ADDITION TO CARBON­CARBON MULTIPLE BONDS

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(Part-2)

Addition to Conjugated Systems (Nucleophilic addition )

Nucleophilic addition to conjugated systems follows the mechanism that resemble the electrophilic addition mechanism

Addition to Conjugated Systems

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Free ­radical addition to conjugated systems follow the mechanism that resemble the electrophilic addition mechanism

Theory of Reactivity of Addition

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The reactivity of the substrate towards addition reaction is influenced by the nature of substituent present

The effect of substituent is so great that there is a generalization as given here.

simple olefines do not react by nucleophilic mechanism, and polyhalo or polycyano olefine do not react by electrophilic mechanism

The reactivity of the substrate towards electrophilic addition reaction is shown with an example

Bromination

Hydration

The reactivity of alkenic and alkynic substrates:

The alkenes more preferred to show electrophilic addition and the alkynes show nucleophilic addition more readily.

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The reactivity of alkenic and alkynic substrates:

• The Free radical additions can occur in any type of substrate
• The nucleophilic radicals behave like nucleophiles and the rate is enhanced in presence of electron withdrawing groups
• The reverse is true for electrophilic radicals
• The deciding factor is the presence of a radical initiator(I_f)

The reactivity of alkenic and alkynic substrates:

Steric influences have profound effect on the reactivity of these multiple bonded substrates

• When unsymmetrical alkenes or alkynes undergo addition, the electrophilic part of the reagent goes to the carbon having more number of hydrogens
• This observation was proposed as a rule- Markovnikov's rule. This addition takes place favouring the formation of a better stabilised intermediate

Orientation of Addition

The orientation of reagents in alkenic and alk y nic substrates have been observed.

• Presence of electron donating substituents in the substrate favours Markovnikov's addition
• Presence of electron withdrawing substituents in the substrate favours anti­Markovnikov's addition

Orientation of Addition

The orientation of reagents in alkenic and alkynic substrates have been observed.

• The nucleophilic attack is studied to a very little extent and it is done only in Michael type of addition
• Presence of electron withdrawing substituents in the substrate favours nucleophilic attack

Orientation of Addition

The orientation of reagents in alkenic and al k yn ic substrates:

Orientation of Addition

The orientation of reagents in alkenic and alky n i c substrates:

• The free­radical attack is influenced very much by the steric factor
• The inductive effect has less impact on this reaction

Stability order of the free radical: 3^o > 2^o > 1^o

Orientation of Addition

The orientation in alkenic substrates have been observed to be intramolecular

• Cyclic products are preferred and Smaller ring types are more favoured than big rings
• It obeying the Baldwin's rule – Exo­Trig addition is more preferred than the Endo­Trig addition

Orientation of Addition

The orientation in conjugated alkenic substrates have been observed to be at the tail­end part

• The attack by the electrophile, nucleophile or free radical will be at the terminal carbon of the conjugated system
• It favours the formation of a stable intermediate

• The attack by the electrophile will be at the middle carbon of the allenic system
• It favours the formation of a stable intermediate, though it is an anti­Markovnikov's addition.

Orientation of Addition

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The orientation in allenic substrates have been observed to be at the middle carbon

Stereochemistry of Orientation

• For cyclic compounds, the addition may be syn or anti
• In syn addition to unsymmetrical cyclic olefins, the two groups approach from the less­hindered side.

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Stereochemistry of orientation

• For cyclic compounds, the addition may be syn or anti for both electrophilic and nucleophilic reagents
• In syn­face addition to unsymmetrical cyclic olefins, the groups approach from the side of electron­ withdrawing group.
• In anti­face addition, the groups approach from the side and opposite of electron­donating group

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Stereochemistry of Orientation

The addition of Br₂ or HOBr are often anti­ because of the formation of cyclic intermediate

Birch Reduction

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Aromatic rings undergo reduction showing 1,4­ addition of hydrogen to give nonconjugated dienes

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It follows radical ion mechanism

Electron donating substituents decrease the rate of the reaction. Electron withdrawing groups increase the rate of the reaction.

The radical ion is proved by the epr studies

Birch Reduction

Aromatic rings undergo reduction showing 1,4­ addition of hydrogen to give non­conjugated dienes

• Ordinary olefines do not undergo this reduction
• Phenylated olefines, internal alkynes and conjugated olefines show this Birch Reduction.
• Benzene shows this reduction to give 1,4­-diene and not 1,3­-diene.

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Hydroboration

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Olefines on hydrobortion give alcohols, ultimately

• Borane always goes to carbon having more number of hydrogens.
• It follows markovnikov's addition. Boron is more electropositive than hydrogen

Hydroboration

Olefines on hydrobortion give alcohols, ultimately

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Borane ­ BH₃ or its substituted derivative – BHR₂ attacks the olefinic carbon that is sterically less crowded

Hydroboration

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Olefines on hydrobortion give alcohols, ultimately

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Enantioselective hydroboration using diisopinocampheylborane

Michael Reaction

Olefines undergo the following addition

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It is a nucleophilic addition

Olefines must have an electron withdrawing group. Otherwise, this reaction is not possible

The nucleophilic reagent also has the electron withdrawing substituents like ­COOR, C=O, CN, NO₂,

S=O, etc.

The reaction is carried out with malonates, cyanoacetates, acetoacetates, other β­ketoesters, etc.

Michael Reaction

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Michael reaction has been carried out traditionally in protic solvents, with catalytic amount of a base.

• Michael reaction has been shown by some alkynic substrates having an electron­withdrawing group
• In some cases, the Michael reaction is possible under acidic conditions

Michael Reaction

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Alkynes undergo the following addition

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