ADDITION TO CARBON-­CARBON

MULTIPLE BONDS

(Part-I)

Introduction

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Two ways of Addition based on the Substrate given as follows

Introduction

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• Four ways of Addition based on the Reagents

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First three are two­step processes and the fourth one is a one­step process.

Electrophilic Addition

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• Electrophilic Reagent approaches the double or triple bond substrate to give positive intermediate

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• The positive intermediate is attacked by t he nucleophilic species

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• This reaction may be uni­ , bi­ or ter­molecular

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Electrophilic Addition

Electrophilic Addition

• The stereochemistry of the addition reaction may be

e ither 'syn' or 'anti' type

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• Mostly open­chain intermediates favours 'syn' addition

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• Cyclic intermediate favours more preferentially the 'anti' addition

Electrophilic Addition

Electrophilic Addition

Electrophilic Addition

Electrophilic Addition

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Year 1911: McKenzie and Fischer independently proved the anti­addition in case of cyclic intermediate formation

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Addition bromine to maleic acid and fumaric acid to give dl­pair and meso forms of ­ 2,3­dibromosuccinic acid, respectively

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Later, chemists proved the formation of 70% of trans­ isomer when dicarboxyacetylene was brominated

Electrophilic Addition

Electrophilic Addition

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It has been found that the addition of bromine is not stereospecifically favouring anti­addition product

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The anti addition depends on the polarity of the solvent

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Solvent of low dielectric favours 90­100% anti­addition

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Solvent of high dielectric favours nonstereospecific addtion

ADDITION TO CARBON­CARBON

MULTIPLE BONDS.....

Electrophilic Addition

Electrophilic Addition

• Nucleophilic Reagent approaches the double or tripple bond substrate to give negative intermediate

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• The positive intermediate is attacked by the electrophilic species

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• This addition reaction is favoured when the substrate has an electron withdrawing group

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• Let us look at the mechanisms

Nucleophilic Addition

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Nucleophilic Addition

Note:

When the substrate contains a good leaving group substitution reaction is more favoured

When electron­withdrawing group is present in the

substrate, addition reaction is more favoured.

Nucleophilic Addition

Nucleophilic Addition

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• Michael reaction is a type of nucleophilic addition

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• This Michael type of reaction can be reversed by heating either with or without alkali

Dr. V Alex Ramani SJC, Trichy 2

Nucleophilic Addition

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• The nucleophilic addition is nonstereospecific and stereoselective

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• The products are formed based on the favourable conformation, assumed by the carbanion

Nucleophilic Addition

• The nucleophilic addition of some reagents – EtOD or Me₃CSD to trans­ethyl but­2­enoate shows anti­

addition product predominantly

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• Nucleophilic addition to C­C triple bond s y s te m s mostly yield anti­addition product as that of electrophlic addition

Free Radical Addition

• Free Radical addition is a type of chain reaction

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• There are three stages – initiation, propagation and termination of the reaction

Free Radical Addition

• Free Radical addition is nonstereospecific

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• But, these reactions are mostly stereoselective. Let us look at some examples

Free Radical Addition

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^ Free Radical addition is becoming stereospecific at low temperature in some cases

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The stereospecificity is disappeared at room tempt.

The addition products are 78% d,l­ and 22% of meso­ isomes

^ It is due to the formation of bridged intermediate

Cyclic Mechanisms

^ Some addition reactions go via four­centre, five­ centre or six­center intermediate mechanisms showing a closed ring intermediate / transition state

Addition to Conjugated Systems

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• Addition to conjugated double bonded systems s h o ws 1,2­addition and 1,4­addition

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• The addition reaction goes via open­chain or c y c l ic intermediate
• Sometimes the reaction is a concerted one

Addition to Conjugated Systems

Mechanism

• Electrophilic addition via open­ chain intermediate

Addition to Conjugated Systems

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^ Potential energy diagram of the 1,2­ and 1,4­ addition reactions

Addition to Conjugated Systems

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^ Formation of some uncommon intermediates du r i ng the addition mechanism

Questions!!!!