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THE CHEMISTRY
OF AMINES
A guide for A level students
KNOCKHARDY PUBLISHING
2008
SPECIFICATIONS
KNOCKHARDY PUBLISHING
AMINES
INTRODUCTION
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AMINES
CONTENTS
• Prior knowledge
• Structure and classification
• Nomenclature
• Physical properties
• Basic properties
• Nucleophilic properties
• Amino acids
• Peptides and proteins
• Amides
• Check list
AMINES
Before you start it would be helpful to…
• know the functional groups found in organic chemistry
• know the arrangement of bonds around atoms
• recall and explain nucleophilic substitution reactions
STRUCTURE & CLASSIFICATION
Structure
Contain the NH2 group
Classification
H
R
N:
H
R
H
primary (1°) amines
R
secondary (2°) amines
R
R
N:
R
R
R
tertiary (3°) amines
N:
+
N
R
R
quarternary (4°) ammonium salts
Aliphatic
methylamine, ethylamine, dimethylamine
Aromatic
NH2 group is attached directly to the benzene ring (phenylamine)
NOMENCLATURE
Nomenclature
Named after the groups surrounding the nitrogen + amine
C2H5NH2
ethylamine
(CH3)2NH
dimethylamine
(CH3)3N
trimethylamine
C6H5NH2
phenylamine (aniline)
PREPARATION
Amines can be prepared from halogenoalkanes
Reagent
Excess, alcoholic ammonia
(WHY USE EXCESS?)
Conditions
Reflux in excess, alcoholic solution under pressure
Product
Amine (or its salt due to a reaction with the acid produced)
Nucleophile
Ammonia (NH3)
Equation
C2H5Br + NH3 (alc) ——> C2H5NH2 + HBr
( or C2H5NH3+Br¯ )
PREPARATION
Amines can be prepared from halogenoalkanes
Reagent
Excess, alcoholic ammonia
(WHY USE EXCESS?)
Conditions
Reflux in excess, alcoholic solution under pressure
Product
Amine (or its salt due to a reaction with the acid produced)
Nucleophile
Ammonia (NH3)
Equation
C2H5Br + NH3 (alc) ——> C2H5NH2 + HBr
( or C2H5NH3+Br¯ )
WHY USE EXCESS AMMONIA?
Ammonia attacks halogenoalkanes because it has a lone pair and is a nucleophile.
The amine produced also has a lone pair C2H5NH2 so can also attack a halogenoalkane;
this leads to the formation of substituted amines.
Using excess ammonia ensures that all the halogenoalkane molecules react with the
ammonia before having the chance to react with any amines produced.
PHYSICAL PROPERTIES
The LONE PAIR on the nitrogen atom in 1°, 2° and 3° amines makes them ...
LEWIS BASES - they can be lone pair donors
BRØNSTED-LOWRY BASES - they can be proton acceptors
RNH2
+
H+
——>
RNH3+
NUCLEOPHILES - provide a lone pair to attack an electron deficient centre
PHYSICAL PROPERTIES
Boiling point
Boiling points increase with molecular mass
Amines have higher boiling
points than corresponding
alkanes because of their
intermolecular hydrogen bonding
Quarternary ammonium
salts are ionic and exist as salts
Solubility
Lower mass compounds are
soluble in water due to hydrogen
bonding with the solvent.
Solubility decreases as the
molecules get heavier.
Soluble in organic solvents.
BASIC PROPERTIES
Bases
The lone pair on the nitrogen atom makes amines basic;
RNH2
+
H+
——>
RNH3+
a proton acceptor
Strength depends on the availability of the lone pair and its ability to pick up protons
• the greater the electron density on the N, the better it can pick up protons
• this is affected by the groups attached to the nitrogen
BASIC PROPERTIES
Bases
The lone pair on the nitrogen atom makes amines basic;
RNH2
+
H+
——>
RNH3+
a proton acceptor
Strength depends on the availability of the lone pair and its ability to pick up protons
• the greater the electron density on the N, the better it can pick up protons
• this is affected by the groups attached to the nitrogen
electron withdrawing substituents (benzene rings)
decrease basicity as the electron density on N is
lowered and the lone pair is less effective
H
C 6H 5
N:
H
BASIC PROPERTIES
Bases
The lone pair on the nitrogen atom makes amines basic;
RNH2
+
H+
——>
RNH3+
a proton acceptor
Strength depends on the availability of the lone pair and its ability to pick up protons
• the greater the electron density on the N, the better it can pick up protons
• this is affected by the groups attached to the nitrogen
electron withdrawing substituents (benzene rings)
decrease basicity as the electron density on N is
lowered and the lone pair is less effective
H
C 6H 5
N:
H
electron releasing substituents (CH3 groups)
increase basicity as the electron density is
increased and the lone pair is more effective
H
CH3
N:
H
BASIC PROPERTIES
Measurement
the strength of a weak base is depicted by its pKb value
the smaller the pKb the stronger the base
the pKa value can also be used;
it is worked out by applying pKa + pKb = 14
the smaller the pKb, the larger the pKa.
