Alcohols - Introduction to Alcohols (A-Level Chemistry)
Introduction to Alcohols
Alcohols
Structure
Alcohols are organic compounds with general formula CnH2n-1OH.
The position of the -OH group in the alcohol molecule will determine whether the alcohol is:
- A primary alcohol – The carbon atom the -OH group is bonded to is in turn bonded to one alkyl group.
- A secondary alcohol – The carbon atom the -OH group is bonded to is in turn bonded to two alkyl groups.
- A tertiary alcohol – The carbon atom the -OH group is bonded to is in turn bonded to three alkyl groups.
Properties of Alcohols
Alcohols are polar molecules. The highly electronegative O atom in the -OH group pulls electron density away from the adjacent H and C atom, causing both to have a partial positive charge.
The dipole on alcohol molecules allows them to form hydrogen bonds with water molecules and with each other. This ability gives alcohols special properties:
- Soluble in water – Hydrogen bonds form between the -OH group in alcohols and water molecules. As a result, alcohols with short carbon chains are soluble in water. However, water solubility of alcohols decreases as the length of the non-polar carbon chain increases.
- Low volatility – Adjacent alcohol molecules are held together by strong hydrogen bonds. A higher amount of energy is required to for alcohols to evaporate compared to non-polar compounds of similar molecular mass.
Alcohol pH
Alcohols are less acidic than water.
This is because the electron donating alkyl group in the alkoxide ion (RO- ) concentrates more negative charge on the negatively charged oxygen atom, so that protons are more strongly attracted to it and undissociated alcohol molecules are formed more readily.
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