Alkyl chlorides, relative rate

Studies of the relative rates of the zinc chloride-catalysed bromination of alkyl-and halogeno-benzenes in nitromethane at 25 °C have lead to the suggestion that the rate-determining step of the reaction is formation of Ji-complex, since low substrate selectivity was found to be coupled with high (i.e. normal) positional selectivity323. Under some conditions (column 1 in Table 75) the low selectivity... 

Olivier and Berger335, who measured the first-order rate coefficients for the aluminium chloride-catalysed reaction of 4-nitroben2yl chloride with excess aromatic (solvent) at 30 °C and obtained the rate coefficients (lO5/ ) PhCI, 1.40 PhH, 7.50 PhMe, 17.5. These results demonstrated the electrophilic nature of the reaction and also the unselective nature of the electrophile which has been confirmed many times since. That the electrophile in these reactions is not the simple and intuitively expected free carbonium ion was indicated by the observation by Calloway that the reactivity of alkyl halides was in the order RF > RC1 > RBr > RI, which is the reverse of that for acylation by acyl halides336. The low selectivity (and high steric hindrance) of the reaction was further demonstrated by Condon337 who measured the relative rates at 40 °C, by the competition method, of isopropylation of toluene and isopropylbenzene with propene catalyzed by boron trifluoride etherate (or aluminium chloride) these were as follows PhMe, 2.09 (1.10) PhEt, 1.73 (1.81) Ph-iPr, (1.69) Ph-tBu, 1.23 (1.40). The isomer distribution in the reactions337,338 yielded partial rate factors of 2.37 /mMe, 1.80 /pMe, 4.72 /, 0.35 / , 2.2 / Pr, 2.55337 339. 

Low substrate selectivity accompanying high positional selectivity was also found in isopropylation of a range of alkyl and halogenobenzenes by /-propyl bromide or propene in nitromethane, tetramethylene sulphone, sulphur dioxide, or carbon disulphide, as indicated by the relative rates in Table 86. The toluene benzene reactivity ratio was measured under a wide range of conditions, and varied with /-propyl bromide (at 25 °C) from 1.41 (aluminium chloride-sulphur... 

Alkyl chlorides, hydrolysis, relative rate constants, 66,67(... 

The usual etherifying agents are the alkyl chlorides or sulfates. The advantage, usually found in organic reactions, of using the alkyl iodides and bromides because of their greater reactivity as compared to the chlorides is overshadowed by their much slower diffusion rate, lower solubility in the alkali and greater rate of saponification. The sulfates are relatively costly. Alkyl sulfites have also been proposed ... 

The diastereomers 251/ewf-251 and 252/ent-252 could be separated and were decom-plexed separately. From the fraction of 251/ewt-251,253 was obtained with 85% ee (e.r. = 92.5 7.5), and the fraction of 252/ent-252 yielded ewt-253 with 88% ee (e.r. = 6 94). A similar situation results from the reaction with tributyltin chloride or alkylation reagents, but the diastereomeric ratio is strongly dependent on the electrophile. The following conclusion is drawn from these and further experiments The enantiomeric ratio is determined by a selection of the chiral base between the diastereotopic methylene groups, since the benzylic carbanionic centres are labile, whereas the diastereomeric ratio results from the relative rate of the electrophile approach syn or anti with respect to the A-methyl group. One question remains—why are opposite d.r. values formed in the alkylation by methyl iodide and ethyl iodide ... 

In 1959, the coordinated mercaptide ion in the gold(III) complex (4) was found to undergo rapid alkylation with methyl iodide and ethyl bromide (e.g. equation 3).9 The reaction has since been used to great effect particularly in nickel(II) (3-mercaptoamine complexes.10,11 It has been demonstrated by kinetic studies that alkylation occurs without dissociation of the sulfur atom from nickel. The binuclear nickel complex (5) underwent stepwise alkylation with methyl iodide, benzyl bromide and substituted benzyl chlorides in second order reactions (equation 4). Bridging sulfur atoms were unreactive, as would be expected. Relative rate data were consistent with SN2 attack of sulfur at the saturated carbon atoms of the alkyl halide. The mononuclear complex (6) yielded octahedral complexes on alkylation (equation 5), but the reaction was complicated by the independent reversible formation of the trinuclear complex (7). Further reactions of this type have been used to form new chelate rings (see Section 7.4.3.1). 

Figure 5.5 Correlation between stability, measured by solvolysis rate in 80 percent aqueous acetone, and selectivity, determined by relative rate of reaction with azide ion (kN) and water (kw), for carbocations derived from alkyl chlorides. Reprinted with permission from D. J. Raber, J. M. Harris, R. E. Hall, and P. v. R. Schleyer, J. Amer. Chem. Soc., 93, 4821 (1971). Copyright by the American Chemical Society.

TABLE 2. Taft s Es values for alkyl and cycloalkyl groups R1 and relative rate constants (R Me2 S i CI) /k ( M e3 S i CI) for the reaction of the two chlorides with lithium silanolates and lithium isopropylate in Et20 at 20 C... 

Data listed in Table 2 include the substituent constants R1 of trialkylchlorosilanes and the relative rate constants fc(R1Me2SiCl)/A (Me3SiCl) for the reactions of the two chlorides with lithium silanolates and isopropylate (equation 39)57. The reaction rates of silanes are influenced almost exclusively by the steric effects of the alkyl groups attached to the silicon atom. The log(A rei) values of the compounds with various R1 groups give a satisfactory correlation with Taft s Es values151. Thus the steric hindrance of silyl groups follows the order listed in entry 4457 of Table 1. 

