MUDDLE YOUR WAY MUDDLE YOUR WAY THROUGH ORGANIC THROUGH ORGANIC

MECHANISMSMECHANISMS

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

A A VERYVERY BRIEF INTRODUCTION BRIEF INTRODUCTION

2008 2008 SPECIFICATIONSSPECIFICATIONS

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 a 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 any grey arrows at the foot of each page

or using the left and right arrow keys on the keyboard

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

MUDDLE YOUR WAY THROUGH MECHANISMSMUDDLE YOUR WAY THROUGH MECHANISMS

MEET THE ATTACKERSMEET THE ATTACKERS

Press the space bar

MEET THE ATTACKERSMEET THE ATTACKERS

I AM A NUCLEOPHILE

I HAVE A LONE PAIR WHICH I CAN USE TO FORM A NEW BOND.

I ATTACK ELECTRON DEFICIENT AREAS (those with a + or )

MEET THE ATTACKERSMEET THE ATTACKERS

Press the space bar

MEET THE ATTACKERSMEET THE ATTACKERS

I AM AN ELECTROPHILE

I HAVE A + CHARGE or a CHARGE. I ATTACK ELECTRON

RICH AREAS SUCH AS C=C DOUBLE BONDS.

MEET THE ATTACKERSMEET THE ATTACKERS

Press the space bar

MEET THE ATTACKERSMEET THE ATTACKERS

AND I AM A FREE RADICAL

I HAVE AN UNPAIRED ELECTRON WHICH I WANT TO PAIR UP. I AM

VERY REACTIVE AND DON’T MIND WHERE I ATTACK… WATCH OUT!

THE BONDING IN A MOLECULEINFLUENCES WHAT WILL ATTACK IT

WHO IS ATTACKED?WHO IS ATTACKED?

THE BONDING IN A MOLECULEINFLUENCES WHAT WILL ATTACK IT

SINGLEA typical covalent bond with one shared pair – nothing to tempt an attacking species

WHO IS ATTACKED?WHO IS ATTACKED?

THE BONDING IN A MOLECULEINFLUENCES WHAT WILL ATTACK IT

SINGLE

MULTIPLE

A typical covalent bond with one shared pair – nothing to tempt an attacking species

Bond has twice as many electrons – species which like electrons will be attracted

WHO IS ATTACKED?WHO IS ATTACKED?

THE BONDING IN A MOLECULEINFLUENCES WHAT WILL ATTACK IT

SINGLE

MULTIPLE

NON-POLAR

A typical covalent bond with one shared pair – nothing to tempt an attacking species

Bond has twice as many electrons – species which like electrons will be attracted

Similar atoms have an equal attraction for the shared pair of the covalent bond

WHO IS ATTACKED?WHO IS ATTACKED?

THE BONDING IN A MOLECULEINFLUENCES WHAT WILL ATTACK IT

SINGLE

MULTIPLE

NON-POLAR

POLAR

A typical covalent bond with one shared pair – nothing to tempt an attacking species

Bond has twice as many electrons – species which like electrons will be attracted

Similar atoms have an equal attraction for the shared pair of the covalent bond

Atoms have different electronegativities and the shared pair will be attracted more to one end – species known as nucleophiles will be attracted to the slightly positive end

WHO IS ATTACKED?WHO IS ATTACKED?

THE BONDING IN A MOLECULEINFLUENCES WHAT WILL ATTACK IT

SINGLE

MULTIPLE

NON-POLAR

POLAR

A typical covalent bond with one shared pair – nothing to tempt an attacking species

Bond has twice as many electrons – species which like electrons will be attracted

Similar atoms have an equal attraction for the shared pair of the covalent bond

Atoms have different electronegativities and the shared pair will be attracted more to one end – species known as nucleophiles will be attracted to the slightly positive end

WHO IS ATTACKED?WHO IS ATTACKED?

ALKANES ARE RELATIVELY UNREACTIVE ORGANIC COMPOUNDS

ALKANES CONTAIN TWO BOND TYPES

C-H and C-C

WHAT ATTACKS ALKANES?WHAT ATTACKS ALKANES?

Both bonds are single – no electron rich areas

Bonds are non-polar - no electron deficient areas

Free radicals are very reactive and do attack

FREE RADICAL SUBSTITUTIONFREE RADICAL SUBSTITUTION

ALKENES ARE MUCH MORE REACTIVE THAN ALKANES

ALKENES CONTAIN A C=C BONDThere will be twice as many electrons between the carbon atoms as there are in a single bond

WHAT ATTACKS ALKENES?WHAT ATTACKS ALKENES?

