A-LEVEL OCR ChEMISTRY NOTES
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- 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
- They react with dilute inorganic acids to form alkylammonium salts
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.
- Cyanide-containing compounds can be toxic or irritants.
- 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