Alkyl chlorides, rates

Secondary and pnmary alcohols do not react with HCl at rates fast enough to make the preparation of the conespondmg alkyl chlorides a method of practical value There fore the more reactive hydrogen halide HBr is used even then elevated temperatures are required to increase the rate of reaction... 

The similar yields of the two alkyl chloride products indicate that the rate of attack by chloride on the secondary carbocation and the rate of rearrangement must be very similar... 

A substantial body of evidence indicates that allylic carbocations are more stable than simple alkyl cations For example the rate of solvolysis of a chlonde that is both tertiary and allylic is much faster than that of a typical tertiary alkyl chloride... 

All lation. In alkylation, the dialkyl sulfates react much faster than do the alkyl haHdes, because the monoalkyl sulfate anion (ROSO ) is more effective as a leaving group than a haHde ion. The high rate is most apparent with small primary alkyl groups, eg, methyl and ethyl. Some leaving groups, such as the fluorinated sulfonate anion, eg, the triflate anion, CF SO, react even faster in ester form (4). Against phenoxide anion, the reaction rate is methyl triflate [333-27-7] dimethyl sulfate methyl toluenesulfonate [23373-38-8] (5). Dialkyl sulfates, as compared to alkyl chlorides, lack chloride ions in their products chloride corrodes and requires the use of a gas instead of a Hquid. The lower sulfates are much less expensive than lower bromides or iodides, and they also alkylate quickly. 

O Spontaneous dissociation of the tertiary alkyl chloride yields an intermediate carbocation in a slow, rate-limiting step. 

The most valuable and comprehensive kinetic studies of alkylation have been carried out by Brown et al. The first of these studies concerned benzylation of aromatics with 3,4-dichloro- and 4-nitro-benzyl chlorides (these being chosen to give convenient reaction rates) with catalysis by aluminium chloride in nitrobenzene solvent340. Reactions were complicated by dialkylation which was especially troublesome at low aromatic concentrations, but it proved possible to obtain approximately third-order kinetics, the process being first-order in halide and catalyst and roughly first-order in aromatic this is shown by the data relating to alkylation of benzene given in Table 77, where the first-order rate coefficients k1 are calculated with respect to the concentration of alkyl chloride and the second-order coefficients k2 are calculated with respect to the products of the... 

TABLE 9.3 Rates of Hydrolysis of Tertiary Alkyl Chlorides at 25° C in 80% Aqueous Ethanol... 

Additional evidence for the SnI mechanism, in particular, for the intermediacy of carbocations, is that solvolysis rates of alkyl chlorides in ethanol parallel carbocation stabilities as determined by heats of ionization mea.sured in superacid solutions (p. 219). It is important to note that some solvolysis reactions proceed by an Sn2 mechanism." ... 

Brown and Grayson reported that the rate of alkylation reactions with benzyl chloride was third order overall first order in aromatic component, first order in AICI3, and first order in benzyl chloride. This indicates that a rate determining nucleophilic attack by the aromatic component on a polar alkyl chloride-aluminum... 

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 ... 

Ni(acac)2 at 55 °C. Primary alkyl chlorides also undergo this reaction, but at much lower rates than iodides, the reaction taking days rather than hours. 

The numerous straightforward examples of internal displacement reactions leading to isolable cyclic products will not be discussed here, but only, for the most part, those ionization reactions in which a cyclic intermediate or transition state is deduced from the rearranged structure of the product. A well-known example is mustard gas and other alkyl chlorides with sulfur on the /3-carbon atom. Although mustard gas is a primary and saturated alkyl chloride, its behavior is like that of a typical tertiary alkyl chloride. It reacts so fast by a first order ionization that the rate of the usual second order displacement reaction of primary alkyl halides is not measureable. Only the ultimate product, not the rate, is determined by the added reagent.228 Since the effect of the sulfur is too large to be explicable in terms of a carbon sulfur dipole or similar explanation, a cyclic sulfonium ion has been proposed as an... 

Kinetics of the solvolysis of acyl chlorides and alkyl chlorides in hydroxylic solvent mixtures have been measured conductimetrically at various temperatures and pressures. The activation parameters A V, AH, and AS were calculated from the rate constants. The authors appear to have been interested mainly in acyl chlorides, but conclude that, whereas p-methylbenzoyl chloride reacts via a dissociative... 

Not all steric effects decrease reaction rates. In the hydrolysis of RC1 by an SnI mechanism (see p. 298), the first step, which is rate-determining, involves ionization of the alkyl chloride to a carbocation ... 

TABLE 9.3 Rates of hydrolysis of tertiary alkyl chlorides at 25°C in 80% aqueous ethanol6... 

A clearer picture emerges from studies of substituent effects on the rate with a single catalyst. A series of alkyl chlorides (C2 to C6) was decomposed on a barium sulphate catalyst [184] and the rate data were correlated by the Taft equation. Large negative values of p were obtained, viz. —34.3 at 220°C and —40.3 at 280°C. Similarly, for a series of three alkyl bromides (ethyl, propyl and isopropyl) on silica—alumina, alumina—... 

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