8B_quiz7-key - Chem 8B Practice Questions related to Quiz#7...

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Chem 8B, Practice Questions related to Quiz #7 - KEY – February 27, 2008 1. a) Both an aldehyde and an ester are considered to be an electrophilic carbonyl compound. Provide a brief explanation why an ester will undergo a substitution reaction, but an aldehyde will not. R H O R OCH 3 O aldehyde ester No leaving group; aldehyde will only undergo an addition reaction Has an electronegative oxygen as a leaving group so an ester can undergo a substitution reaction, such as hydrolysis b) For the pair of compounds shown below, indicate (circle) which compound would have a faster hydrolysis rate (reaction of carbonyl electrophile with water as a nucleophile – an example of a nucleophilic acyl substitution reaction). Provide a brief explanation for your answer. O MeO or CH 3 O O CH 3 O The methoxy (OMe) group is an electron-donating group that donates electrons into the benzene ring. This electron-donating effect will make the oxygen of the ester share its electrons more with the carbonyl, thus making the carbonyl less electrophilic. (In general, electron-donating effects will decrease electrophilicity and electron-withdrawing effects will increase electrophilicity) the molecule that is more electrophilic will have a faster hydrolysis rate (reaction with a water nucleophile). c) Draw a curved-arrow mechanism for the base-catalyzed hydrolysis reaction of an ester and show what two products result from this reaction.
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