Organic Functional Groups

Key Organic Functional Groups

  • Alkanes: Hydrocarbons with single bonds only, general formula CnH2n+2. They are saturated, meaning they contain the maximum possible amount of hydrogen.
  • Alkenes: Hydrocarbons that contain at least one carbon-carbon double bond, general formula CnH2n. They are unsaturated and can undergo addition reactions.
  • Alcohols: Organic compounds with a hydroxyl (–OH) group attached to the carbon atom of an alkyl group. The general formula is R–OH.
  • Aldehydes and Ketones: Carbon compounds with the carbonyl group (>C=O). In aldehydes, the carbonyl group is at the end of the carbon chain, while in ketones, it is in the middle of the chain.
  • Carboxylic acids: Carbon compounds with a carboxyl group (–COOH).
  • Amines: Organic compounds with one or more amino (–NH2) groups.
  • Amides: Organic compounds where the OH of a carboxylic acid has been replaced by an amine, with the general formula RCONR’R’’.
  • Esters: Formed by reaction of an acid and an alcohol, they contain a carbonyl (C=O) group followed by an oxygen atom and an alkyl group (–COO–R).

Identification of Functional Groups

  • Organic functional groups can often be identified by specific chemical tests.
  • Alkenes can be identified using bromine water, which turns from orange to colourless in their presence due to the addition reaction that occurs.
  • The presence of the hydroxyl group in alcohols can be confirmed using acidified potassium dichromate, which turns from orange to green when an alcohol is oxidised.
  • Carboxylic acids can be identified by their reaction with carbonates to produce carbon dioxide gas.
  • The presence of the carbonyl group in aldehydes and ketones can be tested with 2,4-dinitrophenylhydrazine, which forms a yellow to orange precipitate when a carbonyl group is present.

Chemical Reactions of Organic Functional Groups

  • Alkanes mainly undergo substitution reactions, while alkenes typically go through addition reactions due to the presence of the carbon - carbon double bond.
  • Alcohols can be dehydrated to produce alkenes or oxidised to produce either aldehydes or carboxylic acids.
  • Aldehydes can be further oxidised to carboxylic acids, but ketones cannot be further oxidised due to the position of the carbonyl group.
  • Carboxylic acids can react with alcohols to produce esters in a process called esterification.

Importance of Functional Groups in Polymers and Biochemical Molecules

  • In polymer chemistry, functional groups allow individual monomer units to bond together to form long chains or polymers.
  • LDPE and HDPE, both types of polyethene, consist solely of carbon and hydrogen, making them alkanes.
  • Polyesters and polyamides contain ester and amide functional groups respectively.
  • Biomolecules like proteins, nucleic acids, and carbohydrates also contain various functional groups that contribute to their structures and functions.
  • For example, proteins are made up of amino acids, which contain amine and carboxylic acid functional groups. This allows them to link together by the formation of amide bonds (called peptide bonds in this context).