Compound
Formula
pKb
ammonia
NH3
4.76
methylamine
CH3NH2
3.36
methyl group is electron releasing
phenylamine
C6H5NH2
9.38
electrons delocalised into the ring
strongest base
smallest pKb
Comments
methylamine > ammonia > phenylamine
weakest base
largest pKb
CHEMICAL REACTIONS - WEAK BASES
Water
Amines which dissolve in water produce weak alkaline solutions
CH3NH2(g)
Acids
+
H2O(l)
CH3NH3+(aq)
+
OH¯(aq)
Amines react with acids to produce salts.
C6H5NH2(l)
+
HCl(aq) ——> C6H5NH3+Cl¯(aq)
phenylammonium chloride
This reaction allows one to dissolve an amine in water as its salt.
Addition of aqueous sodium hydroxide liberates the free base from its salt
C6H5NH3+Cl¯(aq)
+
NaOH(aq) ——> C6H5NH2(l)
+
NaCl(aq) + H2O(l)
CHEMICAL REACTIONS - NUCLEOPHILIC
Due to their lone pair, amines react as nucleophiles
Reagent
Product
haloalkanes
substituted amines
acyl chlorides
N-substituted amides
Mechanism
nucleophilic substitution
addition-elimination
NUCLEOPHILIC SUBSTITUTION
HALOALKANES
Amines are also nucleophiles (lone pair on N) and can attack halogenoalkanes 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
——> HBr + (C2H5)2NH
diethylamine, 2° amine
NUCLEOPHILIC SUBSTITUTION
HALOALKANES
Amines are also nucleophiles (lone pair on N) and can attack halogenoalkanes 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
——> HBr + (C2H5)2NH
(C2H5)2NH + C2H5Br ——> HBr + (C2H5)3N
diethylamine, 2° amine
triethylamine, 3° amine
NUCLEOPHILIC SUBSTITUTION
HALOALKANES
Amines are also nucleophiles (lone pair on N) and can attack halogenoalkanes 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
——> HBr + (C2H5)2NH
(C2H5)2NH + C2H5Br ——> HBr + (C2H5)3N
(C2H5)3N +
C2H5Br ——> (C2H5)4N+ Br¯
diethylamine, 2° amine
triethylamine, 3° amine
tetraethylammonium bromide
a quaternary (4°) salt
NUCLEOPHILIC SUBSTITUTION
HALOALKANES
Amines are also nucleophiles (lone pair on N) and can attack halogenoalkanes 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
——> HBr + (C2H5)2NH
(C2H5)2NH + C2H5Br ——> HBr + (C2H5)3N
(C2H5)3N +
Uses
C2H5Br ——> (C2H5)4N+ Br¯
diethylamine, 2° amine
triethylamine, 3° amine
tetraethylammonium bromide
a quaternary (4°) salt
Quarternary ammonium salts with long chain alkyl groups are used
as cationic surfactants in fabric softening
e.g. [CH3(CH2)17]2N+(CH3)2 Cl¯
AMINO ACIDS
Structure
Amino acids contain 2 functional groups
amine
NH2
carboxyl
R1
H2N
COOH
C
COOH
R2
They all have a similar structure - the identity of R1 and R2 vary
H
H2N
C
H
H
COOH
H2N
C
CH3
COOH
AMINO ACIDS – OPTICAL ISOMERISM
Amino acids can exist as optical isomers
If they have different R1 and R2 groups
Optical isomers exist when a molecule
Contains an asymmetric carbon atom
H
H2N
Asymmetric carbon atoms have four
different atoms or groups attached
C
COOH
CH3
Two isomers are formed - one rotates plane
polarised light to the left, one rotates it to the right
H
Glycine doesn’t exhibit optical isomerism as
there are two H attached to the C atom
H2N
C
COOH
H
GLYCINE
2-aminoethanoic acid
AMINO ACIDS - ZWITTERIONS
Zwitterion
• a dipolar ion
• has a plus and a minus charge in its structure
• amino acids exist as zwitterions
• give increased inter-molecular forces
• melting and boiling points are higher
R1
H3N+
C
R2
COO¯
AMINO ACIDS - ACID-BASE PROPERTIES
• amino acids possess acidic and basic properties
• this is due to the two functional groups
• COOH gives acidic properties
• NH2 gives basic properties
• they form salts when treated with acids or alkalis.
