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36 GENERAL AND PHYSICO-CHEMICAL.
ions, which transform benzaldehyde into benzoin according to the following
equation:
2C6 H5 .COH =C6H6 .CO.CH(OH).C6H5. 1
If those bodies which accelerate a reaction are to be considered as catalysts,
then certainly the solvents must belong to the catalytes. Attention must be
called to the enormous influence which the solvent can exert upon the velocity
of a reaction under otherwise equal conditions. Thus Menschutkin 2
found for
the reaction
(C2H5) 3 .N+C2H5 .I = (CJBi)4.N.I.f
the following velocity in different solvents:
Hexane 0.00018
Heptane .000235
Xylene 0.00287
Benzene 0.00584
Ethyl alcohol 0.03660
Benzyl alcohol 0. 13300
Recently Bredig and Fajans 3
have been able to show that an optically active
solvent can help in the decomposition of optical antipodes to a varying extent.
Of the optical antipodes of campho-carboxylic acid, the d-form is 17 per cent
more quickly decomposed than the Z-form, when they are dissolved in nicotine
or when nicotine is present, dissolved with the catalyte, while in an optically
indifferent solvent and without any nicotine the catalyte decomposes both forms
with equal rapidity. The reaction proceeds differently with or without catalyst,
and hence the catalyst brings about changes in reaction other than those of velocity.
It is apparent that this does not conform with Ostwald’s definition of a catalyst
(page 33). It must be mentioned that Bredig and Fiske have been able to
perform the asymmetric synthesis of benzaldehyde and hydrocyanic acid by means
of quinine and quinidine as catalysts (page 60).
Catalysis in Heterogeneous Systems. The above-treated catalytic
processes all occur in homogenous systems, i.e., the systems which by
mechanical means cannot be separated into different constituents. In
heterogeneous systems with phases which can be separated from each
other by mechanical means, catalytic reactions can also occur, and
indeed, in such cases the substances taking part [in the reaction and
the catalyst occur in different phases. Such a reaction is the union of
detonating gas, the synthesis of SO3 (from SO2 and O), and the decom-
position of H2O2 by platinum. In case the system is two-phased, and
the reaction takes place only at the boundary between both phases, or
in the one we can differentiate two simple limits:
1. The accumulation of the bodies which are necessary for the
reaction at the proper place takes such a short time that in comparison
1
Stern, ibid., 50, 513 (1905).
2
Ibid., 6, 41 (1890).
8
Ber. d. d. chem. Gesellach., 41, 752 (1908).
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