Chapter 14
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What important properties do alcohols have?
• They can form hydrogen bonds
List 3 differences between the properties of alkanes and alcohols.
• Alcohols have higher boiling points, • are less volatile, • and are more soluble
Why do alcohols have higher boiling points than alkanes?
• They have hydrogen bonds
How does the boiling point of alcohols change as the chain length increases, and why?
• As alcohols get longer, their boiling points become more similar to those of alkanes • This is because the contribution of the -OH decreases
Why are alcohols less volatile than alkanes of the same chain length?
• The higher the boiling point of a substance, the lower its volatility
Why are alcohols more soluble than alkanes?
• Hydrogen bonds form between the alcohol and water molecules, so alcohols are very soluble • Alkanes are non-polar so they are insoluble
How can alcohols be classified?
• According to the number of other carbon atoms the carbon atom they are bonded to is bonded to • In a primary (1°) alcohol, the carbon the hydroxyl group is attached to is bonded to one other carbon (e.g. ethanol) • In a secondary (2°) alcohol, it is bonded to 2 • In a tertiary (3°) alcohol, it is bonded to 3
What happens when alcohols undergo combustion in a plentiful supply of oxygen?
• They produce carbon dioxide and water
How does the chain length of an alcohol affect its combustion?
• The longer the chain, the more energy released per mole
What oxidising agent is used to oxidate alcohols?
• Acidified potassium dichromate (VI) • Written as: K2Cr2O7/ H2SO4
How can you tell if an alcohol is oxidised in the presence of potassium dichromate(VI), and why?
• The orange solution containing the dichromate ions goes from orange to green • This is because the solution containing dichromate (VI) ions is reduced to a solution containing chromium (III) ions
What can primary alcohols be oxidised into, and what causes the difference?
• Aldehydes, when heated gently with acidified potassium dichromate and distilled as it’s formed • Carboxylic acids, when heated under reflux with an excess of potassium dichromate
Why must alcohols be heated gently and distilled when forming aldehydes?
• To prevent further oxidation into a carboxylic acid
Why are alcohols heated strongly under reflux to make carboxylic acids?
• To prevent compounds from escaping (this ensures any aldehyde initially made is fully oxidised)
Why are alcohols heated with excess potassium dichromate to form carboxylic acids?
• To ensure all of the alcohol is oxidised
How would the oxidation of propan-1-ol to form propanal be shown?
• Propan-1-ol + [O] • Arrow: K2Cr2O7/ H2SO4, heat and distill • Propanal + H2O
What does [O] stand for?
• The oxidising agent
How would the oxidation of propan-1-ol to form propanoic acid be shown?
• Propan-1-ol + 2[O] • Arrow: K2Cr2O7/ H2SO4, heat under reflux • Propanoic acid + H2O
What can secondary alcohols be oxidised into?
• Ketones
How can the oxidation of butan-2-ol be shown?
• Butan-2-ol + [O] • Arrow: K2Cr2O7/ H2SO4, heat under reflux • Butanone + H2O
Why are secondary alcohols heated under reflux during their oxidation?
• To ensure the reaction goes to completion
What happens when tertiary alcohols are mixed with potassium dichromate?
• Nothing; they can’t be oxidised • The potassium dichromate remains orange
How do alcohols react to form alkenes?
• The alcohol is heated under reflux with either phosphoric acid or concentrated sulfuric acid catalyst • An alkene and water are produced • It is a dehydration reaction, which is a type of elimination reaction
What do alcohols react with to form haloalkanes, and what type of reaction is this?
• Alcohols react with hydrogen halides to form haloalkanes • Substitution