Amines

Something fishy about amines:

• Have an NH₂, amine group.

• Amines are derivatives of ammonia:

3 H atoms	1 H atom replaced: 1 attached C to N	2 H atoms replaced: 2 attached C's to N	3 H atom replaced: 3 attached C's to N
Ammonia, NH3	Primary amine	Secondary amine	Tertiary amine

• Amines occur in nature and are known for their physiological effects:

Treats drowsiness and fatigue syndrome	Decongestant	'Fight or flight' to cope with sudden stress

• Amines are also known for their unpleasant and often fishy smell.

Naming amines:

• To the longest single alkyl chain add the suffix 'amine':

• If there are 2 alkyl groups on the nitrogen:

Start with the longest alkyl chain - propylamine The shorter alkyl group is prefixed with 'N' - N - methyl propylamine The prefix 'N' tells you that the alkyl group comes off the nitrogen atom. A tertiary amine would have 2 prefixes of 'N' - N,N - methyl ethyl butylamine (for example)

Basicity in amines:

• Amines are weak bases - proton acceptors.

• This is because they have a lone pair of electrons on the nitrogen available to donate in accepting a hydrogen ion:

The inductive effect:

• The basic properties of phenylamine are less than that of ammonia.

• The basic properties of amines are greater than that of ammonia.

• This can be explained by the inductive effect:

• Remember a base is a proton acceptor and the proton is accepted using the lone pair of electrons on the nitrogen.

• Groups attached to the functional group can have an effect on how available the lone pair of electrons for accepting a proton:

Base reactions of amines:

• Just as ammonia forms salts with acids so do amines:

• Phenylamine reacts with nitric acid:

Questions   1 - 2  P37

Amines and their reactions

Preparation of primary aliphatic amines:

• These are made by warming halogenoalkanes with excess ammonia:

             CH₃CH₂CH₂Cl       +       NH₃      à         CH₃CH₂CH₂NH₂       +       HCl

• The excess ammonia reacts with the HCl formed:

             NH₃                  +       HCl      à         NH₄Cl

Preparation of secondary / tertiary aliphatic amines:

• Propylamine can react further (like the ammonia) with more chloropropane:

             CH₃CH₂CH₂Cl       +       CH₃CH₂CH₂NH₂      à         (CH₃CH₂CH₂)₂NH       +       HCl

• And dipropylamine can react even further again:

             CH₃CH₂CH₂Cl       +       (CH₃CH₂CH₂)₂NH      à         (CH₃CH₂CH₂)₃N       +       HCl

• Multiple substitution is avoided by having ammonia in excess.  This minimises the 'chance' of further substitution.

Preparation of aromatic amines:

• Nitrobenzene (and other nitroarenes) can be reduced using a mixture of tin and concentrated hydrochloric acid:

• The excess HCl is neutralised at the end of  the reaction.

• This is an important reaction as it is used in the manufacture of dyes.

Synthesis of dyes from phenylamine:

• There are 2 stages in this process:

        1)  Diazotisation

        2)  Coupling reactions

1)  Diazotisation:

a)  Making Nitrous acid:

• Nitrous acid is unstable so must be made when needed.

NaNO₂                  +             HCl                  à  HNO₂

         Sodium nitrite           Hydrochloric acid                 Nitrous acid             

b)  Diazotisation reaction:

• The diazonium ion, N₂⁺ is unstable and decomposes releasing nitrogen, this is why it needs to be kept below 10^oC.

• The benzene ring in benzenediazonium salts allow the p electrons from the diazonium functional group to be delocalised over the benzene ring.

• This stabilises the benzenediazonium salts enough to be used at low temperatures.

2)  Coupling reaction:

• The benzenediazonium salt is reacted with phenol under alkaline conditions.

• As the 2 molecules are now ‘coupled’ together we call it a coupling reaction.

• The Azo dye is now stable as there is extensive delocalisation over both arenas via the azo group, -N=N-.

• An everyday use of azo dyes is methyl orange indicator.

