Racemisation and Enantiomerization

The rates of racemisation of and /+ ions differ considerably when racemisation takes place in the presence of a second chiral species d (or d+). This was first observed by Ray and Dutt (1941, 1943) who studied (+) and (—)-tris biguanidi-nium cobaltchloride in the presence of (+) tartrate ion. Later Dwyer and Davies (1954) found that the rates of racemisation of (+) and ( —) [Ni(o-phen)3]Cl2 differed in the presence of (+) bromocamphorsulphonate ion (d ) and in that of the cationic species (-J-) cinchoninium ions (d+). 

The racemisations terminated in equilibria in which one component was in excess over the other with equihbrium constants different for the two chiral media. From the temperature dependence of the equilibrium constants they found the differences of heat contents [AH =. Q and 1.8 kj mol i) and Gibbs standard free energies (dG°=0.30 and 0.34 kJ mol-i) respectively for bromocamphorsulphonate and cinchomium ions as the added chiral species. 

The same equilibria are attained if to a solution of racemate in an initially achiral medium (equal concentrations of d and I species) there is added another chiral species d or L. The equilibrium is then displaced in favour of one or other of the constituents of the racemic mixture. This process has recently been termed enantiomerization, although examples of optically labile systems in equilibria sensitive to the presence of other chiral molecules or ions have long been recognized. A typical example of what was earlier termed an asymmetric transformation of the first kind (no second-phase involved) is that of Read and McMath (1925) in which solutions in dry acetone of (—) or ( ) chlorobromomethanesulphonic acid d-l ) together with (—)-hydroxyhydrindamine (l+) showed a change of optical rotation interpreted in terms of an equilibrium 

Later work on enantiomerization by Bosnich and Watts (1975) depends upon equilibrium constant measurements of [Ni(phen)3]Cl2 in (—)-2,3-butanediol in the temperature range 277—373 K. The equilibrium constants as a function of temperature give standard enthalpies and entropies AH° =—495 J mol i and =—1.17 J mol i, with the —) form of the cation the more stable in the (—) solvent. In kinetic terms the (-)-) ion inverts faster than the (—) ion. In 

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