Substitution reactions, partial

These relative rate data per position are experimentally determined and are known as partial rate factors They offer a convenient way to express substituent effects m elec trophilic aromatic substitution reactions... 

Equation (7-85) is a selectivity-reactivity relationship, with lower values of Sf denoting lower selectivity. Lower values ofpt correspond to greater reactivity, with the limit being a partial rate factor of unity for an infinitely reactive electrophile. This selectivity-reactivity relationship is followed for the electrophilic substitution reactions of many substituted benzenes, although toluene is the best studied of these. 

Arynes are intermediates in certain reactions of aromatic compounds, especially in some nucleophilic substitution reactions. They are generated by abstraction of atoms or atomic groups from adjacent positions in the nucleus and react as strong electrophiles and as dienophiles in fast addition reactions. An example of a reaction occurring via an aryne is the amination of o-chlorotoluene (1) with potassium amide in liquid ammonia. According to the mechanism given, the intermediate 3-methylbenzyne (2) is first formed and subsequent addition of ammonia to the triple bond yields o-amino-toluene (3) and m-aminotoluene (4). It was found that partial rearrangement of the ortho to the meta isomer actually occurs. 

Another example of a nucleophilic acyl substitution reaction, this one a substitution by hydride inn to effect partial reduction of a thioester to an aldehyde, occurs in the biosynthesis of mevaldehyde, an intermediate in terpenoid... 

NMR spectroscopy of, 823-824 nucleophilic acyl substitution reactions of, 809-812 occurrence of, 808 partial reduction of, 812 p/C, of, 852... 

Partial rate factors, and hence the isomer distribution in a particular substitution reaction, are also affected by temperature. Increasing temperature has the greatest relative effect on the substitution reaction of highest AG (out of the three possible, alternative attacks on C6H5 Y),... 

There are innumerable industrially significant reactions that involve the formation of a stable intermediate product that is capable of subsequent reaction to form yet another stable product. These include condensation polymerization reactions, partial oxidation reactions, and reactions in which it is possible to effect multiple substitutions of a particular functional group on the parent species. If an intermediate is the desired product, commercial reactors should be designed to optimize the production of this species. This section is devoted to a discussion of this and related topics for reaction systems in which the reactions may be considered as sequential or consecutive in character. 

Good yields of the desired product can be obtained only when k1 > k2. Among the industrially significant reactions of this type are those involving partial oxidation and multiple substitution. Reactions represented by equation 12.3.136 were discussed in detail in Section 9.2, and that material is also pertinent here. 

The associative mechanism resembles a conventional radical (hydrogen atom) substitution reaction where the 7T-bonded benzene molecule is attacked by a hydrogen atom formed by the dissociative adsorption of water or hydrogen gas. The activation energy in this process is essentially due to the partial localization of one tt electron in the transition complex 21, 31). The transition state differs, however, from conventional substitution reactions by being 77-bonded to the catalyst surface ... 

C—Cl bond acquires a partial double bond character due to resonance. As a result, the bond cleavage in haloarene Is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction. 

Although the reaction mixture becomes homogeneous in this example, the submitters report that only partial solution occurs in other successful substitution reactions. The solubilities of the acetylides and the heterogeneous character of the cyclization have been described. ... 

One explanation for the alpha effect is ground-state de-stabilization Repulsive electronic interactions between the alpha atom s lone-pair and the nucleophile occur in the ground-state, and such destabilization is expected to be relieved as a covalent bond is forming in the transition-state of a nucleophilic substitution reaction. Reduced solvation in molecules exhibiting the alpha effect may also play a role in the increased nucleophilicity for example, OH2 displays no effect in the gas phase, but a substantial effect is observed in solution. Another factor may be the abihty of the alpha lone-pair to stabilize any partially positive group formed in the transition state. 

Id (Scheme 3) . This was prepared via the corresponding phosphite, which can be synthesized by reaction of the alcohol with chlorodiethyl phosphite and triethylamine. The phosphite then undergoes nucleophihc substitution reaction with anhydrous H2O2 forming the hydroperoxide Id (enantiomeric ratio S/R 65/35) and isomeric hydroperoxide le in a 2 1 mixture in 74% overall yield starting from the alcohol. Purification was possible by normal-phase HPLC. So in this case transformation of the phosphite to the hydroperoxide proceeds with partially retained configuration. 

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