Integrated SequencePhysics Chemistry Organic Biology

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Special points of emphasis

Stereochemistry

Reactions of Alkyl Halides

Reactions of Carboxylic Acids and Derivatives

Nuclear Physics

An MCAT cliché are questions designed to see if you can distinguish between proposed mechanisms for hydrolysis. Problems often refer indirectly to the original research, in which, to determine the likely mechanism, researchers labeled the alkoxy portion of an ester with 18O. If hydrolysis occurred by an SN2 process, the carboxylate group would leave with the 18O, and the alcohol portion of the reaction products would be lack radioactivity. On the contrary, however, it was found that the alcohol portion did contain the radioactivity, so the acyl substitution mechanism was established as the most likely one. Further support is given by the fact that the reaction occurs with retention of configuration in the alcohol, rather than inversion of configuration, which would occur through an SN2 process. On my MCAT there was a passage looking for me to interpret similar experimental results in a biochemical context. The key to the passage was understanding the acyl transfer mechanism.



Chemical Thermodynamics and the Equilibrium State

Chemical Kinetics

Acids and Bases

Reactions of Carboxylic Acids and Derivatives

Lipids

Ester hydrolysis is promoted both by acid and basic conditions. Acid conditions catalytically enhance the reaction with the addition of a proton to the carbonyl oxygen of the ester group. This enhances the reactivity of the carbonyl group to nucleophilic attack by increasing its electron with-drawing character. Acid catalysis helps a subsequent step as well, with the addition of a second proton to increase the facility of the alkoxy portion to leave as an alcohol.

Alternately, basic conditions may also promote ester hydrolysis. Base promoted ester hydrolysis is also called saponification, occurring with the hydroxide anion acting as the nucleophile. Saponification can be used to produce fatty acid salts (soap) from triglyceride.








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