438 - Practice Problem 4‐38 – Answer Draw a structural...

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Unformatted text preview: Practice Problem 4‐38 – Answer Draw a structural formula for the principal product formed when benzoyl chloride is treated with each reagent. O O Cl O O O Cl O O O Cl S O O Cl N H O O O Cl O O OCH3 O Cl N H Practice Problem 4‐39 – Answer Show the product of treating this anhydride with each reagent. Practice Problem 4‐40 – Answer Predict the distribution of oxygen‐18 in the products obtained from hydrolysis of ethyl benzoate labeled in the ethoxy oxygen under the following conditions. (a) In aqueous NaOH (b) In aqueous HCl Under both basic (a) and acidic (b) reaction conditions, the oxygen‐18 will appear in the ethanol product. Both base‐mediated and acid‐ catalyzed ester hydrolysis involve a tetrahedral carbon addition intermediate that decomposes with loss of ethoxide (a) or ethanol (b). Either way, the oxygen‐18 ends up in the ethanol product. WRITE MECHANISMS ON YOUR OWN TO CONVINCE YOURSELF THAT THIS IS TRUE!!! (c) What distribution would you predict if the reaction were done with the tert‐butyl ester in HCl? The oxygen‐18 will end up in the carboxylic acid product, as demonstrated in the following reaction mechanism. The protonated ester cleaves to produce a carboxylic acid (containing O‐18) and a tert‐butyl cation (which ultimately reacts with H2O to yield tert‐butyl alcohol that does not contain the O‐18 label). Practice Problem 4‐41 – Answer Propose a curved arrow mechanism for the overall transformation shown below (see the course packet for the overall transformation): Practice Problem 4‐42 – Answer The following reactivity order has been found for the saponification of alkyl acetates by aqueous NaOH. Explain. CH3CO2CH3 > CH3CO2CH2CH3 > CH3CO2CH(CH3)2 > CH3CO2C(CH3)3 As steric bulk increases in the alkoxy group, the ability of the hydroxide anion to approach the carbonyl carbon atom is hindered. As a consequence, the nucleophilic acyl substitution reaction becomes more difficult to achieve, and the reaction is disfavored. ...
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