C=C bond is double – an electron rich area

Bonds are non-polar - no electron deficient areas

ELECTROPHILIC ADDITIONELECTROPHILIC ADDITION

HALOGENOALKANES ARE MUCH MORE REACTIVE THAN ALKANES

HALOGENOALKANES CONTAIN A POLAR BONDThe halogen is more electronegative than the carbon attracts the shared electron pair.

WHAT ATTACKS HALOGENOALKENES?WHAT ATTACKS HALOGENOALKENES?

Bond is single – not an electron rich area

Bond is polar – the greater electronegativity of the halogen creates a dipole making the carbon atom electron deficient.

NUCLEOPHILIC SUBSTITUTIONNUCLEOPHILIC SUBSTITUTION

THESE ARE USED TO REPRESENT THE MOVEMENT OF ELECTRONS

CURLY ARROWSCURLY ARROWS

THESE ARE USED TO REPRESENT THE MOVEMENT OF ELECTRONS

CURLY ARROWSCURLY ARROWS

ELECTRONS MOVE FROM AREAS OF HIGH ELECTRON DENSITY TO ONES WITH A LOWER ELECTRON DENSITY

THESE ARE USED TO REPRESENT THE MOVEMENT OF ELECTRONS

CURLY ARROWSCURLY ARROWS

ELECTRONS MOVE FROM AREAS OF HIGH ELECTRON DENSITY TO ONES WITH A LOWER ELECTRON DENSITY

For example…from LONE PAIRS

DOUBLE BONDS

THESE ARE USED TO REPRESENT THE MOVEMENT OF ELECTRONS

CURLY ARROWSCURLY ARROWS

ELECTRONS MOVE FROM AREAS OF HIGH ELECTRON DENSITY TO ONES WITH A LOWER ELECTRON DENSITY

For example…from LONE PAIRS

DOUBLE BONDS

to POSITIVE SPECIESTHE END OF POLAR BONDS

THESE ARE USED TO REPRESENT THE MOVEMENT OF ELECTRONS

CURLY ARROWSCURLY ARROWS

THESE ARE USED TO REPRESENT THE MOVEMENT OF ELECTRONS

ARROWS WITH TWO ‘HEADS’ INDICATE THE MOVEMENT OF TWO (A PAIR OF) ELECTRONS

CURLY ARROWSCURLY ARROWS

THESE ARE USED TO REPRESENT THE MOVEMENT OF ELECTRONS

ARROWS WITH TWO ‘HEADS’ INDICATE THE MOVEMENT OF TWO (A PAIR OF) ELECTRONS

CURLY ARROWSCURLY ARROWS

A PAIR of electrons moves from here… to here

THESE ARE USED TO REPRESENT THE MOVEMENT OF ELECTRONS

ARROWS WITH TWO ‘HEADS’ INDICATE THE MOVEMENT OF TWO (A PAIR OF) ELECTRONS

CURLY ARROWSCURLY ARROWS

A PAIR of electrons moves from here… to here

ARROWS WITH ONE ‘HEAD’ INDICATE THE MOVEMENT OF JUST ONE ELECTRON

THESE ARE USED TO REPRESENT THE MOVEMENT OF ELECTRONS

ARROWS WITH TWO ‘HEADS’ INDICATE THE MOVEMENT OF TWO (A PAIR OF) ELECTRONS

CURLY ARROWSCURLY ARROWS

A PAIR of electrons moves from here… to here

ARROWS WITH ONE ‘HEAD’ INDICATE THE MOVEMENT OF JUST ONE ELECTRON

ONE electron moves from here… to here

THESE ARE USED TO REPRESENT THE MOVEMENT OF ELECTRONS

ARROWS WITH TWO ‘HEADS’ INDICATE THE MOVEMENT OF TWO (A PAIR OF) ELECTRONS

CURLY ARROWSCURLY ARROWS

A PAIR of electrons moves from here… to here

ALWAYS BE PRECISE WITH THE POSITIONING OF ANY ARROWSALWAYS BE PRECISE WITH THE POSITIONING OF ANY ARROWS

ARROWS WITH ONE ‘HEAD’ INDICATE THE MOVEMENT OF JUST ONE ELECTRON

ONE electron moves from here… to here

NUCLEOPHILES:- possess a lone pair of electrons

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

NUCLEOPHILES:- possess a lone pair of electrons

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

HYDROXIDE ION

negative charge

H Olone pair

more lone pairs

NUCLEOPHILES:- possess a lone pair of electrons

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

HYDROXIDE ION

negative charge

H Olone pair

more lone pairs

AMMONIA MOLECULE

Nucleophiles don’t need to have negative charge… BUT they must have a lone pair

lone pair

H

H N

H

NUCLEOPHILES:- possess a lone pair of electrons

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

HYDROXIDE ION

negative charge

H Olone pair

more lone pairs

AMMONIA MOLECULE

Nucleophiles don’t need to have negative charge… BUT they must have a lone pair

lone pair

H

H N

H

ELECTROPHILES:- attract a lone pair of electrons

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

ELECTROPHILES:- attract a lone pair of electrons

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

HYDROGEN ION HThere are no electrons in the outer shell of hydrogen so it has space to accept two electrons

ELECTROPHILES:- attract a lone pair of electrons

