A-Level Chemistry OCR Notes

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Amines

Aliphatic amines are those without aromatic rings present. Primary aliphatic amines can be prepared by reacting ammonia with a halogenoalkane. X = an electronegative species e.g. Cl, Br
CH3CH2CH2Cl + NH3 → CH3CH2CH2NH3Cl
CH3CH2CH2NH3Cl + NH3 ⇌ CH3CH2CH2NH2 + NH4Cl
The second reaction is reversible so excess ammonia will push the equilibrium to the right
Additional substitution of the hydrogen atoms on the nitrogen can occur, resulting in secondary amines, followed by tertiary amines, followed by quaternary ammonium salts (R4N+Cl- where R = any alkyl chain)
Aromatic amines are made by reducing nitroarenes such as nitrobenzene, refluxed at 100°C using a reducing agent of tin and concentrated HCl. The tin and concentrated HCl react to form hydrogen

Amines act as Lewis bases because the nitrogen atom on an amine has a lone pair of electrons which it can donate to a Lewis acid
Amines act as Brønsted-Lowry bases because the nitrogen atom on an amine can accept a proton

CH3CH2NH2 + HCl→CH3CH2NH3+Cl-

Aldehyde & Ketones

Aldehydes and ketones are carbonyl compounds containing the C=O functional group.

Aldehydes can be oxidised to form carboxylic acids by heating under reflux with potassium dichromate (oxidising agent) and concentrated sulphuric acid
Carbonyls have a permanent dipole making them susceptible to nucleophilic addition reactions.
Aldehydes can be reduced to primary alcohols and ketones to secondary alcohols using NaBH4 as the reducing agent e.g. reduction of propanal CH3CH2CHO + 2[H] → CH3CH2CH2OH

Nucleophilic addition of cyanide ions can be used to extend the length of a carbon chain
Reactions of carbonyls with KCN and dilute acid, produces a hydroxynitrile. Aldehydes and unsymmetrical ketones form two optical isomers, as there is an equal chance of either isomer being formed. This is a racemic mixture.

Detecting Carbonyls

Brady’s reagent is used to test for the presence of carbonyls. It consists of a mixture of methanol, sulfuric acid and a solution of 2,4-dinitrophenylhydrazine (2,4-DNP)
- When Brady’s reagent is added to an aldehyde or a ketone, an observation is made as the orange transparent solution forms a yellow/orange precipitate of a 2,4-DNP derivative
- The observation is not seen with carboxylic acids and esters despite these compounds also having C=O bonds
- The specific aldehyde or ketone can be observed by comparing the melting point of the precipitate with a database
Tollen’s reagent (ammoniacal silver nitrate) is a colourless is a weak oxidising agent which reacts with aldehydes, but not ketones, therefore it can be used to distinguish between the two
- When Tollen’s reagent is added to an aldehyde a silver mirroris observed
- The silver ions are reduced to silver metal and the aldehyde is oxidised to a carboxylic acid
- No observation is made when it is added to a ketone

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