Properties of Alcohols

Why are alcohols polar molecules?

The O–H bond in the hydroxyl group is highly polar because oxygen is significantly more electronegative than hydrogen. This creates a permanent dipole on the –OH group, making the whole molecule polar. The C–O bond is also polar for the same reason.

Why do alcohols have higher boiling points than alkanes of a similar molecular mass?

Alcohols can form hydrogen bonds between molecules (intermolecular H bonds) due to the polar O–H group. Hydrogen bonds are much stronger than the London forces that act between non-polar alkane molecules. Because more energy is needed to overcome hydrogen bonds, alcohols have considerably higher boiling points than alkanes of similar size.

How does a hydrogen bond form between two alcohol molecules? What conditions must be met?

A hydrogen bond forms between the slightly positive hydrogen of one molecule's O–H group and a lone pair on the oxygen of a neighbouring molecule's O–H group. For a hydrogen bond to form, hydrogen must be bonded directly to a highly electronegative atom (O, N, or F), which is the case in alcohols.

Why are short-chain alcohols more soluble with water compared to longer-chain alcohols ?

Short-chain alcohols (e.g., methanol, ethanol) can form hydrogen bonds with water molecules via their –OH group, so they mix freely with water. As the carbon chain gets longer, the non-polar hydrocarbon part of the molecule becomes increasingly dominant. This non-polar tail cannot form hydrogen bonds with water and disrupts the water's hydrogen bond network, making longer-chain alcohols progressively less soluble.

Why are alcohols described as having "relatively low volatility" compared to alkanes?

Volatility is the tendency of a substance to evaporate. Alcohols have lower volatility than alkanes of similar molecular mass because hydrogen bonds must be broken to change to a gas from liquid. Since hydrogen bonds are stronger than the London forces in alkanes, more energy is required to break forces — hence alcohols are less volatile.

How are alcohols classified as primary, secondary, or tertiary, and give an example of each?

The classification is based on how many carbon atoms are directly bonded to the carbon atom that carries the –OH group:

• Primary (1°): the carbon bearing –OH is bonded to one other carbon atom. Example: propan-1-ol (CH₃CH₂CH₂OH)

• Secondary (2°): the carbon bearing –OH is bonded to two other carbon atoms. Example: propan-2-ol (CH₃CH(OH)CH₃)

• Tertiary (3°): the carbon bearing –OH is bonded to three other carbon atoms. Example: 2-methylpropan-2-ol ((CH₃)₃COH)

Why does the classification into primary, secondary, and tertiary matter

Because the type of alcohol determines how it behaves in reactions — particularly oxidation. Primary and secondary alcohols can be oxidised but give different products; tertiary alcohols cannot be oxidised under normal conditions.