Thionyl chloride reaction with malic acid

At this point, it is desirable for us to see just how it was originally proved that a given substitution reaction proceeds with inversion of configuration, even before the mechanism was known. Walden presented a number of examples in which inversion must have taken place. For example, (+)-malic acid could be converted to ( + )-chlorosuccinic acid by thionyl chloride and to (— )-chlorosuccinic acid by phosphorus pentachloride ... 

The student may wonder just what the mechanism is in cases where retention of configuration is involved since it certainly is not simple Sn2. As we shall see later, the reaction between malic acid and thionyl chloride is an SNi process (p. 420), while a neighhoring-group mechanism (p. 404) is involved in the treatment of chlorosuccinic acid with silver oxide. 

Once again, one reaction and only one must be an inversion, but which7" It may also be noticed [illustrated by the use of thionyl chloride on (+)-malic acid and treatment of the product with KOH] that it is possible to convert an optically active compound into its enantiomer." ... 

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