Effects of Rings

Rings can either stabilize or destabilize molecules beyond what is to be expected from a simple additivity scheme. Stabilization comes from aromatic ring systems 

All of these effects can be accounted for by extensions of an additivity scheme, when special increments are attributed to monocyclic structures and the combination of two ring systems having one, two, or three atoms in common [23]. Combination of a table containing values for these ring fragments with an algorithm for the determination of the smallest set of smallest rings (SSSR) [24] (see also Section 2.5.1) allows such a procedure to be performed automatically. 

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The quatemization of the nitrogen atom of the thiazole ring (the Menschutkin s reaction) by alkyl halide or methyl tosylate can be used to measure the reactivity of this atom and thus to evaluate steric and electronic effects of ring substituents. 

The effects of ring substituents and counterions on the preparation and reac-tivity of the Af-fluoropyridinium salts have been reported in detail [74, 75, 76]. 

THE EFFECT OF RING-DEUTERIUM SUBSTITUTION ON THE RATE AND PRODUCTS OF THE REARRANGEMENT OF HYDRAZOBENZENE IN 90% ETHANOL AT 0 °C... 

Surprisingly enough, the effect of ring strain on the electronic structure of the —SO and —S02 groups is rather small. Sulphur charges and S—O BOPs are almost constant in the ring compounds and in the dimethyl derivatives. The only exception is a substantially weaker C—S bond (lower BOP) in ethylene episulphoxide. 

The effects of ring substituents on the rate are consistent with a unimolecular rate-determining cleavage. " ... 

Electron-donating groups in the para position of the perbenzoic acid tend to decrease the rate of reaction . The reverse seems to be true when these groups are present in a 4,4 -disubstituted diaryl sulphoxide. The effect of ring size on the oxidation of cyclic sulphoxides is apparently very small in dioxane-water solution under either acidic or basic conditions . This suggests that no major hybridisation change occurs for the sulphur atom in going to the transition state. 

Fig. 2 Effect of ring size on ee in kinetic resolution of racemic allylic ethers. For n = 3, the material is 2,3-benzo-l-oxacydo-2,6-octadiene...

This review will restrict itself to boron-carbon multiple bonding in carbon-rich systems, as encountered in organic chemistry, and leave the clusters of carboranes rich in boron to the proper purview of the inorganic chemist. Insofar as such three-dimensional clusters are considered at all in these review, interest will focus on the carbon-rich carboranes and the effect of ring size and substituents, both on boron and carbon, in determining the point of equilibrium between the cyclic organoborane and the isomeric carborane cluster. A typical significant example would be the potential interconversion of the l,4-dibora-2,5-cyclohexadiene system (7) and the 2,3,4,5-tetracarbahexaborane(6) system (8) as a function of substituents R (Eq. 2). 

Table 3.17. The Effect of Ring Substituents on Type 11 Cleavage...

Type I cleavage of ketones, 75 Type II cleavage of ketones effect of 7-substituents, 122-124 effect of ring substituents, 124 of 5 -(+)-4-methyl-l-phenyl-1- hexanone, 118... 

Case58 investigated the effect of ring currents on NMR shielding constants by means of the DFT calculations. The studied rings included the ones commonly found in proteins and nucleic acids. The shielding constants were calculated for methane molecule placed in several positions relative to the ring. The calculations provided data needed to derive structural parameters from measured chemical shifts in proteins and nucleic acids. 

The very small p- and m-values observed for the fast bromination of a-methoxystyrenes deserve comment since they are the smallest found for this electrophilic addition. The rates, almost but not quite diffusion-controlled, are amongst the highest. The sensitivity to polar effects of ring substituents is very attenuated but still significant that to resonance is nil. These unusually low p-values for a reaction leading to a benzylic carbocation are accompanied by a very small sensitivity to the solvent. All these data support a very early transition state for this olefin series. Accordingly, for the still more reactive acetophenone enols, the bromination of which is diffusion-controlled, the usual sensitivity to substituents is annulled. 

Atkinson, R., Aschmann, S.M., Carter, W.L., Pitts, Jr., J.N. (1983) Effects of ring strain on gas-phase rate constants. 1. Ozone reactions with cycloalkenes. Int. J. Chem. Kinet. 15, 721-731. 

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