THE CHEMISTRY
OF AMINES
A guide for A level students
KNOCKHARDY PUBLISHING
INTRODUCTION
This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may be used for classroom teaching if an interactive white board is available.
Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...
www.knockhardy.org.uk/sci.htm

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AMINES
KNOCKHARDY PUBLISHING
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
AMINES
STRUCTURE & CLASSIFICATION
Structure Contain the NH2 group

Classification




primary (1°) amines secondary (2°) amines






tertiary (3°) amines 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 Aqueous, alcoholic ammonia

Conditions Reflux in aqueous, alcoholic solution under pressure

Product Amine (or its salt due to a reaction with the acid produced)

Nucleophile Ammonia (NH3)

Equation C2H5Br + NH3 (aq / alc) ——> C2H5NH2 + HBr ( or C2H5NH3+Br¯ )
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
C6H5 N:
H
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



electron releasing substituents (CH3 groups)
increase basicity as the electron density is
increased and the lone pair is more effective
CH3 N:
H
H
C6H5 N:
H
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 Comments
ammonia NH3 4.76
methylamine CH3NH2 3.36 methyl group is electron releasing
phenylamine C6H5NH2 9.38 electrons delocalised into the ring


strongest base methylamine > ammonia > phenylamine weakest base
smallest pKb largest pKb
CHEMICAL REACTIONS - WEAK BASES
Water Amines which dissolve in water produce weak alkaline solutions

CH3NH2(g) + H2O(l) CH3NH3+(aq) + OH¯(aq)



Acids 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 Mechanism
haloalkanes substituted amines nucleophilic substitution

acyl chlorides N-substituted amides 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 diethylamine, 2° amine


(C2H5)2NH + C2H5Br ——> HBr + (C2H5)3N 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 diethylamine, 2° amine


(C2H5)2NH + C2H5Br ——> HBr + (C2H5)3N triethylamine, 3° amine


(C2H5)3N + C2H5Br ——> (C2H5)4N+ Br¯ 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 diethylamine, 2° amine


(C2H5)2NH + C2H5Br ——> HBr + (C2H5)3N triethylamine, 3° amine


(C2H5)3N + C2H5Br ——> (C2H5)4N+ Br¯ tetraethylammonium bromide
a quaternary (4°) salt


Uses 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 COOH




They all have a similar structure - the identity of R1 and R2 vary
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


Asymmetric carbon atoms have four
different atoms or groups attached


Two isomers are formed - one rotates plane
polarised light to the left, one rotates it to the right



Glycine doesn’t exhibit optical isomerism as
there are two H attached to the C atom
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
• 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.
AMINO ACIDS - ACID-BASE PROPERTIES
AMINO ACIDS - ACID-BASE PROPERTIES
Basic properties:

with H+ HOOCCH2NH2 + H+ ——> HOOCCH2NH3+

with HCl HOOCCH2NH2 + HCl ——> HOOCCH2NH3+ Cl¯




Acidic properties:

with OH¯ HOOCCH2NH2 + OH¯ ——> ¯OOCCH2NH2 + H2O

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 NH2 becomes NH3+Cl¯
H+ NH2 becomes NH3+
NaOH COOH becomes COO¯ Na+
OH¯ COOH becomes COO¯
PEPTIDES - HYDROLYSIS
Peptides are broken down into their constituent amino acids by hydrolysis

Which amino acids are formed?
PEPTIDES - HYDROLYSIS
Peptides are broken down into their constituent amino acids by hydrolysis

+
+
PEPTIDES - HYDROLYSIS
Peptides are broken down into their constituent amino acids by hydrolysis

Which amino acids are formed?
PEPTIDES - HYDROLYSIS
Peptides are broken down into their constituent amino acids by hydrolysis

2 x
+
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 -CONH2



Nomenclature 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 + NH3 ——> CH3CONH2 + HCl
ethanoyl chloride ethanamide
AMIDES - CHEMICAL PROPERTIES
Hydrolysis
general reaction CH3CONH2 + H2O ——> CH3COOH + NH3
acidic soln. CH3CONH2 + H2O + HCl ——> CH3COOH + NH4Cl
alkaline soln. CH3CONH2 + NaOH ——> CH3COONa + NH3


Identification 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
REVISION CHECK
What should you be able to do?
Recall the structural types and nomenclature of amines
Explain and understand the basic properties of amines
Recall and explain the nucleophilic properties of amines
Recall the structure and general properties of amino acids, proteins and peptides
Understand why amino acids can exhibit optical activity
Work out the products from the hydrolysis of a peptide
Recall the structure, nomenclature, preparation and simple properties of amides
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WELL DONE!
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© 2003 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
THE CHEMISTRY
OF AMINES
THE END