Question

How does the molecular structure, intermolecular forces, and chemical bonds affect the properties of alcohols?

Answer 1

1.Molecular structure

Alcohols are soluble in water. This is due to the hydroxyl group in the alcohol which is able to form hydrogen bons with water molecules. Alcohols with a smaller hydrocarbon chain are very soluble. As the length of the hydrocarbon chain increases, the solubility in water decreases. With four carbon in the hydrocarbon chain and higher, the decrease in solubility becomes visible as the mixture forms two immiscible layers of liquid. The reason why the solubility decreases as the length of hydrocarbon chain increases is because it is requires more energy to overcome the hydrogen bonds between the alcohol molecules as the molecules are more tightly packed together as the size and mass increases.

In the image above, the partially negative oxygen atom in the ethanol molecule forms a hydrogen bond with the partially positive hydrogen atom in the water molecule.

2.Intermolecular forces

This graph shows the comparison of boiling points of methane with methanol, ethane with ethanol, propane with propanol, and butane with butanol.
From the graph we can see that the boiling point of an alcohol is always much higher than the boiling point of the corresponding alkane with the same hydrocarbon chain. The boiling point of alcohols also increase as the length of hydrocarbon chain increases.
The reason why alcohols have a higher boiling point than alkanes is because the intermolecular forces of alcohols are hydrogen bonds, unlike alkanes with van der Waals forces as their intermolecular forces.

The image below shows ethanol molecules with a hydrogen bond.

Alcohols turns from liquid to solid at room temperature and pressure (rtp) as the length of the hydrocarbon chain in the alcohol increases.

The boiling points of the first 11 alcohols are as follows:

The factors affecting the boiling/melting points of alcohols are not only hydrogen bonds, but also van der Waals dispersion forces and dipole-dipole interactions.
The hydrogen bonds and dipole-dipole interactions will remain relatively the same throughout the series of alcohols. The van der Waals dispersion forces increase as the length of hydrocarbon chain increases. This is due to the increase in number of electrons in the molecules, which in turns increases the strength and size of the temporarily induced dipole-dipole attraction. Hence, more energy is required to overcome the intermolecular forces, resulting in the increase in boiling/melting points.

3. Chemical bonds

Combustion breaks the covalent bonds of the molecules, so as the size and mass of the molecules increases, there are more covalent bonds to break in order to burn that alcohol. Hence, more energy is required to break the bonds, therefore the flammability of alcohols decrease as size and mass of molecules increases.The general molecular equation for the reaction is:

CnH2n+1OH + (1.5n)O2 → (n+1)H2O + nCO2

e.g. combustion of ethanol:

C2H5OH (l) + 3 O2 (g) → 2 CO2 (g) + 3 H2O (g); (ΔHc = −1371 kJ/mol).