Polar nucleophilic reactions, competition mechanism

On the contrary, in the case of 1-iodonorbomane (a tertiary hahde), the result of the reaction with trimethylstannyl reagents (Me3SnM, M = Li, Na), both in the absence and in the presence of trapping agents, confirmed that the nucleophilic substitution process is governed by competition between polar and radical mechanisms. ... 

Examples of the solvent-dependent competition between nucleophilic substitution and / -elimination reactions [i.e. SnI versus Ei and Sn2 versus E2) have already been given in Section 5.3.1 [cf. Table 5-7). A nice example of a dichotomic y9-elimination reaction, which can proceed via an Ei or E2 mechanism depending on the solvent used, is shown in Eq. (5-140a) cf. also Eqs. (5-20) and (5-21) in Section 5.3.1. The thermolysis of the potassium salt of racemic 2,3-dibromo-l-phenylpropanoic acid (A), prepared by bromine addition to ( )-cinnamic acid, yields, in polar solvents [e.g. water), apart from carbon dioxide and potassium bromide, the ( )-isomer of l-bromo-2-phenylethene, while in solvents with low or intermediate polarity e.g. butanone) it yields the (Z)-isomer [851]. 

Most of the work concerned with micellar catalysis of nucleophilic substitution refers to reactions of the Aac2 and SN2 types and will not be reviewed here. To date only a few systems have been examined in which a micellar medium affects the partitioning of solvolytic reactions between unimolecular and bimolecular mechanisms. The effects of cationic (hexadecyltrimethylammonium bromide = CTAB) and anionic (sodium lauryl sulfate = NaLS) micelles on competitive SN1 and SN2 reactions of a-phenylallyl butanoate 193) have been investigated189. The rate of formation of the phenylallyl cation 194) is retarded by both surfactants probably as a consequence of the decreased polarity of the micellar pseudo phase. The bimolec-... 

Key point. Alkyl halides are composed of an alkyl group bonded to a halogen atom (X = F, Cl, Br, I). As halogen atoms are more electronegative than carbon, the C-X bond is polar and nucleophiles can attack the slightly positive carbon atom. This leads to the halogen atom being replaced by the nucleophile in a nucleophilic substitution reaction, and this can occur by either an SN1 (two-step) mechanism or an Sn2 (concerted or one-step) mechanism. In competition with substitution is elimination, which results in the loss of HX from alkyl halides to form alkenes. This can occur by either an El (two-step) mechanism or an E2 (concerted) mechanism. The mechanism of the substitution or elimination reaction depends on the alkyl halide, the solvent and the nucleophile/base. 

Kinetics have been measured for the dediazoniation reactions of 2- and 3-methyl-benzenediazonium cations in water and in aqueous methanol. The results are consistent with a heterolytic mechanism involving rate-determining formation of reactive aryl cations which show little discrimination for nucleophiles present. Results for the dediazoniation of 4-nitrobenzenediazonium ions in aqueous acid catalysed by Cu(II) chloride are also consistent with a heterolytic pathway at low acidity and with conditions designed to avoid the formation of Cu(I) the reaction may yield chloroarenes in high yield. Activation parameters have been reported for the decomposition of some arenediazonium tetrafluoroborates in polar solvents. The results are generally consistent with heterolysis, although in ethanol there is competition from a radical mechanism, with characteristically different activation parameters, which leads to hydrodediazoniation f... 

Zollinger [15], to interpret the product yield results. The yields of phenol product, z-ArOH, from reaction with water and halo product, z-ArX, from reaction with Br or Cl are assumed to be determined primarily by the position of equilibrium between the ion-molecule and ion-ion pairs. The dediazoniation rate constant for each pair is probably of secondary importance for these dediazoniations because dediazoniation reactions are notoriously insensitive to the polarity of the reaction medium (see later). For several decades the basic consensus on the dediazoniation mechanism has been the rate-determining loss of N2 to give a highly reactive aryl cation intermediate that is trapped extremely rapidly and competitively by available nucleophiles [15]. However, more recent ah initio calculations provide support for a bimolecular mechanism in which C—N bond cleavage is almost complete and bond formation with the nucleophile has barely begim [16]. Because both mechanisms lead to the same definition for the selectivity of the reaction toward competing nucleophiles [Eq. (1)], the bimolecular pathway for dediazoniation is not included in Scheme 1. 

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