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The chemistry of amines a guide for a level students

THE CHEMISTRY

OF AMINES

A guide for A level students

2008 SPECIFICATIONS

KNOCKHARDY PUBLISHING


The chemistry of amines a guide for a level students

KNOCKHARDY PUBLISHING

AMINES

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

Navigation is achieved by...

either clicking on the grey arrows at the foot of each page

orusing the left and right arrow keys on the keyboard


The chemistry of amines a guide for a level students

AMINES

  • CONTENTS

  • Prior knowledge

  • Structure and classification

  • Nomenclature

  • Physical properties

  • Basic properties

  • Nucleophilic properties

  • Amino acids

  • Peptides and proteins

  • Amides

  • Check list


The chemistry of amines a guide for a level students

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


The chemistry of amines a guide for a level students

H

H

R

R

R N:

R N:

R N:

+

R NR

H

R

R

R

STRUCTURE & CLASSIFICATION

StructureContain the NH2 group

Classification

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

tertiary (3°) aminesquarternary (4°) ammonium salts

Aliphaticmethylamine, ethylamine, dimethylamine

AromaticNH2 group is attached directly to the benzene ring (phenylamine)


The chemistry of amines a guide for a level students

NOMENCLATURE

NomenclatureNamed after the groups surrounding the nitrogen + amine

C2H5NH2ethylamine

(CH3)2NHdimethylamine

(CH3)3Ntrimethylamine

C6H5NH2phenylamine (aniline)


The chemistry of amines a guide for a level students

PREPARATION

Amines can be prepared from halogenoalkanes

ReagentExcess, alcoholic ammonia(WHY USE EXCESS?)

ConditionsReflux in excess, alcoholic solution under pressure

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

NucleophileAmmonia (NH3)

EquationC2H5Br + NH3 (alc) ——> C2H5NH2 + HBr ( or C2H5NH3+Br¯ )


The chemistry of amines a guide for a level students

PREPARATION

Amines can be prepared from halogenoalkanes

ReagentExcess, alcoholic ammonia(WHY USE EXCESS?)

ConditionsReflux in excess, alcoholic solution under pressure

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

NucleophileAmmonia (NH3)

EquationC2H5Br + 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.


The chemistry of amines a guide for a level students

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


The chemistry of amines a guide for a level students

PHYSICAL PROPERTIES

Boiling pointBoiling 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

SolubilityLower mass compounds are

soluble in water due to hydrogen

bonding with the solvent.

Solubility decreases as the

molecules get heavier.

Soluble in organic solvents.


The chemistry of amines a guide for a level students

BASIC PROPERTIES

BasesThe lone pair on the nitrogen atom makes amines basic;

RNH2 + H+ ——> RNH3+ a proton acceptor

Strengthdepends 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


The chemistry of amines a guide for a level students

BASIC PROPERTIES

BasesThe lone pair on the nitrogen atom makes amines basic;

RNH2 + H+ ——> RNH3+ a proton acceptor

Strengthdepends 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

C6H5 N:

H


The chemistry of amines a guide for a level students

BASIC PROPERTIES

BasesThe lone pair on the nitrogen atom makes amines basic;

RNH2 + H+ ——> RNH3+ a proton acceptor

Strengthdepends 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

H

C6H5 N:

H

H

CH3 N:

H


The chemistry of amines a guide for a level students

BASIC PROPERTIES

Measurementthe 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 pKbComments

ammonia NH34.76

methylamine CH3NH23.36methyl group is electron releasing

phenylamine C6H5NH29.38electrons delocalised into the ring

strongest basemethylamine > ammonia > phenylamineweakest base

smallest pKblargest pKb


The chemistry of amines a guide for a level students

CHEMICAL REACTIONS - WEAK BASES

WaterAmines which dissolve in water produce weak alkaline solutions

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

AcidsAmines 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)


The chemistry of amines a guide for a level students

CHEMICAL REACTIONS - NUCLEOPHILIC

Due to their lone pair, amines react as nucleophiles

Reagent Product Mechanism

haloalkanes substituted aminesnucleophilic substitution

acyl chloridesN-substituted amides addition-elimination


The chemistry of amines a guide for a level students

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)2NHdiethylamine, 2° amine


The chemistry of amines a guide for a level students

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° amineand, eventually an ionic quarternary ammonium salt.

