The reactions of amines

We are going to be looking at two primary amines in this experiment. Primary amines contain the -NH₂ group. We shall be looking at butylamine, C₄H₉NH₂, where the attached group is an alkyl group. This is an example of an alkylamine. We shall also be looking at an arylamine where the attached group is an aryl group. The simplest of these is phenylamine, C₆H₅NH₂. This is used in the dye industry, and used to be called aniline. As it is toxic, we shall be using an alternative called ethyl 4-aminobenzoate. It's alternative name is benzocaine, and it is used as an anaesthetic.

ethyl 4-aminobenzoate

We shall also be looking at ammonia, NH₃. This is because it is best to consider the amines as substituted ammonia molecules. Butylamine is an ammonia molecule in which one of the hydrogen atoms has been replaced by a butyl group. You should be able to see the similarities in their chemical reactions in the following experiments.

1 Solubility and pH

You should be able to predict the solubility of the three amines in water. Bear in mind the size of the hydrophobic hydrocarbon section of each molecule, and the fact that the NH₂ groups can hydrogen bond to the water. Check your predictions in the following video.

Video - the solubility and pH of the amines

The equilibrium, which causes the amines to be alkaline, must produce the hydroxide ion, OH^-(aq). Write down an equilibrium to show this occurring with ammonia, then try to write down the analogous equilibrium for butylamine. You should have estimated the pH of each of the solutions from the video. Try to explain why the pHs differ for the three compounds. You can check your answers here.

2 Formation of salts

Amines smell of rotting fish

On adding hydrochloric acid to the amines, their characteristic fishy odour disappears, although this does not come across well on video! As the ammonia and butylamine are readily soluble in water and hydrochloric acid, there is nothing additional to see when these are added to dilute hydrochloric acid. Look at the video and assess the solubility of the ethyl 4-aminobenzoate in dilute hydrochloric acid compared to that in the water shown in the last experiment. Try to explain any contrast, and suggest why the amine smell disappears on addition to dilute hydrochloric acid.

Video - the solubility of ethyl 4-aminobenzoate in dilute hydrochloric acid

Again the amines react in an analogous way to that of ammonia with hydrochloric acid. You should be able to write down an equation for the reaction of ammonia solution with dilute hydrochloric acid. Then try to write down a similar equation for the reaction of butylamine with dilute hydrochloric acid. You can check your answers by looking at the formation of ammonium salts

3 Reaction with transition metal ions

A number of different coloured complex ions are formed in these reactions when the various amines are added to some 0.1 M copper(II) sulfate. The formation of coloured complexes on addition of copper(II) sulfate solution is often used as a test for amines. This is a popular exam question. Make a note of the complex colours and write down an ionic equation for the reaction mentioned in the video. Check your answers here.

Video - the reactions of amines with copper(II) sulfate solution

4 Nucleophilic reactions of amines with acid derivatives

We have already seen the reactions of ethanoyl chloride in an earlier experiment. Review its chemistry and then try to write equations for the reactions seen in the video. What explanation can you give as to the difference in reactivity of the compounds? What are the white fumes seen in the first experiment? Check your answers below.

Video - reactions with ethanoyl chloride

Results - reactions with ethanoyl chloride