14.1 properties of alcohols

naming alcohols

The suffix -ol is added to the stem containing the largest carbon chain. The position of the functional group along the chain is indicated using numbers

physical properties of alcohols

compared to alkanes they are:

• less volatile

• have higher melting points

• greater water solubility

• differences become smaller as the length of the carbon chain increases

reasons of the physical properties of alcohols

• The alkanes have non-polar bonds because the electronegativity of hydrogen and carbon are very similar

• the alkane molecules are therefore non-polar

• the inter molecular forces between non-polar molecules are very weak London forces

• alcohols have a polar O-H bond because of the different in electronegativity of the oxygen and hydrogen atoms

• alcohol molecules are therefore polar

• the intermolecular forces will be very weak London forces but there will also be much stronger hydrogen bonds between the polar O-H group

why is there a difference in volatility of boiling points between alkanes and alcohols

• in the liquid state, intermolecular hydrogen bonds hold the alcohol molecules together

• These bonds must be broken in order to change the liquid alcohol into a gas

• this requires more energy than overcoming the weaker London forces in alkanes, so alcohols have a lower volatility that the alkanes with the same number of carbons

why is there a difference in the solubility of water between alkanes and alcohols

• a compound that can form hydrogen bonds with more is far more water soluble that a compound that cannot

• alkanes are non-polar compounds and can not form hydrogen bonds with water

• alcohols such as methanol and ethanol are completely soluble in water, as hydrogen bonds form between the polar -OH group of the alcohol and the water molecules

• as the hydrocarbon chain length increases in size, the influence of the -OH group becomes relatively smaller, and the solubility of longer-chain alcohols becomes more like that of hydrocarbons - solubility decreases.

classifying alcohols

alcohols can be classed as primary, secondary or tertiary. This classification depends on the number of hydrogen atoms and alkyl groups attached to the carbon atom that contains the alcohol functional group

primary alcohols

in primary alcohols, the -OH group is attached to a carbon atom which is attached to two hydrogen atoms and 1 alkyl group. (methanol is the exception and its still classed as a primary alcohol)

secondary alcohols

the -OH group is attached to a carbon atom that is attached to one hydrogen atom and two alkyl groups. Propan-2-ol and pentan-3-ol are both examples of secondary alcohols

tertiary alcohols

the -OH group is attached to a carbon atom that is attached to no hydrogen atoms and three alkyl groups. 2-methylpropan-2-ol is an example