Alkyl chlorides, relative rate constants

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

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. 

A large number of accurate rate constants are known for addition of simple alkyl radicals to alkenes.33-33 Table 2 summarizes some substituent effects in the addition of the cyclohexyl radical to a series of monosubstituted alkenes.36 The resonance stabilization of the adduct radical is relatively unimportant (because of the early transition state) and the rate constants for additions roughly parallel the LUMO energy of the alkene. Styrene is selected as a convenient reference because it is experimentally difficult to conduct additions of nucleophilic radicals to alkenes that are much poorer acceptors than styrene. Thus, high yield additions of alkyl radicals to acceptors, such as vinyl chloride and vinyl acetate, are difficult to accomplish and it is not possible to add alkyl radicals to simple alkyl-substituted alkenes. Alkynes are slightly poorer acceptors than similarly activated alkenes but are still useful.37... 

Considerations for synthetic planning are remarkably similar to the tin hydride method. The initial radical can either add to the alkene or add to its own precursor (32). To maximize the addition, it is advisable to use a reactive alkene (typically in excess) and to keep a relatively low concentration of the thiohydroxamate (32) hence the slow addition of the acid chloride. At present, rate constants for the addition of a primary and a tertiary alkyl radical to a thiohydroxamate are known (Scheme 46)38-40 and these are useful in selecting alkene acceptors and in planning reaction conditions. 

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). 

Previous investigations (Brady, 1949 Grayson, 1952 Bonner, 1952) in the Purdue laboratories were concerned with the influence of alkyl groups on the rate of ionization of phenyldimethylcarbinyl chloride. These studies indicated that first-order rate constants could be determined with high accuracy for the solvolysis reaction. Moreover, the entropies of activation were invariant in this series of halides. These considerations led to further study of other substituted phenyldimethylcarbinyl chlorides in an attempt to gain a further understanding of the influence of substituent groups on relative reactivity and as a possible model reaction for the assessment of parameters for electron-deficient reactions. 

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],... 

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.)...

B. Kinetics of Reaction with Alkyl Halides (14). Prior to the work of Danen and Warner, (14,15) few rate constants for the reaction of O2 with alkyl halides had been reported. In an electrochemical study, Merritt and Sawyer (16) had determined the pseudo-first-order rate constants at 28 C for three butyl chlorides in DMSO solvent. In a similar manner, Dietz, et al., (J) had reported a pseudo-first-order rate constant for 1-bromobutane reacting with electrogenerated O2 in DMF containing tetra-n-butylammonium perchlorate. San Filippo and coworkers (S) had determined the relative reactivity of several alkyl halides but had not reported any absolute rate constants. 

The self-ionisation of aluminium chloride and bromide in nitrobenzene has been studied in great detail [15], and the rates of the forward and back reactions have been determined so that all the relevant equilibrium constants are known. The whole body of evidence available shows that self-ionisation of the initiator, with or without other ionogenic reactions in the initiator solutions, can be regarded as well established for all aluminium halides and as highly probable for the alkyl aluminium halides. Moreover, the ionogenic reactions are relatively slow and - except under the dirtiest conditions - the concentration of ions in the initiator solution will be very much less than [A1X3]0. 

To assess the accuracy and precision of these other factors influencing relative reactivity. Recently, solvent effects were shown to produce important variations in the relative reactivity of very similar molecules. Thus the relative influence of m-methyl and m-t-butyl substituents on the rate of solvolysis of benzhydryl chloride depends on the solvent (Shiner and and Verbanic, 1957). Less remarkable but equally important variations in reactivity were detected among the p-alkylated benzhydryl chlorides (Shiner and Verbanic, 1957 Berliner and Chen, 1958). A full analysis of solvent influences (Clement et al., 1960) requires much detailed... 

Ionization of an alkyl halide requires formation and separation of positive and negative charges, similar to what happens when sodium chloride dissolves in water. Therefore, SN1 reactions require highly polar solvents that strongly solvate ions. One measure of a solvent s ability to solvate ions is its dielectric constant (e), a measure of the solvent s polarity. Table 6-6 lists the dielectric constants of some common solvents and the relative ionization rates for fm-butyl chloride in these solvents. Note that ionization occurs much faster in highly polar solvents such as water and alcohols. Although most alkyl halides are not soluble in water, they often dissolve in highly polar mixtures of acetone and alcohols with water. 

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