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.
Introduction to Alcohols
Introduction to Alcohols

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.

Introduction to Alcohols
Introduction to Alcohols

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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