Reactions: Rings, Acids and Amines

Benzene reacts with a nitrating mixture of concentrated nitric acid and concentrated sulfuric acid at a temperature of 50c. The sulfuric acid acts as a catalyst. If the temperature is above 50c then more than one nitro group is substituted onto the benzene ring.

The equation is:

C6H6 + HNO3 (H2SO4 @ 50c) ----> C6H5NO2 + H2O

Benzene does not react with halogens on its own, so requires halogen carriers as a catalyst.

To react with chlorine, AlCl3, FeCl3 is needed at room temperature and pressure.

For the reaction of bromine, FeBr3 or AlBr3 is also needed. The halogen carrier generates a bromonium ion:

Br" + FeBr3 -----> Br+ + FeBr4-

The Br+ electrophile now reacts with benzene in a reaction mechanism. The H+ that has been substituted from the benzene ring then reacts with FeBr4- to form HBr and reforms FeBr3.

Aldehydes and ketones are reduced to alcohols by using sodium tetrahydridoborate, NaBH4.

CH3CH2CHO + 2[H] -----> CH3CH2CH2OH

CH3COCH2CH2CH3 + 2[H] -----> CH3CHOHCH2CH2CH3

Aldehydes and ketones can be detected using the reagent 2,4-dinitrophenylhydrazine (2,4-DNP). A solution of 2,4-DNP in a mixture of methanol and sulfuric acid is known as Brady's reagent. When Brady's reagent is added to an aldehyde or ketone, a yellow or orange precipitate is formed. There is no precipitation with a carboxylic acid or an ester.

Once identified as an aldehyde or ketone, Tollens' reagent (a weak oxidising agent) can be used to distinguish between the two. Aldehydes are oxidised to carboxylic acids by the reagent whereas ketones are not. Tollens' is made by adding aqueous sodium hydroxide to aqueous silver nitrate until a brown precipitate of silver oxide is formed. Dilute aqueous ammonia is added until the precipitate dissolves. When added to an aldehyde, a silver mirror is formed, whereas when added to a ketone there is no reaction.

Once identified as an aldehyde or ketone, it can be positvely identified. The yellow/orange impure product is filtered and recrystallised to produce a pure sample of yellow/orange crystals, which are filtered and left to dry. The melting point is measured and recorded and then compared to a database to identify the compound.

Esters can be synthesised by adding an alcohol to a carboxylic acid in the presence of concentrated sulfuric acid.

They can also be synthesised by gently heating an acid anhydride with an alcohol.

In acid hydrolysis, the ester is heated under reflux with dilute sulfuric acid or hydrochloric acid. The ester is broken down by water, with the acid acting as the catalyst.

In alkaline hydrolysis, aqueous sodium hydroxide is refluxed with the ester.

Nitrobenzene is reduced by using a mixture of tin and concentrated hydrochloric acid, heated under reflux.

Diazotisation is when a mixture of phenylamine and nitrous acid is kept below 10c to form a diazonium salt.

A coupling reaction occurs when the diazonium salt is reacted with another aromatic compound under alkaline conditions.