1. The inversion of configuration in an S_N2 reaction is often called a Walden inversion, named after its discoverer, Paul Walden. In the cycle shown above, the overall conversion of one enantiomer of malic acid to the other one must require an inversion of configuration. Similarly, the same is true of the chloro acids. More generally, each interconversion of enantiomers must require an odd number of inversions. The PCl₅ reaction requires a single inversion which means that the Ag₂O reaction involves an even number of inversions of configuration, namely two in this instance. (-)-Malic acid is of the (S)-configuration.

a) Show how malic acid, like any alcohol, might react with PCl₅ and then undergo inversion to form a chloride. Remember that phosphoric acid is a strong acid and its conjugate base and analogs thereof are also good leaving groups.

b) Silver oxide is an anhydrous form of AgOH. The carboxylic acid group closest to the hydroxyl group plays a role in the process. The reaction medium is mildly alkaline. Using these data, show how there is net retention of configuration.

c) Draw these four enantiomers as Fischer projections with the CO₂H closest to the OH or Cl in the topmost position. (-)-Malic acid is of the (S)-configuration.