reasoning that a "vanishingly small" heterochiral dimer concentration would lead to (2•PR•PS) ~ 0. This clearly incorrect assumption renders the kinetic model invalid. Thus the proposal that monomers dominate over dimers is supported neither by the experimental data nor the kinetic model. Another model invoking monomers as the active catalyst was developed by authors who did not carry out experimental work but built a mechanistic model based solely on a fit to a single kinetic profile taken from an NMR study by Brown and coworkers. Brown was investigating symmetry breaking in the absence of added product as catalyst. The background reaction forms product that goes on to autocatalyze further turnovers. While racemic product is expected in the absence of an apparent chiral source, often non-zero enantiomeric excesses have been observed in such cases, leading to studies aimed at understanding factors that control how the system may be "tipped" to cause symmetry breaking. It is a challenge for any kinetic model to impose accurate conditions for the autocatalyst concentration in the absence of a defined chiral source in this autocatalytic system, because the direction and extent of the initial symmetry breaking is notoriously susceptible to unmeasurable phenomena including the presence of cryptochiral impurities. Nevertheless, the authors of Ref. 41a aimed to model both symmetry breaking and autocatalysis simultaneously, fitting the single data set shown in Figure to the monomer model of Scheme 8, which contains six adjustable parameters. The fitted parameters allowed calculation of the dimerization equilibria constants shown in Scheme 7. The results of their fit are given in Figure . This model raises a number of serious concerns because of its glaring inconsistencies with experimental data. While the kinetic model was built entirely from fitting to the single data set shown in part b) of Figure , the same set of kinetic rate constants clearly fails to describe a second data set from the same publication, where the identical reaction was carried out using different initial substrate concentrations. It must be emphasized that for a model to have chemical meaning, the same elementary step rate constants must be able to describe data sets from reactions carried out under different conditions, because kinetic constants are not concentration-dependent. It is equally critical to note that this proposed monomer model does not accurately predict the experimentally measured ee value found at the end of the reaction under either set of reaction conditions. Further, the kinetic parameters returned a value of Kdimer These authors also stated that they were able to reproduce the 1:1 ratio of heterochiral:homochiral dimers that was found from experimental NMR studies. This claim is deceptive in that the 1:1 ratio observed spectroscopically in the Blackmond/Brown model was for the case of racemic product mixtures, while the monomer model simulation of Ref. 41b