3.11 Amines Flashcards
Why do 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.
Why are primary aliphatic amines weak bases?
Because only a low concentration of hydroxide ions is produced.
(see page 1 in the chemrevise revision guide)
Write the equilibrium reaction of CH3NH2 + H2O
(see page 1 in the chemrevise revision guide)
Why are primary aliphatic amines stronger bases than ammonia?
Because 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.
(see page 1 in the chemrevise revision guide)
Why are secondary amines 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 (as the inductive effect of alkyl groups is greater than that of H atoms)
Why are tertiary amines weaker bases than secondary amines?
The tertiary amines and corresponding ammonium salts are less soluble in water.
Why do primary aromatic amines such as phenylamine 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.
Draw the display formula for phenylamine.
(see page 1 in the chemrevise revision guide)
What is the overall order of base strength from weak to strong of these:
Ammonia Tertiary amines Secondary amines Primary amines Aromatic amines
aromatic amines < ammonia < primary amines < tertiary amines < secondary amines
All amines will react with acids to become __________
All amines will react with acids to become ammonium salts.
The ionic salts in this reaction means that the compounds are soluble in the acid.
e.g. phenylamine is not very soluble in the water but phenylammonium chloride is soluble.
These ionic salts will be solid crystals, if the water is evaporated, because of the strong ionic interactions.
(see page 1 in the chemrevise revision guide for some example equations)
Write the equation of the formation of methyl ammonium chloride from methylamine.
(see page 1 in the chemrevise revision guide)
What will addition of NaOH do to an ammonium salt?
It will convert it back to the amine.
How can a basic buffer be made from an amine?
Give 3 examples.
Basic buffers can be made from combining a weak base with a salt of that weak base.
e.g.
Ammonia and ammonium chloride
Methylamine and methylammonium chloride
Ethylamine and ethylammonium chloride
What mechanism reaction is primary amines formed by?
And between which two compounds?
Nucleophilic substitution between halogenoalkanes and ammonia in a one step reaction.
Why is the nucleophilic substitution reaction between halogenoalkanes and ammonia not a good method for making a primary amine?
What can we do to reduce this problem?
(after the nucleophilic substitution reaction) 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.
Therefore it is 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.
Using an excess of ammonia can limit further subsequent reactions and will maximise the amount of primary amine formed.