R1
H2N
C
R2
COOH
AMINO ACIDS - ACID-BASE PROPERTIES
Basic properties:
with H+
HOOCCH2NH2
+ H+
——>
HOOCCH2NH3+
with HCl
HOOCCH2NH2
+ HCl
——>
HOOCCH2NH3+ Cl¯
Acidic properties:
+ OH¯
——> ¯OOCCH2NH2 + H2O
with OH¯
HOOCCH2NH2
with NaOH
HOOCCH2NH2 + NaOH ——> Na+ ¯OOCCH2NH2 + H2O
PEPTIDES - FORMATION & STRUCTURE
Amino acids can join together to form peptides via an amide or peptide link
2 amino acids joined
dipeptide
3 amino acids joined
tripeptide
many amino acids joined
polypeptide
a dipeptide
PEPTIDES - HYDROLYSIS
Peptides are broken down into their constituent amino acids by hydrolysis
• attack takes place at the slightly positive C of the C=O
• the C-N bond is broken
• hydrolysis with water is very slow
• hydrolysis in alkaline/acid conditions is quicker
• hydrolysis in acid/alkaline conditions (e.g. NaOH) will produce salts
with
HCl
H+
NaOH
OH¯
NH2
NH2
COOH
COOH
becomes
becomes
becomes
becomes
NH3+Cl¯
NH3+
COO¯ Na+
COO¯
PEPTIDES - HYDROLYSIS
Peptides are broken down into their constituent amino acids by hydrolysis
H
H2N
H
C
CO NH
CH3
C
H
CH3
CO NH
C
CH3
Which amino acids are formed?
COOH
PEPTIDES - HYDROLYSIS
Peptides are broken down into their constituent amino acids by hydrolysis
H
H2N
C
H
CO NH
CH3
C
CH3
CO NH
C
COOH
CH3
H
H
H
H2N
C
CH3
COOH
+
H2N
C
H
CH3
COOH
+
H2N
C
CH3
COOH
PEPTIDES - HYDROLYSIS
Peptides are broken down into their constituent amino acids by hydrolysis
H
H2N
H
C
CO NH
CH3
C
H
H
CO NH
C
CH3
Which amino acids are formed?
COOH
PEPTIDES - HYDROLYSIS
Peptides are broken down into their constituent amino acids by hydrolysis
H
H2N
C
H
CO NH
CH3
H
CO NH
C
H2N
C
CH3
COOH
CH3
H
H
H
2x
C
COOH
+
H2N
C
H
COOH
PROTEINS
• are polypeptides with high molecular masses
• chains can be lined up with each other
• the C=O and N-H bonds are polar due to a difference in electronegativity
• hydrogen bonding exists between chains
dotted lines ---------- represent hydrogen bonding
AMIDES
Structure
derivatives of carboxylic acids
amide group is
Nomenclature
-CONH2
White crystalline solids named from the corresponding acid
(remove oic acid, add amide)
CH3CONH2
ethanamide (acetamide)
C2H5CONHC6H5
N - phenyl propanamide - the N tells you the
substituent is on
the nitrogen
Nylons are examples of polyamides
Preparation
Acyl chloride + ammonia
CH3COCl
+
ethanoyl chloride
NH3
——>
CH3CONH2 + HCl
ethanamide
AMIDES - CHEMICAL PROPERTIES
Hydrolysis
general reaction
acidic soln.
alkaline soln.
Identification
CH3CONH2
CH3CONH2
CH3CONH2
+
+
+
H2O ——> CH3COOH + NH3
H2O + HCl ——> CH3COOH + NH4Cl
NaOH ——> CH3COONa + NH3
Warming an amide with dilute sodium hydroxide solution and
testing for the evolution of ammonia using moist red litmus paper
is used as a simple test for amides.
Reduction
Reduced to primary amines: CH3CONH2
+
4[H]
——>
CH3CH2NH2 + H2O
THE CHEMISTRY
OF AMINES
THE END
© 2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING