Amines

Primary aliphatic amines act as Bronsted-Lowry bases because the lone pair of electrons on the nitrogen is readily available for forming a dative covalent bond with a H+ and so accepting a proton. They are weak bases as only a low concentration of hydroxide ions is produced.

Primary aliphatic amines are stronger bases than ammonia as the alkyl groups are electron releasing and push electrons towards the nitrogen atom and so making the lone pair of electrons on the nitrogen more readily available to accept protons.

Secondary amines are stronger bases than primary amines because they have more alkyl groups that are substituted onto the N atom in place of H atoms. Therefore more electron density is pushed onto the N atom (as the inductive effect of alkyl groups is greater than that of H atoms), so making the lone pair of electrons on the nitrogen more readily available. One might expect using the same trend that tertiary amine would be the strongest amine base but the trend does not hold. The tertiary amines and corresponding ammonium salts are less soluble in water and this makes them less strong bases than the secondary amines. (This point will not be examined)

Primary aromatic amines such as phenylamine do not form basic solutions because the lone pair of electrons on the nitrogen delocalise with the ring of electrons in the benzene ring. This means the lone pair on the N is less able to accept protons.

weakest Aromatic amines < ammonia< primary amines< tertiary amines< secondary amines Stronger bases

CH3NH2 (aq) + H+ (aq) –> CH3NH3+(aq) (CH3CH2)2 NH (aq) + H+ (aq) –> (CH3CH2)2NH2+(aq) (CH3CH2)3 N (aq) + H+ (aq) –> (CH3CH2)3NH+(aq) Amines as bases react with acids to form ammonium salts.

The ionic salts formed in this reaction means that the compounds are soluble in the acid. e.g. phenylamine is not very soluble in water but phenylammonium chloride is soluble. Addition of NaOH to an ammonium salt will convert it back to the amine These ionic salts will be solid crystals, if the water is evaporated, because of the strong ionic interactions.

Making a basic buffer from an amine Basic buffers can be made from combining a weak base with a salt of that weak base e.g.Ammoniaand ammoniumchloride Methylamine and methylammonium chloride Ethylamine and ethylammonium chloride

Primary amines can be formed by the nucleophilic substitution reaction between halogenoalkanes and ammonia in a one step reaction. However, as the lone pair of electrons is still available on the N in the amine formed, the primary amine can react in the same nucleophilic way in a successive series of reactions forming secondary, tertiary amines and quaternary ammonium salts. This is therefore not a good method for making a primary amine because of the further reactions. It would mean the desired product would have to be separated from the other products.

CH3CH2Br + 2NH3 –> CH3CH2NH2 + NH4Br Ammonia dissolved in ethanol is the initial nucleophile

In the first step of the mechanism the nucleophile attacks the halogenoalkane to form an intermediate In the second step of the mechanism a second ammonia removes a proton from the intermediate (acts as base) to form the amine Using an excess of ammonia can limit the further subsequent reactions and will maximise the amount of primary amine formed

The primary amine formed in the reaction above has a lone pair of electrons on the nitrogen and will react further with the halogenoalkane. In this second step of the mechanism either ammonia or the amine can remove a proton from the intermediate (acts as base) to form the amine.

The same reaction mechanism occurs with the secondary amine reacting to form a tertiary amine

Using an excess of the halogenoalkane will promote the formation of the quaternary salt Only the first step of the mechanism occurs when forming the quaternary salt Quaternary ammonium salts are not amines

Using a large excess of ammonia will maximise the amount of primary amine formed. Using an excess of the halogenoalkane will promote the formation of the quaternary salt.

Quaternary salts can be used as cationic surfactants Surfactants reduce the surface tension of liquids. The positive nitrogen is attracted toward negatively charged surfaces such as glass, hair, fibres and plastics. This helps in their uses as fabric softeners, hair conditioners and sewage flocculants.

Step 1. convert halogenoalkane to nitrile by using KCN in aqueous ethanol (heat under reflux) CH3CH2Br + CN- –> CH3CH2CN + Br - Step 2. reduce nitrile to amine by using LiAlH4 in ether or by reducing with H2 using a Ni catalyst CH3CH2CN + 4[H] –> CH3CH2CH2NH2 A disadvantage of this method is that it is a two step reaction that may therefore have a low yield. Also KCN is toxic.

Reagent: Sn and HCl or Fe and HCl Conditions: Heating Mechanism: Reduction As the reaction is carried out in HCl the ionic salt C6H5NH3+Cl- will be formed. Reacting this salt with NaOH will give phenylamine. This reduction reaction can also be done with catalytic hydrogenation (H2 using a Ni catalyst).

Change in functional group: acyl chloride –> secondary amide Reagent: primary amine Conditions: room temp.

Aliphatic amines and phenylamine can react with acyl chlorides and acid anhydrides to form amides in a nucleophilic addition-elimination reaction- see chapter on reactions of C=O bond for more details.

Change in functional group: acid anhydride  secondary amide Reagent: primary amine Conditions: room temp.