In A-Level Chemistry - Organic chemisrtry, we look at different functional groups in hydrocarbons. Let's look at the homologous series of alcoal
Alcohol
General formula: CnH(2n+1)OH
Alcohols can be primary, secondary or tertiary. Primary alcohols are alcohols where the oxygen is attached to a carbon that is only attached to one other carbon (it is at the end of a chain). Secondary alcohols are alcohols where two carbons are attached to the carbon adjoined to the oxygen. 3 or more carbons are attached to the carbon directly attached to the oxygen in tertiary alcohols.
Properties of alcohols
- Have low volatility due to their ability to form hydrogen bonds between molecules.
- The smaller alcohols can dissolve in water because they can form hydrogen bonds with water. The longer the chains the less solublethe alcohol is.
- Ethanol is the alcohol in alcoholic drinks; commonly used as a solvent in the form of methylated spirits.
Reactions of alcohols
Dehydration reactions – removal of a water molecule
Alcohol → Alkene
Conditions and reagents: warm under reflux; sulphuric acid
Substitution reactions
Alcohol → haloalkane
Needed: concentrated sulphuric and sodium halide
NaCl + H₂SO₄ → NaHSO₄ + Hcl
CH₃CH₂OH + Hcl → CH₃CH₂Cl + H₂O
Oxidising alcohols
In complete combustion the only products are water and carbon dioxide. Potassium dichromate (K₂Cr₂O₇) is the oxidising agent for alcohols.
Primary alcohol → aldehyde
Conditions and reagents: warm and distil; potassium dichromate and dilute sulphuric acid.
Observation: the orange dichromateion reduces to a green Cr3+ ion.
Distillation:
It is used as a separation technique for organic products. Often electric heaters are used to heat organic chemicals because they are highly flammable and could set on fire with a naked flame.
Water goes in and out of a liebig condenser to cool the gas so that it can condense into a liquid.
Full oxidation
Primary alcohol → carboxylic acid
Conditions and reagents: heat under reflux; potassium dichromate and dilute sulphuric acid.
Reflux:
Used for heating organic reactions for long periods of time; the condenser prevents organic vapours from escaping by condensing them back to liquids. The end of the condenser must never be sealed as the build up of gas pressure could cause the apparatus to explode. Anti-bumping granules are used to prevent vigorous uneven boiling by making small bubbles for instead of large bubbles.
Oxidation of secondary alcohols
Secondary alcohols → ketone
Conditions and reagents: heat under reflux; potassium dichromate and dilute sulphuric acid
Tertiary alcohols cannot be oxidised by potassium dichromate.
Drafted by Eunice (Chemistry)
References
https://www.masterorganicchemistry.com/2014/09/17/alcohols-1-nomenclature-and-properties/
https://docbrown.info/page06/alcohols5.htm
https://igcseandialchemistry.com/alcohol-organic-chemistry/