Amines Flashcards
Basic properties- Primary aliphatic 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.
CH3NH2 +H2O ⇌ CH3NH3+ +OH-
Basic properties- Ammonia
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.
NH3 (aq) +H2O (l) ⇌ NH4+(aq) +OH-(aq)
Basic properties- Secondary amines
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).
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)
Basic properties- aromatic amines
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 N is less able to accept protons.
Overall order of base strength
weak Aromatic amines ammonia primary amines tertiary amines secondary amines strong
Reactions with acids
All amines will react with acids to become ammonium salts
Amines act as 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)
Adding NaOH to ammonium salt
converts it back to ammonia
Nucleophilic addition
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.
Nucleophilic addition equation
CH3CH2Br + 2NH3 -> CH3CH2NH2 + NH4Br
Nucleophilic addition mechanism step 1
3HN: to C
makes C-NH3+ + Br-
Nucleophilic addition mechanism step 2
:NH3 to H
H-NH2 to N
makes C-NH2 + NHBr4
why is excess ammonia used
Using an excess of ammonia can limit the further subsequent reactions and will maximise the amount of primary amine formed
Reaction of forming a secondary amine
:NH2 to C
C-Br to Br
:NH3 to H
H-NH+ to N
Reaction of forming a tertiary amine
:NH to C
C-Br to Br
:NH3 to H
H-N+ to N+
Reaction of forming a quaternary ammonium salt
:N to C
C-Br to Br
