This post details some core information regarding the classification and properties of alcohols.
Classification of Alcohols
First lets define what an alcohol is:
Alcohol – an alcohol is an alkane in which one or more of the hydrogen groups has been replaced by a hydroxyl group (-OH)
Alcohols can be classified depending on the positioning of the -OH group on the alkane. There are three classifications:
Primary Alcohol – the alcohol group is attached to a carbon atom with no or only 1 alkyl groups.
NOTE: The R in the diagrams below represents an alkyl group.
IMPORTANT – the attached alkyl group may be simple or complex, however it does NOT matter. When classifying an alcohol we are only concerned with the number of alkyl groups attached to the carbon atom with the hydroxyl group.
Secondary Alcohol – the alcohol group is attached to a carbon atom with 2 alkyl groups.
Tertiary Alcohol – the alcohol group is attached to a carbon atom with 3 alkyl groups.
Examples
Properties
Alcohols have the ability for form hydrogen bonds with neighbouring molecules. The hydroxyl group (-OH) has a hydrogen atom attached to an oxygen atom. The oxygen atom is more electronegative than the hydrogen atom and this difference in electronegativity gives the oxygen atom a partially negative charge and the hydrogen atom a partially positive charge. This gives alcohols the ability to form hydrogen bonds with neighbouring molecules which has an influence on their properties.
Boiling Points and Volatility
The boiling points of alcohols are significantly higher that alkanes of similar molecular mass. For example, the boiling point of ethane is approximately 184K, compared to a boiling point of approximately 351K for ethanol. This is due to the hydrogen bonding, because hydrogen bonds are much stronger than the C-H bonds in the alkane and there require more energy to break.
The more carbon atoms in the alcohol the higher the boiling point.
Alcohols also have a lower volatility than alkanes of similar molecular mass.
Solubility
Small alcohols can be completely dissolved in water as they are able to form hydrogen bonds between the polar hydroxyl group of the alcohol and water molecules. The first three alcohols (methanol, ethanol and propanol) are completely soluble in water. However, as the length of the alkane chain increases solubility decreases.
The alkane chain is non-polar and can’t form hydrogen bonds with water. The longer the chain, the weaker the forces of attraction between the alcohol and the water (as there are fewer hydrogen bonds relative to the size of the molecule) and so the lower the solubility.
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