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

HYDROGEN ION HThere are no electrons in the outer shell of hydrogen so it has space to accept two electrons

HYDROGEN CHLORIDE

contains a POLAR BOND; the end will attract the electron pair H Cl

ELECTROPHILES:- attract a lone pair of electrons

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

HYDROGEN ION HThere are no electrons in the outer shell of hydrogen so it has space to accept two electrons

HYDROGEN CHLORIDE

contains a POLAR BOND; the end will attract the electron pair H Cl

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

‘BALANCING THE BOOKS’

H O H C C Br

H

3

H

H C C O H

H

3

H

Br

This is the basic mechanism for the nucleophilic substitution of bromoethane.

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

‘BALANCING THE BOOKS’

H O H C C Br

H

3

H

H C C O H

H

3

H

Br

This is the basic mechanism for the nucleophilic substitution of bromoethane. To see how it works, it helps to show the electrons involved.

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

‘BALANCING THE BOOKS’

H O H C C Br

H

3

H

H C C O H

H

3

H

Br

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

‘BALANCING THE BOOKS’

H O

The hydroxide ion has a – ive charge, The oxygen has eight electrons in its outer shell. Because it has a lone pair, the ion will act as a nucleophile

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

‘BALANCING THE BOOKS’

H O H C C Br

H

3

HIn bromoethane, C2H5Br, the C-Br bond is polar because the electronegativity of Br is greater than C. It will susceptible to attack by nucleophiles.

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

‘BALANCING THE BOOKS’

H O H C C Br

H

3

HA pair of electrons moves from th hydroxide ion to the slightly positive carbon in bromoethane.

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

‘BALANCING THE BOOKS’

H O H C C Br

H

3

HCarbon has a full outer shell of eight electrons so as two come in, two must leave.

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

‘BALANCING THE BOOKS’

H O H C C Br

H

3

HThe C-Br bond breaks as the shared pair of electrons move onto the bromine.

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

‘BALANCING THE BOOKS’

H O H C C Br

H

3

H

H C C O H

H

3

H

Br

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

‘BALANCING THE BOOKS’

H O H C C Br

H

3

H

H C C O H

H

3

H

Br

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

‘BALANCING THE BOOKS’

H O H C C Br

H

3

H

H C C O H

H

3

H

Br

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

more lone pairs‘BALANCING THE BOOKS’

H O H C C Br

H

3

H

H C C O H

H

3

H

Br

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

more lone pairs‘BALANCING THE BOOKS’

H O H C C Br

H

3

H

H C C O H

H

3

H

Br

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

more lone pairs‘BALANCING THE BOOKS’

H O H C C Br

H

3

H

H C C O H

H

3

H

Br

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

NUCLEOPHILIC SUBSTITUTIONNUCLEOPHILIC SUBSTITUTION

negative charge

H O

lone pair

more lone pairs

‘BALANCING THE BOOKS’

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

more lone pairs‘BALANCING THE BOOKS’

H O H C C Br

H

3

H

H C C O H

H

3

H

Br

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

negative charge

lone pairmore lone pairs

‘BALANCING THE BOOKS’

H O H C C Br

H

3

H

H C C O H

H

3

H

Br

DRAWING CURLY ARROWSDRAWING CURLY ARROWS

When moving electrons about, it is essential to check that the charges on the reactants and products balance.

NUCLEOPHILIC SUBSTITUTIONNUCLEOPHILIC SUBSTITUTION

negative charge

H O

lone pair

more lone pairs

‘BALANCING THE BOOKS’

© 2008 JONATHAN HOPTON & KNOCKHARDY PUBLISHING© 2008 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

THE ENDTHE END

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