Alcohols

• alcohol homologous series:

  • contain the -OH functional group
  • hydroxyl group
  • responsible for both the chemical and physical properties of the alcohols
• • simplest alcohol = methanol (CH3OH)
  • used as a high-performance fuel bc of its efficient combustion
  • also an important chemical feedstock - starting material in many industrial syntheses
  • can be converted into polymers, paints, solvents, insulation, adhesives and many other useful products
• • second member of the alcohol homologous series, ethanol is used primarily in alcoholic drinks and as a fuel - also finds use as a solvent and a feedstock
• Naming alcohols:

  • suffix = -ol added to stem name of longest carbon chain
  • position of alcohol functional group in chain indicated using a number
• Naming 2-methylbutane-2,3-diol

  1. there are 4 carbons in the longest chain - stem is butane
  2. there are 2 -OH functional groups, so the suffix is -diol
  3. the -OH groups are on carbons 2 and 3, so the infix is -2,3-
  4. the suffix -diol does not start with a vowel - alkane chain name is not shortened
  5. there is a methyl group (-CH3) on carbon 2 - prefix 2-methyl is added to name
  6. 2-methylbutane-2,3-diol
• when you compare the physical properties of alcohols with alkanes of the same number of carbon atoms - differences:

  • alcohols are less volatile
  • have higher melting points
  • greater water solubility

  than corresponding alkanes

  • differences become smaller as length of the carbon chain increases
• differences can be explained by exploring the polarity of the bonds in both the alkanes and alcohols, and the effect that these bonds have on the strength of the IMF
• • alkanes has non-polar bonds bc the electroneg of carbon and hydrogen are very similar
  • the alkane molecules are therefore non-polar
  • the IMF between non-polar molecules are very weak London forces
  • alcohols have polar O-H bonds bc of the difference in electroneg of the oxygen and hydrogen atoms
  • alcohol molecules therefore non-polar
  • IMF will be very weak London forces but will also be much stronger hydrogen bonds between polar O-H groups
• Volatility and boiling points:

  • in the liquid state, intermolecular hydrogen bonds hold alcohol molecules together
  • these bonds must be broken in order to change the liquid alcohol into a gas
  • requires more energy than overcoming the weaker London forces in alkanes
  • alcohols have lower volatility than alkanes with same no. carbon atoms
• solubility in water:

  • compound that can form hydrogen bonds with water is far more water-soluble than compound that cannot
  • alkanes are non-polar molecules - cannot form H bonds with water
  • alcohols such as methanol and ethanol are completely soluble in water
  • H bonds form between polar -OH group of alcohol and water molecules
• compounds with low bps are volatile - easily converted from a liquid to a gas - higher the bp lower the volatility
• as the length of the hydrocarbon chain 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
• alcohols can be classified as primary, secondary or tertiary:

  • classification depends on the number of hydrogen atoms and alkyl groups are attached to the carbon atom that contains the alcohol functional group
• Primary alcohols:

  • methanol and ethanol are the 2 simplest alcohols in the alcohol homologous series - both primary
  • in a primary alcohol - the -OH group is attached to a carbon atom that is attached to 2 hydrogen atoms and one alkyl group
  • methanol - 3 H atoms and no carbon atoms attached is an exception - still classed as primary alcohol
• Secondary alcohols:

  • the -OH group is attached to a carbon atom that is attached to 1 hydrogen atom and 2 alkyl groups
  • e.g. propan-2-ol and pentan-3-ol
• tertiary alcohols:

  • -OH group is attached to a carbon atom that is attached to no hydrogen atoms and three alkyl groups
  • 2-methylpropan-2-ol and 2-methylbutan-2-ol
• Combustion of alcohols:

  • alcohols burn completely in a plentiful supply of oxygen to produce carbon dioxide and water
  • reaction is exothermic releasing a large quantity of energy in the form of heat
  • as the number of carbon atoms in the alcohol chain increases, the quantity of heat released per mole also increases
• Oxidation of alcohols:

  • primary and secondary alcohols can be oxidised by an oxidising agent
  • usual oxidising mixture is a solution of potassium dichromate (VI), K2Cr2O7 acidified with dilute sulfuric acid, H2SO4
  • if alcohol is oxidised - orange solution containing dichromate (VI) ions is reduced to green solution containing chromium (III) ions
• Oxidation of primary alcohols:

  • primary alcohols can be oxidised to either aldehydes or carboxylic acids
  • product of oxidation depends on the reaction conditions used bc aldehydes themselves are also oxidised to carboxylic acids
• Preparation of aldehydes:

  • one gentle heating of primary alcohols with acidified potassium dichromate, an aldehyde is formed
  • to ensure that the aldehyde is prepared rather than the carboxylic acid, the aldehyde is distilled out of the reaction mixture as it forms
  • prevents any further reaction with the oxidising agent
  • dichromate (VI) ions change from orange to green
• Preparation of carboxylic acids:

  • if primary acid heated strongly enough, under reflux with an excess of acidified potassium dichromate (VI) - carboxylic acid is formed
  • use of an excess of acidified potassium dichromate (VI) ensures that all the alcohol is oxidised
  • heating under reflux ensures that any aldehyde formed initially in the reaction also undergoes oxidation to the carboxylic acid
• conditions of the oxidation of a primary alcohol, such as whether a reagent is in excess, the conditions and the technique used influence the product formed:

  • when preparing the aldehyde, use distillation to remove the aldehyde from the reaction mixture
  • when preparing the carboxylic acid, heat the alcohol under reflux
• Oxidation of secondary alcohols:

  • oxidised to ketones - not possible to further oxidise ketones using acidified dichromate (VI) ions
  • to ensure the reaction goes to completion, the secondary alcohol is heated under reflux with the oxidising mixture
  • dichromate (VI) ions once again turn orange to green
• Tertiary alcohols do not undergo oxidation reactions:

  • the acidified dichromate (VI) remains orange when added to tertiary alcohols
• Dehydration of alcohols:

  • dehydration is any reaction in which a water molecule is removed from the starting material
  • an alcohol is heated under reflux in the presence of an acid catalyst such as conc. sulfuric acid or conc. phosphoric acid
  • product of the reaction is an alkene
  • dehydration of an alcohol is an example of an elimination reaction
• substitution reactions of alcohols:

  • react with hydrogen halides to form haloalkanes
  • when preparing a haloalkane, the alcohol is heated under reflux with sulfuric acid and a sodium halide e.g. NaBr the hydrogen halide (hydrogen bromide) is formed in situ
  • the HBr (e.g.) formed reacts with the alcohol to produce the haloalkane