Questions  1 - 3  P39    Qu 9 P41   Qu 2a (ii) (iii)  P42    Qu 11-13  P44,45

Amino acids

• Have an NH₂, amine and a COOH, carboxylic acid group:

• They are the building blocks for proteins which are held together by peptide links.

• The body has 20 different amino acids which, as proteins can be enzymes, hormones, antibodies.

• These proteins are responsible for body functions such as carrying oxygen in the blood, formation of bones etc.

• The 20 Naturally occurring amino acids have the general formula of:

• This arrangement is called a amino acids due to its optical isomerism (covered later)

• The R group can contain OH, SH, COOH or NH₂ group.

• Glycine is the simplest amino acid which means it will not contain an R group:

• Other examples of a amino acids are shown below with different R groups attached:

Zwitterion and the isoelectric point:

• Amino acids contain an acidic carboxylic acid group and a basic amine group.

• These can interact with each other to form a zwitterion:

A proton is transferred from the COOH to the NH2 The zwitterion has no overall charge as the COO- cancels out the NH3+ The isoelectric point is the pH at which there is no 'net' electric charge. This + / - charge increases the intermolecular forces between amino acids considerably.  They are often described as having unusually high melting points. You may expect this to be pH7. In actual fact is is usually around the pH6 region. This is due to the fact that the COOH is actually slightly more acidic than the NH2 is basic. The rest of the molecule also has an influence on how acidic and how basic the COOH and NH2 are which means that every amino acid will have a slightly different isoelectric point.

Comparison of melting points:

Acid and base properties of amino acids:

• Amino acids are amphoteric.

• This means that they will react with both:

            Acids:  Due to the basic NH₂ present.

            Alkalis:  Due to the acidic COOH present.

pH < Isoelectric point:	pH > Isoelectric point:
pH < Isoelectric point: In acidic conditions there is an abundance of H+ions. The amino acid acts as a base and accepts as many H+ions as possible.	pH > Isoelectric point: In Alkaline conditions there is a defficit of H+ions. The amino acid acts as an acid and donates as many H+ions as possible.

Qu 1 - 3  P51

Polypeptides and proteins

Amino acids and condensation reactions:

• When 2 amino acids join together we call it a dipeptide. 

• When 3 amino acids join together we call it a tripeptide.

• When many join together we call it a polypeptide.

• Polypeptides are synthetic.

• Proteins are natural and are usually larger than polypeptides. 

• The term ‘Peptide linkage’ (or bond) is the name for the amide link which in poly peptides or proteins.

• The reaction is called a condensation reaction as water is given off:

• The functional group (in purple) is an amide group (CONH)

• A different dipeptide can be made by joining them the other way round:

Forming polypeptides and proteins:

• This is a long chain of amino acids joined by peptide linkages:

• Each linkage forms a molecules of water.

• A polypeptide generally has >50 amino acids.

Hydrolysis of polypeptides and proteins:

• Polypeptides and proteins can be hydrolysed back into their constituent amino acids.

• This is the reverse of the reaction above.
• This is done by using an acid or alkaline catalyst. (like ester hydrolysis). 

Acid hydrolysis:

• Heat under reflux with 6M HCl fro 24hrs
• As this is in acidic conditions, the acid amino acid ion is formed:

Alkali hydrolysis:

• Solution of aq NaOH above 100^oC
• As this is in alkaline conditions, the alkali amino acid ion (salt) is formed:

Qu  1 - 3  P51

Condensation polymerisation:  1)  Polyesters

• This is the joining of s monomers while eliminating a small molecule - H₂O or HCl

• The functional group on one monomer joins with a different functional group in another molecule.

• There are 2 types of condensation polymerisations covered

• Polyesters - from alcohols and carboxylic acids

• Polyamides - from amines and carboxylic acids

1)  polyesters:

• A polyester is made by condensing an alcohol and carboxylic acid.

• The joining link is an ester functional group, hence polyester.