C2H5NH2 + C2H5Br ——> HBr + (C2H5)2NHdiethylamine, 2° amine

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


The chemistry of amines a guide for a level students

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)2NHdiethylamine, 2° amine

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

(C2H5)3N + C2H5Br ——> (C2H5)4N+ Br¯tetraethylammonium bromide

a quaternary (4°) salt


The chemistry of amines a guide for a level students

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)2NHdiethylamine, 2° amine

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

(C2H5)3N + C2H5Br ——> (C2H5)4N+ Br¯tetraethylammonium bromide

a quaternary (4°) salt

UsesQuarternary ammonium salts with long chain alkyl groups are used

as cationic surfactantsin fabric softening e.g. [CH3(CH2)17]2N+(CH3)2 Cl¯


The chemistry of amines a guide for a level students

R1

H

H

H2N C COOH

H2N C COOH

H2N C COOH

CH3

H

R2

AMINO ACIDS

StructureAmino acids contain 2 functional groups

amine NH2

carboxyl COOH

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


The chemistry of amines a guide for a level students

H

H

H2NCCOOH

H2NCCOOH

CH3

H

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


The chemistry of amines a guide for a level students

R1

H3N+ C COO¯

R2

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


The chemistry of amines a guide for a level students

R1

H2N C COOH

R2

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.


The chemistry of amines a guide for a level students

AMINO ACIDS - ACID-BASE PROPERTIES

Basic properties:

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

with HClHOOCCH2NH2 + HCl ——> HOOCCH2NH3+ Cl¯

Acidic properties:

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

with NaOH HOOCCH2NH2 + NaOH ——> Na+ ¯OOCCH2NH2 + H2O


The chemistry of amines a guide for a level students

PEPTIDES - FORMATION & STRUCTURE

Amino acids can join together to form peptides via an amide or peptide link

2 amino acids joineddipeptide

3 amino acids joinedtripeptide

many amino acids joinedpolypeptide

a dipeptide


The chemistry of amines a guide for a level students

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 HClNH2becomes NH3+Cl¯

H+NH2becomes NH3+

NaOHCOOHbecomes COO¯ Na+

OH¯COOHbecomes COO¯


The chemistry of amines a guide for a level students

H

H

CH3

NH C COOH

H2N C CO

NH C CO

CH3

CH3

H

PEPTIDES - HYDROLYSIS

Peptides are broken down into their constituent amino acids by hydrolysis

Which amino acids are formed?


The chemistry of amines a guide for a level students

H

H

CH3

NH C COOH

H2N C CO

NH C CO

CH3

CH3

H

H

H

CH3

H2N C COOH

H2N C COOH

H2N C COOH

CH3

CH3

H

PEPTIDES - HYDROLYSIS

Peptides are broken down into their constituent amino acids by hydrolysis

+

+


The chemistry of amines a guide for a level students

H

H

H

NH C COOH

H2N C CO

NH C CO

CH3

CH3

H

PEPTIDES - HYDROLYSIS

Peptides are broken down into their constituent amino acids by hydrolysis

Which amino acids are formed?


The chemistry of amines a guide for a level students

H

H

H

H

H

NH C COOH

H2N C CO

NH C CO

H2N C COOH

H2N C COOH

CH3

CH3

H

CH3

H

PEPTIDES - HYDROLYSIS

Peptides are broken down into their constituent amino acids by hydrolysis

2 x

+


The chemistry of amines a guide for a level students

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


The chemistry of amines a guide for a level students

AMIDES

Structurederivatives of carboxylic acids

amide group is -CONH2

NomenclatureWhite crystalline solids named from the corresponding acid

(remove oic acid, add amide)

CH3CONH2ethanamide (acetamide)

C2H5CONHC6H5N - phenyl propanamide -the N tells you the

substituent is on

the nitrogen

Nylons are examples of polyamides

PreparationAcyl chloride + ammonia

CH3COCl + NH3 ——> CH3CONH2 + HCl

ethanoyl chloride ethanamide


The chemistry of amines a guide for a level students

AMIDES - CHEMICAL PROPERTIES

Hydrolysis

general reaction CH3CONH2 + H2O ——> CH3COOH + NH3

acidic soln. CH3CONH2 + H2O + HCl ——> CH3COOH + NH4Cl

alkaline soln. CH3CONH2 + NaOH ——> CH3COONa + NH3

IdentificationWarming 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 a guide for a level students

THE CHEMISTRY

OF AMINES

THE END

© 2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING


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