Entering group effect

Entering group effect platinum complexes substitution reactions, 494 EXAFS spectroscopy cadmium complexes, 929 copper(II) complexes, 720 zinc complexes, 929... 

Some typical values of /ip, are Cl" (3.04) NH3 (3.07) Nj" (3.58), r (5.46) CN (7.14) PPhj (8.93). Clearly, if one is testing for entering group effects in Pt(II) chemistry, one should not choose Cl and NH3 as test nucleophiles because their nucleophilicities are almost identical and one would not see much entering or leaving group effect This scale wotks well for Pt(II) reactions, but is at best a qualitative indicator for other metals and not even that for the fust-row transition-metal ions. 

The reactions of Ni(II) are slower than those of the other M(II) ions in this group, and this metal ion has been studied most extensively because the rates fall conveniently in the range of the stopped-flow technique. The substitution kinetic results on Ni(OHj)5 have been reviewed by Wilkins. Some representative results are given in Table 3.20. The first impression of these data is that there are significant entering group effects on the rate constant, so that the mechanism appears to be I,. However,... 

Steric effects play a major role in determining the ortho para ratio in Friedel-Crafts alkylations. The amount of ortho substitution of toluene decreases as the size of the entering alkyl group increases along the series methyl, ethyl, /-propyl. No ortho product is found when the entering group is /-butyl. ... 

The position of substitution in disubstituted thiophenes can, in most cases, easily be deduced from the directing effect of each substituent. Thus with a - -M-substituent in the 2-position and a —M-substituent in the 5-position, both substituents direct the entering group to the 3-position as is exemplified by the nitration of methyl 2-bromo-5-thiophenecarboxylate to methyl 2-bromo-3-nitro-5-thio-phenecarboxylate (109) or in the chlororaethylation of methyl 2-methyl-5-thiophenecarboxylate to methyl 2-methyl-3-chloromethyl-5-thiophenecarboxylate (110). °... 

The calculations so far have been concerned with the permanent charge distribution which the molecule would have when isolated, and which would be observed in its dipole moment. The factor that actually determines the orientation of the entering group, however, is the charge distribution at the instant of attack, which differs from the permanent charge distribution in consequence of polarization by the entering group. In most cases it is not necessary to take this latter effect into account explicitly, since it represents only a small correction. However, in certain cases it is apparently of decisive importance.7... 

EFFECT OF ENTERING AND LEAVING GROUPS 4.5.1 The entering group... 

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