Amines

1
Amines

• Ammonia derivatives

2
Specification from OCR

• Explain the basicity of amines in terms of proton
  acceptance by the nitrogen lone pair.
• Describe the reactions of amines with acids to
  form salts
• Describe the preparation of
• i) aliphatic amines by substitution of
  halogenalkanes with excess ethanolic ammonia
• ii) aromatic amines by reduction of nitroarenes
  using tin and conc. hydrochloric acid
• Describe the synthesis of an azo-dye
• State the use of reactions in the formation of
  dyestuffs

3
Amines

• Amines are essentially molecules of ammonia
• One or more of the hydrogen atoms have been
  replaced with an alkyl group.

4
Amines

• Replace one hydrogen atom with an alkyl group
  primary amine, replace 2 secondary amine etc.

5

The LONE PAIR on the nitrogen atom in amines
makes them ...

• BRØNSTED-LOWRY BASES - they can be proton
  acceptors
• RNH2 H gt RNH3
• GOOD NUCLEOPHILES able to attack the positive
  end of a polarised bond.

6
Amines as bases

• Bases are proton acceptors.
• Amines dont actually accept protons, they donate
  a lone pair to the hydrogen atom to form a dative
  bond.
• Ammonia and bases can do this with any suitable
  acid to give a salt.

7
Replacing a hydrogen in ammonia has the following
effect

• Causes increased electron donation in the C-N
  bond
• Becomes polar, and nitrogen becomes slightly
  negative
• Lone pair on nitrogen slightly repelled
• Can be donated to a proton more easily
• 1 amines are more basic than ammonia

8
What about secondary and tertiary?

• So following the same argument, 2 amines will be
  more basic still, as the lone pair will be
  repelled even more.
• In phenylamine, the lone pair becomes involved in
  the aromaticity, so it is less basic. The lone
  pair as part of the rings delocalised system, it
  is less readily donated to a proton.
• A tertiary amine will be more basic still.

9
Reaction of amines with acids

• A standard Acid Base reaction.

10
The preparation of amines
11
1. Reduction of nitrobenzene to give phenylamine
Conditions are reflux, this is important in the
production of compounds called azo-dyes.
12
Substitution of haloalkanes with excess ethanolic
ammonia
AMMONIA Reagent Aqueous, alcoholic ammonia (in
EXCESS) Conditions Reflux in aqueous , alcoholic
solution under pressure Product Amine Nucleophile
Ammonia (NH3) Equation e.g. C2H5Br
2NH3 (aq / alc) gt C2H5NH2
NH4Br (i) C2H5Br NH3 (aq / alc)
gt C2H5NH2 HBr (ii) HBr NH3
(aq / alc) gt NH4Br Mechanism Notes
The equation shows two ammonia molecules. The
second one ensures that the HBr is removed.
13
NUCLEOPHILIC SUBSTITUTION
AMMONIA Why excess ammonia? The second ammonia
molecule ensures the removal of HBr which would
lead to the formation of a salt. A large excess
ammonia ensures that further substitution doesnt
take place - see below Problem Amines are also
nucleophiles (lone pair on N) and can attack
another molecule of halogenoalkane to produce a
2 amine. This too is a nucleophile and can
react further producing a 3 amine and,
eventually an ionic quarternary ammonium
salt. C2H5NH2 C2H5Br gt HBr
(C2H5)2NH diethylamine, a 2
amine (C2H5)2NH C2H5Br gt HBr
(C2H5)3N triethylamine, a 3
amine (C2H5)3N C2H5Br gt (C2H5)4N
Br tetraethylammonium
bromide a quaternary (4) salt
14
Diazonium Salts

• Diazonium there are 2 nitrogen atoms joined
  together in the positive ion.
• In French, nitrogen is still called by its old
  name azote which means unable to support life.

15
Formation of a Diazonium salt.

• Formed by reacting phenylamine with sodium
  nitrite and hydrochloric acid below 10?C.
• These reagents form in situ nitrous acid HNO2.

16
Diazonium Salts

• Notice the triple bond between the nitrogen atoms
• The positive charge is on the nitrogen that is
  attached to the benzene ring

17
Why are they important? They look pretty weird!

• They are essential in the dye industry.
• A Diazonium salt is produced then reacted with a
  phenol. If the correct phenol is used, almost any
  colour can be produced.
• OCR specify this as a reaction you need to know.

