Boiling Points of Functional Groups

What is a Functional Group?

A functional group affects the chemical property of a compound. In organic chemistry, a functional group is the group of atoms in a molecule that determines the chemical behaviour of the molecule. A covalent bond bonds the atoms in a functional group and molecule.

Hydroxyl, sulfhydryl, carbonyl, carboxyl, amino, methyl and phosphate are the seven main functional groups.

What is Boiling Point?

The boiling point is the temperature at which the vapour pressure of the liquid is equal to the atmospheric pressure. At boiling point, liquid changes from its liquid state to the gaseous state. The boiling point at one atmospheric pressure is known as the normal boiling point.

Table of Content

• Factors affecting the Boiling Point
• Boiling Points of Functional Groups
• Frequently Asked Questions – FAQs

Factors affecting the Boiling Point

The boiling point is the temperature at which the vapour pressure of the liquid is equal to the atmospheric pressure. At boiling point, liquid changes from its liquid state to the gaseous state. The boiling point at one atmospheric pressure is known as the normal boiling point.

Various factors determine the boiling point of a liquid.

Pressure

Pressure is directly proportional to the boiling point of a liquid. The larger the pressure more would be its boiling point. For example, the boiling point of water is equal to 100 degrees celsius at normal sea level. If we artificially decrease the pressure or increase the altitude, the boiling point will decrease. The boiling point of water at Mt Everest is equivalent to 72 degrees celsius.

Adding impurities

Adding impurities can often increase the boiling point of a liquid. The larger the contaminants, the higher its boiling point. For example, the boiling point of water is equal to 100 degrees celsius at normal sea level. We can artificially increase the boiling point of the solution by adding impurities like salt. When salt is added to the pure water, its boiling point increases.

Structure

The structure of the molecule also affects the boiling point of a liquid. The boiling point increases with the increase in the number of carbon atoms attached and decreases with the molecule’s branching. For example, the boiling point of ethyl alcohol is less than that of butyl alcohol, or the boiling point of n-pentane is larger than iso propane.

Intramolecular force of attraction

The intramolecular force of attraction between the molecule plays an essential role in affecting the boiling point of the liquid. The more significant the force of attraction in between the molecule, the more would be the boiling point of the liquid.

• The force of attraction increases in the order – Electrostatic force of attraction > Hydrogen bonding > dipole attraction > Van der Waals force of attraction.

For example, the boiling point of ethanol is 78.5 degrees celsius at normal sea level, while the boiling point of methyl ether is -25 degrees celsius. The rise in the boiling point of ethanol is due to strong hydrogen bonding.

Boiling Points of Functional Groups

The larger the force of attraction between the molecules more would be its boiling point. Or the larger is the electronegativity difference between the molecule more would be its polarity or boiling point. In general, the force of attraction increases in the order – Electrostatic force of attraction > Hydrogen bonding > dipole-dipole attraction > Van der Waals force of attraction.

When different groups are attached to the hydrocarbon, it changes the intramolecular force of attraction between the molecule, thus changing its boiling point.

• Hydrocarbons are non-polar. There is a weak van der waals force of attraction in between molecules. Thus its boiling point is pretty low.
• As we increase the number of carbon atoms in the molecule, the van der waals force of attraction increases. Thus, its boiling point increases.
• Further on increasing the branching in the molecule, the molecule tends to go away from each other. Thus, its boiling point decreases.
• Unsaturated molecules having double or triple bonds have strong attraction forces between them, thus high boiling point than saturated molecules.
• Haloalkanes have dipole-dipole attraction between them. Thus its boiling point is relatively high than alkanes.
• Ether, Ester, and Amine have dipole-dipole attraction between them. Thus its boiling point is relatively high than alkanes and haloalkanes.
• Aldehyde, Ketone are highly polar due to permanent dipole-dipole attraction. Thus its boiling point is relatively higher than alkane, haloalkanes, ether, ester, and amine.
• Alcohol, Carboxylic Acid and Amide have strong hydrogen bonding. Thus their boiling point is the largest.

The boiling point of organic molecules increases in the order:

Alkanes > Alkenes > Alkynes > Haloalkanes > Ether > Ester > Amine > Aldehyde > Ketone > Alcohol > Carboxylic Acid > Amide.

S. No.	Functional Group	Boiling Point	Force of Attraction	Polar Rank	Structure
1.	Amide	222o	Hydrogen Bonding, Dipole-Dipole Attraction and Van der Waals force of attraction	1
2.	Carboxylic Acid	118o	Hydrogen Bonding, Dipole-Dipole Attraction and Van der Waals force of attraction	2
3.	Alcohol	117o	Hydrogen Bonding, Dipole-Dipole Attraction and Van der Waals force of attraction	3
4.	Ketone	56o	Inter Molecular Hydrogen Bonding, Permanent Dipole-Dipole Attraction and Van der Waals force of attraction	4, 5
5.	Aldehyde	49o	Inter Molecular Hydrogen Bonding, Permanent Dipole-Dipole Attraction and Van der Waals force of attraction	4, 5
6.	Amine	49o	Hydrogen Bonding, Dipole-Dipole Attraction and Van der Waals force of attraction	6
7.	Ester	32o	Dipole-Dipole Attraction and Van der Waals force of attraction	7
8.	Ether	11o	Dipole-Dipole Attraction and Van der Waals force of attraction	8
9.	Alkane	-42o	Van der Waals force of attraction	9