Wallach s Rule

C.P. Brock, W.B. Schweizer, J.D. Dunitz, On the validity of Wallach s rule on the density and stability of racemic crystals compared with their chiral counterparts, J. Am. Chem. Soc. 113 (1991) 9811-9820. 

The close-packing criterion for crystal stability generally yields lower free energies for a structure composed of a racemic assembly over that composed of homochiral molecules. This view has been expressed in the empirical rule of Wallach [14], which states that the combination of two opposite enantiomers to form a racemate is accompanied by a volume contraction. The many exceptions to this rule have been discussed in a systematic manner [15]. The modification to Wallach s rule contributed by Walden [16] is more generally valid it states that if an enantiomer has a lower melting point than its corresponding racemate, then the crystals of the latter will have the higher density. 

Perfection of Wallach s isoprene rule for terpinoids Robinson3... 

At the turn of the twentieth century, structural investigations of many terpenoids led Otto Wallach to formulate the isoprene rule, which postulated that most terpenoids could be constructed hypothetically by repetitively joining isoprene units. This principle provided the first conceptual framework for a common structural relationship among terpenoid natural products. Wallach s idea was refined in... 

This chapter contains a discussion of possible ways of separating lipophilic mixtures of natural products, the components of which are built up from isoprene units in accordance with Ruzicka s isoprene rule [219]. Wallach [321] has divided the hydrocarbons and their derivatives into the following groups ... 

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