18
Formation of the Diazonium salt.

• The Diazonium salt is unstable above 10C, so the
  reaction is normally carried out in ice.
• An aliphatic Diazonium salt is very unstable, so
  only aromatics are used.
• The lone pairs present in the salt can
  participate in the benzene ring, making it more
  stable. More correctly this is due to overlap of
  p-orbitals in the diazo group with the p-system
  in the ring.
• So phenylamine would give benzenediazonium
  chloride.

19
Formation of the Diazonium salt.
20
Formation of the Diazonium salt.

• The conditions are below 10?C and remember the
  HNO2 (nitrous acid) is prepared in situ by
  reacting sodium nitrite with hydrochloric acid.
• The diazonium salt can then do one of two things
  depending on the temperature

21
Reactions of aromatic diazonium salts
22
Hydrolysis

• The following occurs if a solution of a diazonium
  salt is warmed up

23
Coupling reactions

• The mechanism is for interest only, you do not
  need to know it.

24
General method for synthesis of azo dyes

• Add a cold aqueous solution of sodium nitrite
  slowly (with cooling and stirring) to a cold
  solution of the amine compound in excess
  hydrochloric acid
• The temperature must not rise above 5C.
• This solution (still cold) should then be added
  slowly with stirring to a solution of the
  coupling compound.
• This should be kept below 5C the whole time.

25
Amino Acids

• These are bi-functional compounds. The contain 2
  functions groups
• A primary amine (in most cases) NH2
• The carboxylic acid group COOH
• An amino acid must contain at least both of these
  functional groups.

26
Amino Acids

• The simplest amino acid is glycine.

27
Amino Acids

• All the amino acids (the twenty vitally important
  ones biologically) are 2-amino acids.
• The amine and acid groups are both attached to
  the same carbon.
• All can be names systematically, but in most
  cases the old names are used.
• Alanine is also known as 2-aminopropanoic acid,
  but alanine is the acceptable name to use.

28
Alanine
29
General Formula
30
Physical Properties

• White solids
• With relatively high melting points glycine (the
  simplest) has a melting point of 235C.
• Normally readily soluble in water
• Almost totally insoluble in non-polar solvents

31
Acid Base Properties

• They are very largely ionic compounds.
• The carboxyl group can lose a proton
• The amine group can gain a proton
• The result is a ZWITTERION. From the German for
  hermaphrodite, hybrid or mongrel!

32
Zwitterions

• Glycine mainly exists as.

33
Zwitterions

• The strong attractions in the crystal cause the
  high melting point
• In aqueous solution depending on the pH, they
  form either the neutral form, or the carboxylate
  will lose a proton, or the amino group will gain
  a proton.

34
Zwitterions
35
Isoelectric Point

• For each amino acid there is a definite pH the
  isoelectric point at which the acid and basic
  ionisations are equal.
• The molecule is effectively neutral it carries
  equal and opposite charges
• This is rarely near pH 7 because the molecule
  ionisation tendencies are affected by the other
  groups in the molecule.

36
Isoelectric Point

• Aspartic acid which has 2 COOH groups is
  acid in aqueous solution.
• Lysine with more amino than carboxyl groups is
  alkaline.
• Due to this dual functionality, they are able to
  act as buffer solutions (able to maintain a
  reasonably constant pH with small additions of
  acid or alkali).
• They also have optical activity.

37
How amino acids join together

• Amino acids join together in specific ways to
  form specific proteins.
• One amino acid can join to another to form a
  substituted amide.

38
How amino acids join together

• This kind of bond between 2 amino acids is called
  a peptide bond or a peptide link.

39
How amino acids join together
40
How amino acids join together

• Two joined amino acids dipeptide
• Three tripeptide
• Many polypeptide
• At some point a polypeptide becomes a protein.
  This can be put at 40 amino acids.

41
Acid Hydrolysis of proteins

• Proteins and peptides can be hydrolysed with hot
  concentrated (6 mol dm-3) HCl.
• The protein is refluxed for about 24 hours.
• This hydrolysis is the exact reverse of the
  formation of the peptide bond.
• A molecule of water is in effect added across the
  linkage to regenerate the original amino acid and
  carboxyl groups.

42
Acid Hydrolysis of proteins