organicsynthesis knockhardy publishing 2008 specifications

ORGANICORGANIC

SYNTHESISSYNTHESIS

KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING2008 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 with an interactive white board.

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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ORGANIC SYNTHESISORGANIC SYNTHESIS

CONTENTS• Introduction

• Functional groups

• Chiral synthesis - introduction

• Nucleophilic addition

• Nucleophilic substitution

• Synthetic methods



Involves the preparation of new compounds from others.



Many industrial processes involve a multi stage process where functional groups are converted into other functional groups.




When planning a synthetic route, chemists must consider...





• the reagents required to convert one functional group into another

• the presence of other functional groups - in case also they react







• the conditions required - temperature, pressure, catalyst

• the rate of the reaction

• the yield - especially important for equilibrium reactions

• atom economy







• the conditions required - temperature, pressure, catalyst

• the rate of the reaction

• the yield - especially important for equilibrium reactions

• atom economy

• safety - toxicity and flammability of reactants and products

• financial economy - cost of chemicals, demand for product

• problems of purification

• possibility of optically active products


Functional groupsFunctional groups

Common functional groups found in organic molecules include...

alkene

hydroxyl (alcohols)

halogenoalkane

carbonyl (aldehydes & ketones)

amine

nitrile

carboxylic acid

ester

CHIRAL SYNTHESISCHIRAL SYNTHESIS

Rationale

Pharmaceutical synthesis often requires the production of just one optical isomer. This is because...


Rationale


• one optical isomer usually works better than the other

• the other optical isomer may cause dangerous side effects

• laboratory reactions usually produce both optical isomers

• naturally occurring reactions usually produce just one optical isomer


Rationale


• one optical isomer usually works better than the other

• the other optical isomer may cause dangerous side effects

• laboratory reactions usually produce both optical isomers

• naturally occurring reactions usually produce just one optical isomer

Example Aldehydes and ketones undergo nucleophilic addition with cyanide (nitrile) ions;

CH3CHO + HCN ——> CH3CH(OH)CNethanal 2-hydroxypropanenitrile


Example

Aldehydes and ketones undergo nucleophilic addition with cyanide ions



Example



Problem - the C=O bond is planar

- the nucleophile can attack from above and below

- there is an equal chance of each possibility

- a mixture of optically active isomers is produced- only occurs if different groups are attached to the C=O


Example







CN¯ attacksfrom above


Example







CN¯ attacksfrom below


Example CH3CHO + HCN ——> CH3CH(OH)CNethanal 2-hydroxypropanenitrile



MIRROR IMAGES



ANIMATIONANIMATION


Consequences • isomers have to be separated to obtain the effective one

• separation can be expensive and complicated

• non-separation leads to





larger doses needed

possible dangerous side effects

possible legal action





larger doses needed



Solution • use natural chiral molecules as starting materials

• use stereoselective reactions which give one isomer

• use catalysts which give a specific isomer

• use enzymes or bacteria which are stereoselective





larger doses needed



Solution • use natural chiral molecules as starting materials

• use stereoselective reactions which give one isomer

• use catalysts which give a specific isomer

• use enzymes or bacteria which are stereoselective

Other examples Nucleophilic substitution of halogenoalkanes

NUCLEOPHILIC SUBSTITUTIONNUCLEOPHILIC SUBSTITUTION

Problems There are two possible mechanisms

SN2

This produces just one optical isomer with reversed optical activityCalled SN2 because two species are involved in the rate determining step.



SN1

This produces a racemic mixture of two optical isomersCalled SN1 because one species is involved in the rate determining step.



SN2

This produces just one optical isomer with reversed optical activityCalled SN2 because two species are involved in the rate determining step.

SN1

This produces a racemic mixture of two optical isomersCalled SN1 because one species is involved in the rate determining step.

MODERN SYNTHETIC METHODSMODERN SYNTHETIC METHODS

The following methods can be used to synthesise a single optical isomer

Enzymes / bacteriaEnzymes / bacteria

Chiral chemicalsChiral chemicals

Chiral catalystsChiral catalysts

Natural chiral moleculesNatural chiral molecules

©2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING©2009 JONATHAN HOPTON & KNOCKHARDY PUBLISHING

THE ENDTHE END

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organicsynthesis knockhardy publishing 2008 specifications

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