6.2.1: Basicity and the preparation of amines Flashcards
What are amines?
- A class of compounds related to ammonia, NH3.
- They are organic molecules where one or more of the hydrogen atoms on ammonia have been replaced by alkyl chains
What are the three types of amine?
Primary:
-One hydrogen atom has been substituted (RNH2)
Secondary:
-Two hydrogen atoms have been substituted (RNHR’)
Tertiary:
-All three hydrogen atoms have been substituted (RNR’R’’)
Describe a Lewis base and assess whether or not ammonia and amines fit this description.
- Has a lone pair of electrons for donation.
- Ammonia and amines have a lone pair of electrons on the nitrogen atom, making them lewis bases.
Describe a Bronsted-Lowry base and assess whether or not ammonia and amines fit this descriprion.
- A proton acceptor.
- Ammonia and amines can accept protons on the nitrogen atom and are therefore also Bronsted-Lowry bases.
Using the Bronsted Lowry model, what happens when an amine reacts with an acid?
- It accepts a proton.
- A covalent bond is formed by the nitrogen atom donating its lone pair of electrons to the proton to form a dative covalent bond).
What happens when a base reacts with a dilute strong inorganic acid such as HCl?
-When HCl(aq) reacts with a base, a chloride salt and water are produced.
What happens when an amine reacts with a dilute strong inorganic acid?
give eqn. for reaction of ethylamine with HCl
- An alkylammonium salt is made.
- This is formed by the proton in the acid being replaced by an alkylammonium ion,
CH3CH2NH2 + HCl –> CH3CH2NH3 + Cl-
What is an alkylammonium salt?
-A compound where all of the hydrogen(s) have been substituted by alkyl chains.
Which salt is created when these acids react with amines?
- Hydrochloric acid
- Nitric acid
- Sulfuric acid
- Alkylammonium chloride
- Alkylammonium nitrate
- Alkylammonium sulfate
Why is the balanced symbol equation for the reaction of H2SO4 with an amine more complex than when HCl or HNO3 is used?
- Because when an amine reacts with sulfuric acid, two molecules of amine are required for every molecule of sulfuric acid.
- This is because the alkylammonium sulfate produced contains two alkylammonium ions for every one sulfate ion.
- Alkylammonium ions have a single +ve charge whilst a sulfate ion has a double -ve charge.
How could you prepare an amine?
Type of reaction:
Reactants:
Conditions:
- Nucleophilic substitution.
- A haloalkane, ammonia, ethanol.
- Reactants heated together (Reflux cannot be used as the ammonia is so volatile that it would escape out of the condenser rather than react, instead, a sealed tube should be used)
A haloalkane, such as 1-chloropropane, will undergo nucleophilic substitution in a two-stage process to form a primary amine. What are the two stages?
Stage 1:
-The ammonia reacts with the haloalkane to make an ammonium salt.
- CH3CH2CH2Cl + NH3 –> CH3CH2CH2NH3Cl
Stage 2:
-An additional ammonia molecule reacts to form the propylamine product and ammonium chloride salt.
- CH3CH2CH2NH3Cl +NH3 CH3CH2CH2NH2 + NH4Cl
Why does using excess ammonia in the second stage of preparing a primary amine increase the yield of the desired primary amine product?
-The reaction is reversible so excess ammonia will drive the reaction to the right and increase the yield of the desired primary amine product.
How could a secondary amine be formed (e.g. from propylamine)?
-Additional substitution of the hydrogen atoms on the nitrogen atom can occur.
-In this reaction, initially N-dipropylamine, a secondary amine is made.
CH3CH2CH2Cl + CH3CH2CH2NH2 (CH3CH2CH2)2NH2 + HCl
How could a tertiary amine be formed (e.g. from N-dipropylamine)?
-Further substitution of the hydrogen atoms on the nitrogen atom produces N-tripropylamine, a tertiary amine.
CH3CH2CH2Cl + (CH3CH2CH2)2NH
(CH3CH2CH2)3N +HCl
What is the final possible stage when preparing aliphatic amines?
(e.g. N-tripropylamine)
-The final stage is a quaternary ammonium salt where each hydrogen on the ammonium ion has been replaced with an alkyl chain.
CH3CH2CH2Cl + (CH3CH2CH2)3N
(CH3CH2CH2)4N+Cl-
Why can further substitution reactions occur in the preparation of aliphatic amines?
-Because amines have lone pairs of electrons on the nitrogen atom that can act as a nucleophile.
How could you increase the yield of the quaternary ammonium salt?
-By using excess haloalkane.
How can you produce aromatic amines?
Type of reaction:
Reactants:
Conditions:
-Reduction
-Nitroarenes (such as nitrobenzene), Tin, conc. HCl
(The reducing agent is made in situ by using a mixture of tin and concentrated HCl)
-Reflux at 100 degrees celsius.
-A strong alkali, such as NaOH, is added after about half an hour.
Why is a strong alkali, such as sodium hydroxide, added after about half an hour when preparing an aromatic amine?
-To remove the excess HCl and produce an amine.
How do you separate the aromatic amine after producing it?
-It’s a multi-stage process including steam distillation, solvent extraction and further distillation.
Draw the eqn. for the reduction of nitrobenzene to make phenylamine.
Check in book.