• A polyester is can be made in one of 2 ways:

a)    2 monomers:  Diol and Dicarboxylic acids-

• Terylene is made from the reaction between the monomers ethane-1,2-diol and benzene-1,4-dicarboxylic acid.

• It is described as a condensation reaction as water is eliminated as the ester link is formed.

• Used in the manufacture of carpets.

b)    1 monomer:  Hydroxycarboxylic acid

• Poly(lactic acid), PLA is made from the reaction between the same monomer containing an OH and a COOH group.

• It is described as a condensation reaction as water is eliminated as the ester link is formed.

Qu 1-3   P55

Condensation polymerisation:  2)  Polyamides: 

• Polyamides are made by an amine / carboxylic acid condensation reaction.

• The resulting link is an amide:

• A polyester is can be made in one of 2 ways:

a)    2 monomers:  Diamine and Dicarboxylic acids-

• Formed from the reaction between the 2 monomers containing 2 NH₂ and 2 COOH groups.

• Nylon is made from the reaction between the monomers 1,6-diaminohexanel and hexane-1,4-dioic acid.

• It is described as a condensation reaction as water is eliminated as the amide link is formed.

• Used in the manufacture of clothing.

• Another is kevlar - very strong polymer used in fire and bullet proof vest and crash helmets:

b)    1 monomer:  Hydroxycarboxylic acid

• A monomer with an amine group at one end and a carboxylic acid group at the other.

• Polypeptides and proteins - from the reaction between the same monomer containing an NH₂ and a COOH group.

• It is described as a condensation reaction as water is eliminated as the amide link is formed.

Qu 1-3   P57

Addition and condensation polymerisation

Addition polymers:

• This was covered in AS under the alkenes chapter:

• These are made from one monomer only - containing a C=C

• Only one product is formed.

• Using different alkene molecules, different addition polymers can be made:

Condensation polymerisation:

• These polymers are when monomers are joined with the elimination of a small molecule, H₂O or HCl.

• The monomers must have 2 functional groups.

Comparison of addition and condensation polymers:

	Addition polymerisation	Condensation polymerisation
		Polyester	Polyamide
Functional groups	C=C	COOH  /  OH	COOH / NH2
No monomers	1	1 or 2	1 or 2
Products	poly(alkene)	polyester + water	polyamide + water
linkage	C-C

Workes example  P59

Qu  1,2  P59

Breaking down condensation polymers

• If condensation polymerisation eliminates water then they can be hydrolysed with the addition of water (acid / base)

• 2 condensation polymers have been covered and both can be hydrolysed:

1)  Hydrolysis of polyesters:

• In carbonyl compounds we saw that esters can be hydrolysed in acidic or basic conditions.

• This gave the corresponding alcohol and carboxylic acid (or salt of acid - base)

• Polyesters can be hydrolysed in exactly the same way with hot aqueous acid / aqueous alkali.

• The monomers making up the polymer are produced (or salt if base hydrolysis used):

2)  Hydrolysis of polyamides:

• Polyesters can also be hydrolysed with hot aqueous acid / aqueous alkali.

• The monomers making up the polymer are produced - ammonium in acidic conditions - carboxylate salts in basic:

Degradable polymers:

• Most plastic packaging is addition polymers which will not degrade in landfill sites.

• Environmental demand has produced biodegradable plastics.

• These have chemical bonds that will hydrolyse similar to polyesters and polyamides.

• A polymer based on tapiocha starch will decompose in 28 days when buried.

Photodegradable polymers:

• These become weak and brittle when exposed to light.

• The polymers are blended with light sensitive catalysts which will catalyse the decomposition of the polymer.

• Another way is to encorperate C=O bonds within the polymer structure.

• These absorb UV light and break.

• Photodegradable plastics break to form waxy shorter hydrocarbon molecules before bacteria breaks them further into CO₂ and H₂O.

Qu  1-3  P61

Qu 9 P41   Qu 2a (ii) (iii)  P42    Qu 11-13  P44,45

Qu  1,2,4,5,6  P 69   /   Qu 1a,2,3,4,5,6,9,10,11,12   P70-73