Classifying Alcohols (AQA A-Level Chemistry): Revision Notes

3.5.1 Classifying Alcohols

What is an Alcohol?

Common Example

When non-chemists refer to "alcohol," they typically mean ethanol ($CH₃CH₂OH$), which is the alcohol present in alcoholic beverages. However, in chemistry, the term "alcohol" encompasses a broad range of compounds that all contain the $-OH$ functional group.

Classification of Alcohols

Alcohols are categorised into three main types: primary, secondary, and tertiary. The classification depends on the carbon to which the $-OH$ group is attached and the number of alkyl groups (R groups) connected to that carbon.

Primary Alcohols ($1°$)

A primary alcohol has the hydroxyl group attached to a carbon that is connected to only one other carbon (or no other carbon if it's methanol).

• General Structure: $RCH₂OH$
• Example: Ethanol ($CH₃CH₂OH$)
• Methanol ($CH₃OH$) is also a primary alcohol because the carbon with the $-OH$ group is not attached to any other carbon atoms.

Secondary Alcohols ($2°$)

A secondary alcohol has the hydroxyl group attached to a carbon that is connected to two other carbon atoms.

• General Structure: $R₂CHOH$
• Example: Propan-2-ol ($CH₃CH(OH)CH₃$)

Tertiary Alcohols ($3°$)

A tertiary alcohol has the hydroxyl group attached to a carbon that is connected to three other carbon atoms.

• General Structure: $R₃COH$
• Example: 2-methylpropan-2-ol $(CH₃)₃COH$