AMINES

What do amines act as?

• primary aliphatic amines act as bronsted-lowry bases as 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’re weak bases as only a low concentration of hydroxide ions is produced

Why are primary aliphatic amines stronger bases than ammonia?

• the alkyl groups are electron releasing and push electrons towards the nitrogen atom

• makes the lone pair of electrons on the nitrogen more readily available to accept protons

Why’re secondary amines stronger bases than primary amines?

• they have more alkyl groups that are substituted onto the N atom in place of H atoms

• greater electron density is pushed onto the N atom due to the inductive effect of alkyl groups so the lone pair of electrons on the nitrogen is more readily available

What is the base strength of aromatic amines?

primary amines such as phenylamine don’t form basic solutions as the lone pair of electrons on the nitrogen delocalise with the ring of electrons in the benzene ring, making the lone pair on the N less able to accept protons

What is the overall order of base strength?

1. secondary amines

2. tertiary amines

3. primary amines

4. ammonia

5. aromatic amines

What happens when amines react with acids?

• they form ammonium salts

• CH3NH2 + HCl → CH3NH3+Cl-

• addition of NaOH to an ammonium salt will convert it back to the amine

• ionic salts will be solid crystals

• compounds are soluble in the acid

How can you make a basic buffer from an amine?

• combine a weak base with a salt of that weak base

• ammonia and methylammonium chloride

How can primary amines be formed?

• nucleophilic substitution reaction between halogenoalkanes and ammonia in a 1 step reaction

• primary amine can react in the same nucleophilic way forming secondary and tertiary amines and quaternary ammonium salts

What is the mechanism for nucleophilic substitution of a halogenoalkane and ammonia?

How can you limit further subsequent reactions in the nucleophilic substitution reaction?

use excess of ammonia

How can you promote for the formation of a quaternary salt in nucleophilic substitution reaction?

use excess of the halogenoalkane

What can quaternary ammonium salts be used as?

• cationic surfactants

• reduce the surface tension of liquids

• positive nitrogen is attracted towards negatively charged surfaces

• used as fabric softeners, hair conditioners and sewage flocculants

What’re the 2 steps in preparing amines from nitriles?

1. convert halogenoalkane into a nitrile

2. reduce nitrile into amine

What occurs in step 1 of preparing amine from nitrile?

• convert halogeno alkane to nitrile

• use KCN in aqueous ethanol

• heat under reflux

What occurs in step 2 of preparing amine from a nitrile?

• reduce nitrile to amine

• use LiAlH4 in ether or reduce using H2 and a Ni catalyst

What’re advantages to the 2 step method of producing an amine?

gives a higher yield as further substitutions cannot occur

What’re the disadvantages to the 2 step method of producing an amine?

• involves 2 steps so may have a low yield

• KCN is toxic

How do you reduce a nitroarene into an aromatic amine?

• reagent - Sn and HCl or Fe and HCl

• conditions - heating

• mechanism - reduction