Effects ortho

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

Fujita and Nishioka have attempted to place ortho effects on the same numerical scale as meta and para effects. They assume that the normal ortho electronic effect can be represented by the standard substituent constant appropriate to the reaction (cr, cr", cr, cr°), that the steric effect is given by E , and that the proximity effect is measured by the Swain-Lupton Then a multiple LFER is written... 

The idea that hydrogen bonding, as a special ortho effect of the substrate, may be involved in the transition state of the reactions with amines was first proposed by Chapman et al. Attempting to test this hypothesis, Hawthorne investigated the hydrogen-isotope effect in the reaction of o- and p-chloronitrobenzene with... 

Where ortho effects and special entropy factors control the relation of the reaction rates, it seems more appropriate to evaluate relative activation from the energies of activation. 

The ortho effect has its origin in the ability of an ortho substituent to form a hydrogen bond with a hydrogen on the amine nucleophile58-61. The present... 

Pedersen and coworkers10 studied the El mass spectra of several alkyl 2-hydroxyphenyl sulfoxides (10) and found that, contrary to methyl phenyl sulfoxide2,11 and the corresponding sulfones10, they do not show any abundant skeletal rearrangement ions (see Section III). This is obviously due to an ortho effect as shown in structure 10. 

The loss of Cl- from the molecular ion of ortho-chlorodiphenyl sulfoxide (o-ll) has been found to be significantly greater than from the meta- and para-isomers (equation 3)12. This observation is best explained by an ortho effect in accord with a tight activated complex. 

The other fragmentation pathways are typical for diaryl sulfoxides1-4-6,1. A corresponding ortho effect was found in chlorodiphenyl ethers and sulfides but not in sulfones12 (12) were the sulfinate ester rearrangements1-4,6,11 and the consequent formation of the m/z 125 and m/z 159 ions suppress the other possible fragmentations of the molecular ions (equation 4). It is also noteworthy that the ratio [m/z 125] [m/z 159] increases with increasing distance between the chlorine and the sulfur (equation 4). 

The main primary fragment ions of diaryl sulfoxides 13 and 14 have the structures 16a or 16b(C8H7S02 + m/z 167) and the ion m/z 152 (17) can be obtained from both by the loss of CH3 (equation 5)13. Ions 16a and 16b are formed from the sulfenate ester structure of the molecular ions of 13 and 14 through a cyclization process and a simultaneous loss of the other O Ar part. A similar ortho effect is not possible in 15 and hence its most intense ion is M+ (23% of the total ionization in comparison with 2.7 and 0.6% for 13 and 14, respectively) and its primary fragments are typical for a normal diaryl sulfoxide. 

Efremov and coworkers investigated the mass spectra of 18 methyl-substituted diphenyl (63)40 and substituted phenyl mesityl sulfones (64)41. The mass spectra of practically all the compounds showed by the rearrangement ions [M — OH]+, [M —H20]+ and [M — (H20 + OH)]+, the relative abundances of which depend on the position of the substituent in the phenyl moiety (ortho effect). It was also evident that in 63 the introduction of the first methyl substituent clearly decreases the contribution of the sulfone sulfinate isomerization (equation 30) to their fragmentation whereas the further substitution had little or no effect on the isomerization process in both 6340 and 6441. 

Hojo and colleagues155,156 have carried out numerous studies of ortho-effects, in particular on the ionizations of benzoic acids in DMSO-water mixed solvents. The ortho-effects are assessed by measuring the pKa values of ortho- and para- substituted benzoic acids in solvents containing from 0 to 95% v/v DMSO and expressing the results as equation 15 ... 

Several studies confirm an ortho effect leading to a dominating aryl sulphur bond cleavage. For example, the introduction of a bulky ortho substituent17 will provide the formation of the aliphatic sulphinic acid. A series of cyclic sulphones was studied18,19 at the mercury cathode and the results (see Table 2) appear to be fully in agreement with those expected when considering the preliminary works presented above. 

This is not the same as the ortho effect discussed on page 688. 

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