The initial products of organic reactions are formed under conditions of kinetic control - the products are formed in proportions governed by the relative rates of the parallel (forward) reactions leading to their formation. Subsequently, product composition may become thermodynamically controlled (equilibrium controlled), i.e. when products are in proportions governed by the equilibrium constants for their interconversion under the reaction conditions. The reaction conditions for equilibrium control could involve longer reaction times than those for kinetic control, or addition of a catalyst. The mechanism of equilibrium control could simply involve reversal of the initial product-forming reactions (as in Scheme 2.4, see below), or the products could interconvert by another process (e.g. hydrolysis of an alkyl chloride could produce a mixture of an alcohol and an alkene, and the HsO"1" by-product could catalyse their interconversion). 

How do the other groups bonded to the electrophilic carbon affect the rate of the SN1 reaction Table 8.2 lists the relative rates of the SN1 reaction for a number of compounds, compared to the rate for isopropyl chloride taken as 1. Methyl chloride and ethyl chloride are not listed in the table because methyl and simple primary alkyl chlorides do not react by the mechanism. Even under the most favorable SN1 conditions, these unhindered compounds react by the SN2 mechanism. 

Some actual data may help at this point. The rates of reaction of the following alkyl chlorides with Kl in acetone at 50 C broadly confirm the patterns we have just analysed. These are relative rates with respect to n-BuCI... 

Table 17.10 Relative rates of Sn2 reactions of alkyl chlorides with the Iodide ion...

Traditionally, relative stabilities of carbocations have been derived from the comparison of the rates of solvolysis reactions following the SN1 mechanism, for which the designation Dm + An has recently been proposed [36], The comparison of solvolytic rate constants for substrates of a large structural variety is hampered by the fact that the published solvolysis rates refer to different solvents, different temperatures, and precursors with different leaving groups. Dau-Schmidt has, therefore, converted solvolysis rates of a manifold of alkyl chlorides and bromides to standard conditions, i.e., soiv of RC1 in 100% EtOH at 25° C (Scheme 6) [37]. Although from a theoretical point of view, ethanol is not an ideal solvent for observing unassisted SN 1-type reactions (nucleophilic solvent participation), it has been selected as the reference solvent because most available experimental data have been collected in solvents of comparable nucleophilicity, a fact which made conversions to 100% ethanol relatively unproblematic [38],... 

On the basis of these considerations it has been concluded that under given reaction conditions (Lewis acid/solvent) the reactivity maximum is found for an alkylating system (RA7R+) that is approximately half-ionized [60,61]. Scheme 11 suggests that the electrophilic reactivity of RA" increases with increasing stabilization of R+ if only small equilibrium concentrations of carbocations are involved. In accord with this analysis, the relative alkylating abilities of alkyl chlorides have been found to be proportional to their ethanolysis rates (Fig. 2) [62]. The only compound that deviates from this correlation is trityl chloride which alkylates considerably more slowly than expected from its solvolysis rate. 

Figure 2 Correlation of the relative reactivities of alkyl chlorides la-x toward allyltrimethylsilane (CH2CI2, —70° C) with their ethanolysis rate constants (25° C). The value for PluCCl has not been used for calculating the correlation equation log fcrd = 1.036 log (EtQH) + 10.1 (r = 0.971). (From Ref. 62, reprinted with permission of VCH Verlagsgesellschaft.)...

In some systems it is necessary to add a large amount of salts to obtain polymers with low polydispersities. This happens when salts participate in ligand/anion exchange (special salt effect) and when they enhance ionization of covalent compounds through the increase of ionic strength. The special salt effect may either reduce or enhance ionization. Strong rate increases observed in the polymerization of isobutyl vinyl ether initiated by an alkyl iodide in the presence of tetrabutylammonium perchlorate or triflate can be explained by the special salt effect [109]. The reduction in polymerization rate of cyclohexyl vinyl ether initiated by its HI adduct in the presence of ammonium bromide and chloride can be also ascribed to the special salt effect [33]. The breadth of MWD depends on the relative rate of conversion of ion pairs to covalent species and is affected by the structure of the counterions. 

Ferrocene reacts with acetyl chloride and aluminum chloride to afford the acylated product (287) (Scheme 84). The Friedel-Crafts acylation of (284) is about 3.3 x 10 times faster than that of benzene. Use of these conditions it is difficult to avoid the formation of a disubstituted product unless only a stoichiometric amount of AlCft is used. Thus, while the acyl substituent present in (287) is somewhat deactivating, the relative rate of acylation of (287) is still rapid (1.9 x 10 faster than benzene). Formation of the diacylated product may be avoided by use of acetic anhydride and BF3-Et20. Electrophilic substitution of (284) under Vilsmeyer formylation, Maimich aminomethylation, or acetoxymercuration conditions gives (288), (289), and (290/291), respectively, in good yields. Racemic amine (289) (also available in two steps from (287)) is readily resolved, providing the classic entry to enantiomerically pure ferrocene derivatives that possess central chirality and/or planar chirality. Friedel Crafts alkylation of (284) proceeds with the formation of a mixture of mono- and polyalkyl-substituted ferrocenes. The reaction of (284) with other... 

Table 17.8 Rates of solvolysis of alkyl chlorides in 50% aqueous ethanol at 44.6 °C Compound Relative rate Comments...

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