Lecture14outline

Lecture14outline - Acetals are not formed under basic...

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I. Review • Aldehydes and ketones react with water in an equilibrium process to form hydrates • The extent to which this equilibrium favors the hydrate is dependent upon the ability of the groups bonded to the carbonyl to stabilize the partial positive charge on carbon • Electron withdrawing groups strongly favor hydrate formation while alkyl groups and other electron donating groups strongly disfavor hydrate formation • The propensity of a carbonyl to undergo hydration parallels the reactivity of the carbonyl toward reactions with all nucleophiles • Acid increases the rate of hydration because protonation of the carbonyl enhances its electrophilicity • Base increases that rate of hydration because hydroxide is more nucleophilic than water • Alcohols react with aldehydes and ketones to form hemiacetals, which are alkylated analogs of hydrates • In the presence of acid hemiacetals react further to yield oxocarbenium ions (oxygen- stabilized carbocations) • Oxocarbenium ions react with alcohols to form acetals
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Unformatted text preview: Acetals are not formed under basic conditions because the hemiacetal cannot break down to form an electrophile in the absence of acid Acetals and aldehydes or ketones are almost identical with respect to thermodynamic stability, so reaction conditions must be selected carefully to prepare one or the other Removal of water is necessary for efficient acetal formation Since acetals are ethers, they are inert toward nucleophilic addition reactions Acetals can be readily converted back to ketones and aldehydes with acid and water, making them excellent protecting groups Alcohol sequestration is useful for acetal hydrolysis The mechanism of acetal cleavage can be deduced by applying the principle of microscopic reversibility to the formation mechanism II. More reactions of ketones and aldehydes The Baeyer-Villiger reaction Imine and enamine formation, and the Wolff-Kischner reduction Sulfur chemistry...
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