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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