅): Boiling point ≈ 77.1°C

Amines

Functional Group: Amino (-NH₂)

Characteristics:

• Polar molecules capable of hydrogen bonding.
• Boiling points are higher than hydrocarbons but lower than alcohols of similar molecular weight.

Example:

• Methylamine (CH₃NH₂): Boiling point ≈ -6.3°C
• Ethylamine (C₂H₅NH₂): Boiling point ≈ 16.6°C

Amides

Functional Group: (-CONH₂)

Characteristics:

• Intermolecular Forces: Hydrogen bonding and dipole-dipole interactions.
• Boiling Point Trend: Among the highest in organic compounds.

Example: 

Formamide (HCONH₂) has a BP of 210°C.

5.0Comparative Analysis of Boiling Points

The type and strength of intermolecular forces present influence the boiling points of organic compounds. Generally, the order of boiling points for compounds with similar molecular weights is:

Carboxylic Acids > Alcohols > Amines > Aldehydes ≈ Ketones > Esters > Ethers > Alkanes ≈ Alkenes ≈ Alkynes

This trend is due to the varying strengths of intermolecular forces:

• Carboxylic Acids: Strong hydrogen bonding and dimer formation.
• Alcohols: Hydrogen bonding.
• Amines: Hydrogen bonding (weaker than in alcohols).
• Aldehydes and Ketones: Dipole-dipole interactions.
• Esters and Ethers: Dipole-dipole interactions (weaker than in aldehydes and ketones).
• Alkanes, Alkenes, Alkynes: Van der Waals forces.

6.0Practice Problems

1. Arrange the following compounds in increasing order of boiling points:

• Methanol (CH₃OH)
• Ethanol (C₂H₅OH)
• Propanol (C₃H₇OH)

Answer: Methanol < Ethanol < Propanol

2. Which compound has a higher boiling point and why?

• Acetone (CH₃COCH₃)
• Ethanol (C₂H₅OH)

Answer: Ethanol has a higher boiling point due to hydrogen bonding, whereas acetone exhibits only dipole-dipole interactions.

3. Predict the boiling point trend for the following compounds:

• Butane (C₄H₁₀)
• But-1-ene (C₄H₈)
• But-1-yne (C₄H₆)

Answer: Butane > But-1-ene > But-1-yne