OCR AS Chemistry

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4.4.1 Properties of Alcohols

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Properties of Alcohols

Intermolecular Forces 

  • Alcohols are compounds that contain at least one hydroxy (-OH) group
  • The general formula of alcohols is CnH2n+1OH
  • Alcohols can be prepared by a wide range of chemical reactions

Hydrogen bonding

  • Hydrogen bonding occurs between molecules where you have a hydrogen atom attached to one of the very electronegative elements - fluorine, oxygen or nitrogen
  • In an alcohol, there are O-H bonds present in the structure
  • Therefore hydrogen bonds set up between the slightly positive hydrogen atoms (δ+ H) and lone pairs on oxygens in other molecules
  • The hydrogen atoms are slightly positive because the bonding electrons are pulled away from them towards the very electronegative oxygen atoms
  • In alkanes, the only intermolecular forces are temporary induced dipole-dipole forces
  • Hydrogen bonds are much stronger than these and therefore it takes more energy to separate alcohol molecules than it does to separate alkane molecules
  • Therefore, the boiling point of alkanes is lower than the boiling point of the respective alcohols
    • For example, the boiling point of propane is -42 oC and the boiling point of propanol is 97 oC

Temporary induced dipole-dipole forces

  • As with all molecules, there are also temporary induced dipole-dipole forces between alcohols
  • The boiling points increase as the number of carbon atoms in the chains increases. It takes more energy to overcome the temporary induced dipole-dipole forces and so the boiling points rise
  • This is because the dipole is caused by the changing position of the electron cloud, so the more electrons there are, the larger the temporary induced dipole-dipole forces will be

Solubility

  • The small alcohols are completely soluble in water
  • If they are mixed with water, there will be one solution
  • However, solubility falls as the length of the hydrocarbon chain in the alcohol increases
  • The hydrocarbon chains are forcing their way between water molecules and so breaking hydrogen bonds between those water molecules
  • The -OH end of the alcohol molecules can form new hydrogen bonds with water molecules, but the hydrocarbon can not form hydrogen bonds

Classification of Alcohols

  • Primary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to one other carbon atom (or alkyl group). 
  • Secondary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to two other carbon atoms (or alkyl groups)
  • Tertiary alcohols are alcohols in which the carbon atom bonded to the -OH group is attached to three other carbon atoms (or alkyl groups)

Hydroxy Compounds Classifying Alcohols, downloadable AS & A Level Chemistry revision notes

Classifying primary, secondary and tertiary alcohols and alcohols with more than one alcohol group

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Philippa

Author: Philippa

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.