A-level Applied Science/Synthesising Organic Compounds/Organic reactions
Each functional group has its characteristic reactions. Synthetic chemists use their understanding of how chemical changes take place (reaction mechanisms) to devise efficient synthetic routes for the preparation of organic compounds. In order to give details of the main types of chemical reaction you should be able to recognise and give examples of various reaction types:
Oxidation is defined as when a molecule or atom:
- Loses electrons
- Loses hydrogen atoms
- Gains oxygen atoms
(These are really the same process. Hydrogen normally exists chemically as the hydrogen ion, H+. Losing a hydrogen atom is equivalent to losing a hydrogen ion and an electron. Oxygen normally exists as the oxide, O−2. When a molecule gains an oxygen atom, it has to lose two electrons to convert oxygen to oxide.) Oxidising agents include potassium manganate (VII) (‘permanganate’, KMnO4) and potassium dichromate (VI) (K2Cr2O7). The metals in these compounds have lost many electrons (7 and 6 respectively). They oxidise other compounds to recover their own lost electrons.
Reduction is the opposite of oxidation (fill in the blanks):
Reducing agents include hydrogen itself and hydrogen-rich metallic compounds such as lithium aluminium hydride (LiAlH4) and sodium borohydride (NaBH4).
- Alkenes can be oxidised to alcohols
- Alcohols can be oxidised to aldehydes, ketones and carboxylic acids.
- Aldehyes can be oxidised to carboxylic acids.
- Carboxylic acids can be reduced to alcohols.
Hydrolysis (as a substitution reaction)
The hydroxide ion (OH-) can replace the –NH2 group of an amine or a primary amide. Secondary amides and esters can be split into carboxylic acids and alcohols or amines by the addition of a water molecule. This is the reverse of the condensation process that forms amides and esters. An acid or base catalyst is used. If a base catalyst is used, the salt of the carboxylic acid is obtained, not the acid itself. If an acid catalyst is used to hydrolyse an amide, the ammonium salt is generated, not the amine.
Esters are formed when an alcohol and a carboxylic acid react. To catalyse the reaction an acid is added. Concentrated sulphuric acid is used because it also removes the water formed and so helps drive the reaction forwards.
Esterification links two molecules to form a larger one, but many more molecules can be incorporated if required. With a diol and a dicarboxylic acid, for example, we can make a polyester:
Secondary amides form from amines and carboxylic acids in the same way as esters form from alcohols and carboxylic acids:
Not surprisingly, we can make polyamides from diamines and dicarboxylic acids:
Polyamides are also made from amino acids. Proteins are polyamides. Artificial polyamides are commonly known as nylons.
These polymeric substances are useful as fibres and gears. Your notes should include balanced chemical equations for the various types of reaction. You should note the reagents that are used in the reactions.