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Chapter 20

CHEM267 Chapter Notes - Chapter 20: Nucleophilic Addition, Grignard Reaction, Steric Effects

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Monica Barra

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Chem 267 Chapter 20 Notes Winter 2012
20.4 Introduction to Nucleophilic Addition Reactions
The carbon of a carbonyl group is especially electrophilic due to both resonance and inductive effects
(both resultling in a positive charge on C); making it susceptible to nucleophilic attack
RULE: nucleophilic attack occurs at an angle of 10 to the plane of the carbonyl group
Nucleophilic attack forms a tetrahedral intermediate
RULE: aldehydes are more reactive than ketones to nucleophilic attack
The nucleophilic attack of a carbonyl group has 2 steps: nucleophilic attack and proton transfer
o Basic conditions = nucleophile attacks the carbon, the double bond moves up to the oxygen
(giving O a ve charge), and the oxygen is protonated
o Acidic conditions = oxygen is first protonated (giving O a +ve charge), nucleophile attacks the
carbon, then the double bond moves up to the oxygen
NOTE: Equilibrium of a nucleophilic addition reaction is dependent on the ability of the nucleophile to
function as a leaving group
o Ex. a Grignard reagent is a good nucleophile, but not a good leaving group; therefore,
equilibrium will greatly favour the productions (occur in 1 direction)
o Ex. a halide is a good nucleophile and leaving group; equilibrium favours the reverse rxn
20.5 Oxygen Nucleophiles
Hydrate Formation
When an aldehyde or ketone is treated with water, the carbonyl group is converted to a hydrate
Equilibrium favours the carbonyl group rather than the hydrate (except for simple aldehydes)
Reaction is relatively slow under neutral conditions, but enhanced in the presence of acid or base
o Base catalyzed = nucleophilic attack on C (from base), then protonation of O
Negatively charged intermediate is formed (-ve charge on O)
o Acid catalyzed = protonation of O (from acid), nucleophilic attack on C, deprotonation
Positively charged intermediate is formed (+ve charge on O)
Acetal Formation
When an alcohol attacks an aldehyde or ketone, an acetal is formed
Acidic conditions = aldehyde/ketone will react with 2 molecules of alcohol to form acetal
o Common acids used = p-Toluenesulphonic acid (TsOH) and sulfuric acid (H2SO4)
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