28.3 Amines as nucleophiles and their synthesis Flashcards
How do amines act as nucleophiles
The lone pair on them will attack positively charged carbon atoms.
So amines, like ammonia will act like nucleophiles
When are primary aliphatic amines produced
Eg with chloroethane
When halogenoalkanes are reacted with ammonia, there is nucleophilic substitution of the halide by NH2
NH3 + CH2ClCH3 —> (CH3CH2NH3+) + Cl-
(CH3CH2NH3+) + NH3 —> (CH3CH2NH2) + NH4+ + Cl-
The primary amine produced from the last reaction is also a nucleophile as it has a lone pair on nitrogen.
How does this react further
CH3CH2NH2 + CH2ClCH3 –> CH3CH2NH2CH2CH3 + Cl-
CH3CH2NH2CH2CH3 + Cl- + NH3 —>
CH3CH2NHCH2CH3 + NH4+ + Cl-
So it produces a secondary amine
How does the secondary amine produced react to make a tertiary amine
The process repeats:
CH3CH2NHCH2CH3 + CH3CH2NH2 —>
[CH3CH2NHCH2CH3CH2CH3]+
[CH3CH2NHCH2CH3CH2CH3]+
+ CH3CH2Cl —->
CH3CH2NCH2CH3CH2CH3 + NH4+ + Cl-
This is a quaternary salt, and the chain reaction stops because there is no longer a lone pair on the nitrogen
How are the products of this reaction separated
Fractional distillation
So this isn’t a very efficient way of producing an amine, though the products may be separated by fractional distillation
Explain the reaction
Initially ammonia acts as a nucleophile, but then it acts as a base to accept protons
How are primary amines made
Reduction of nitriles
They are made in a two step process by nucleophilic substitution with halogenoalkanes and ammonia
- Halogenoalkanes react with a cyanide ion in aqueous ethanol.
The cyanide ion replaces the halide ion by nucleophilic substitution - This forms a nitrile which has functional group C≡N
Nitriles can be reduced to primary amines, eg with a nickel hydrogen catalyst.
So CH3-C≡N + 2H2 —> CH3-CH2NH2
This gives a purer product than a bromoalkane and ammonia because only the primary amine can be formed. It won’t keep reacting
How is phenylamine made
Made from benzene, which is reacted with a concentrated solution of nitric and sulfuric acid. This produces nitrobenzene
Then nitrobenzene is reduced to phenylamine using tin and HCl as the reducing agent.
Tin and HCl react to form hydrogen which reduces the nitrobenzene by removing oxygen atoms of the NO2 group and replacing them with hydrogen atoms
So do + 6[H] to change the NO2 on benzene to NH2
How are amides made
Amines will react with acyl chlorides and acid anhydrides.
These are nucleophilic substitution reactions (sometimes called addition elimination reactions)
Describe mechanism for amides being made using acyl chloride
CH3-C=OCl + :NCH3HH
So the lone pair on nitrogen goes to the C on acyl chloride which breaks one of the C=O bonds
This joins the NCH3HH to the acyl chloride and then the C-O:- means the lone pair goes back to the bond to make it double again, which breaks the other C-Cl bond
Now it is CH3C=ONCH3H2 bonded where the N has a positive charge so electrons from one of the NH bonds go to the N
So this makes final CH3C=ONCH3 + H+
So overall the amine adds on to the acid chloride and HCl is eliminated
Describe economic importance of amines
Amines are used in the manufacture of synthetic materials such as nylon and polyurethane and dyes and drugs
How are quaternary ammonium compounds used in conditioners
Hair and fabric conditioners contain them.
They have a long hydrocarbon chain and a positively charged organic group so they form cations
At the end of the chain is three CH3 molecules.
Both wet fabric and wet hair get negatively charged.
So the positive charges of the cations attract to them and bind to the surface to form a coating that prevents build up of static electricity.
This keeps surface of fabric smooth and prevents flyaway hairs
In aqueous solution the ions cluster with their charged ends in the water and their hydrocarbon tails on the surface
