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Chapter 20

CHEM 267: Chapter 20 Textbook Notes

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University of Waterloo
CHEM 267
Monica Barra

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 107° 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 (2 S4 ) 1  NOTE: in the presence of acid, the carbonyl group is protonated, making the carbon even more electrophilic; this is necessary because the nucleophile (alcohol) is weak + +  NOTE: the H–A that protonates the initial carbonyl group is most likely a protonated alcohol (R2H ) nd  :A in the 2 proton transfer step is most likely a molecule of alcohol present in solution  The proton (H) is used in step 1 and returned to the alcohol in step 3 (catalytic nature)  In the formation of acetal, the loss of LG and nucleophilic attack do not happen at the same time! o It is anNS 1 reaction, not Nn S 2 (too much steric hindrance)  The formation of the hemiacetal and acetal both begin and end with a proton transfer!  For simple aldehydes, equilibrium favours formation of the acetal  For ketones, the equilibrium favours reactants rather than products o Le Chatelier’s Principle = formation of acetal can be accomplished by removing one of the products (water)  Notice that 2 equivalents of alcohol are required for acetal formation o Alternative: a compound containing 2 OH groups can be used, forming a cyclic acetal Acetals as Protecting Groups  Acetal formation is a reversible process  RULE: acetal formation is favoured by removing water o To convert acetal back to aldehyde/ketone, add water in the presence of an acid catalyst  If you only want 1 substituent to be reduced (ex. ester to alcohol), but not the other substituent (ex. ketone), a protecting group can be
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