Substitution mechanisms polar

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

The hydrostannation reaction can proceed either by a free-radical mechanism, or, with polar-substituted alkenes or alkynes, by a polar mechanism, respectively resulting in anti-Markownikoff or Markow-nikoff orientation. Both t3rpes of reaction are particularly suitable for preparing functionally substituted, organotin compounds. 

Several research groups ha ve been involved in the study of ET reactions from an electrochemically generated aromatic radical anion to alkyl halides in order to describe the dichotomy between ET and polar substitution (SN2). The mechanism for indirect reduction of alkyl halides by aromatic mediators has been described in several papers. For all aliphatic alkyl halides and most benzylic halides the cleavage of the carbon-halogen bond takes place concertedly with the... 

However, the competition between Srn 1 and polar abstraction mechanisms is complicated in certain reactions by the formation of disulfides which is inhibited by radical and radical anion traps, and requires photolysis [23, 24]. These results implicate a third possibility, the chain SET redox mechanism (Srt2, i.e. substitution, electron transfer, bimolecular), Scheme 10.34. This alternative mechanism occurs when the intermediate radical anion can be intercepted by the thiolate (Equation 10.23) prior to the dissociation required in the SrnI mechanism (Equation 10.17 in Scheme 10.29). It becomes possible when either... 

For reactions which do not proceed via radical intermediates, the question of detail remains does the polar substitution take place via an SN2-like substitution at the halogen, or is there an intermediate - an ate complex Gilman was the first to suggest a mechanism for halogen-... 

The temperature dependence of the rate coefficient was given by the following Arrhenius equation kx = 10(1394 02)exp [(-174,000 2000)/8.3147] s 1. The relative rate ratio isopropyl chloroformate ethyl chloroformate was 160 at 280 °C. The enhanced rate due to an a-substitution was associated with the degree of polarity of the alkyl halides7. Therefore, the transition state for chloroformate ester decomposition must also be polar. The mechanism was believed to involve a cyclic six-membered transition state in which formation of HC1 is assisted, as shown below. 

The reaction with dimethyl acetylenedicarboxylate proceeds less cleanly for 1,3-dimethylindole, possibly because this indole is a better electron donor [40c]. Seven products are obtained these are the 2-1-2 cycloadduct and the geometrical isomers of 17-19 shown in Scheme 8. The substitution products 17 dominate in polar, protic solvent and are possibly formed via photochemical electron transfer from 1,3-dimethylindole to the alkyne this substitution mechanism is discussed further in Section V. In nonpolar or aprotic media, 17 is still formed, although only as a minor product under these conditions, where electron transfer could be endothermic, it is possible that 17 is formed by a route involving intramolecular disproportionation of the triplet 1,4-biradical 20 that is also the likely precursor of the 2-1-2 photocycloadduct. The geometrical isomers 18 and 19 are the major products formed when the indole concentration is high Davis and Neckers speculate that these arise from addition of biradical 20 to 1,3-dimethylindole [40c]. However, the lifetimes of... 

The reactions of HTIB with alkenes (Scheme 3.73) can be rationalized by a polar addition-substitution mechanism similar to the one shown in Scheme 3.70. The first step in this mechanism involves electrophilic flnfi-addition of the reagent to the double bond and the second step is nucleophilic substitution of the iodonium fragment by tosylate anion with inversion of configuration. Such a polar mechanism also explains the skeletal rearrangements in the reactions of HTIB with polycyclic alkenes [227], the participation of external nucleophiles [228] and the intramolecular participation of a nucleophilic functional group with the formation of lactones and other cyclic products [229-231]. An analogous reactivity pattern is also typical of [hydroxy(methanesulfonyloxy)iodo]benzene [232] and other [hydroxy(organosulfonyloxy)iodo]arenes. 

Towards the end of Chapter 10 we introduced a classic example of a nucleophilic substitution reaction, namely the hydrolysis of a halogenoalkane by a warm aqueous solution of sodium hydroxide. As a result of the polarization of the carbon-halogen bond, the carbon atom is an electron-deficient centre susceptible to attack by a nucleophile such as the hydroxide ion (OH ). Primary halogenoalkanes are thought to undergo a substitution mechanism that involves a single reaction step. This one-stage reaction involves the simultaneous attack of the nucleophile and departure of the halide ion. We will use as an example the reaction between bromomethane and sodium hydroxide solution ... 

Until now, we have not paid much attention on the tole of the solvent in nucleophilic substitution teac-tions, but the choice of solvent can tip the balance in favot of one substitution mechanism ot anothet. We noted that secondaty haloalkanes can react by either an Sj l or an Sj 2 mechanism. In these cases, the polarity of the solvent plays an important role. The S l process forms a carbocation intermediate. Because a polar solvent stabilizes charged species better than a non polar solvent, a polar solvent increases the rate of Sj 1 reactions. Reactions that occur via an mechanism are also affected by solvent polarity,... 

For unactivated aromatic and heteroaromatic substrates, where a polar substitution is not favorable, nucleophilic substitution is feasible through processes that involve electron transfer (ET) steps. In these reactions, an aromatic compound bearing an adequate leaving group is substituted at the ipso position by a nucleophile in a unimolecular radical nucleophilic substitution mechanism (or S,y.jl), which is a chain process that involves radicals and radical anions as intermediates. 

Because of the chemical similarity between benzoyl nitrate and the acetyl nitrate which is formed in solutions of nitric acid in acetic anhydride, it is tempting to draw analogies between the mechanisms of nitration in such solutions and in solutions of benzoyl nitrate in carbon tetrachloride. Similarities do exist, such as the production by these reagents of higher proportions of o-substituted products from some substrates than are produced by nitronium ions, as already mentioned and further discussed below. Further, in solutions in carbon tetrachloride of acetyl nitrate or benzoyl nitrate, the addition of acetic anhydride and benzoic anhydride respectively reduces the rate of reaction, implying that dinitrogen pentoxide may also be involved in nitration in acetic anhydride. However, for solutions in which acetic anhydride is also the solvent, the analogy should be drawn with caution, for in many ways the conditions are not comparable. Thus, carbon tetrachloride is a non-polar solvent, in which, as has been shown above,... 

Solvent Effects on the Rate of Substitution by the S 2 Mechanism Polar solvents are required m typical bimolecular substitutions because ionic substances such as the sodium and potassium salts cited earlier m Table 8 1 are not sufficiently soluble m nonpolar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate Other than the requirement that the solvent be polar enough to dis solve ionic compounds however the effect of solvent polarity on the rate of 8 2 reactions IS small What is most important is whether or not the polar solvent is protic or aprotic Water (HOH) alcohols (ROH) and carboxylic acids (RCO2H) are classified as polar protic solvents they all have OH groups that allow them to form hydrogen bonds... 

The mechanism of the reaction is unknown. The stereospecificity observed with (E)- and (Z)-l-methyl-2-phenylethylene points to a one-step reaction. The very low Hammett constant, -0.43, determined with phenylethylenes substituted in the benzene ring, excludes polar intermediates. Yields of only a few percent are obtained in the reaction of aliphatic alkenes with (52). In the reaction of cyclohexene with (52), further amination of the aziridine to aminoaziridine (99) is observed. Instead of diphenylazirine, diphenylacetonitrile (100) is formed from diphenylacetylene by NH uptake from (52) and phenyl migration. 

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