Solvolysis of 2-chloro-2-methylpropane

Thomson O1" Click Organic Process to view animations showing the S 1 reaction of 2-methyl-2-propanol with HCI and the SN1 solvolysis of 2-chloro-2-methylpropane. 

This maximum is thought, from viscosity and enthalpy of mixing studies (Fig. 5.42), to be due to the formation of a H-bonded monomer-water complex. Previous investigations of the solvolysis of 2-chloro-2-methylpropane had led us to conclude that low intensity ultrasound (< 2 W cm ) was capable of destroying H-bonds and if this were... 

The solvolysis of 2-chloro-2-methylpropane is 335000 times faster in water than in the less polar solvent ethanol [40] cf. reaction type (a) in Table 5-4. 

Table 5-13. Rate constants and activation parameters for the SnI solvolysis of 2-chloro-2-methylpropane in six solvents at 25 °C [40],...

Fig. 5-13. Correlation between Ig A i [40] and the Kirkwood function (cr — l)/(2 r + 1) for the solvolysis of 2-chloro-2-methylpropane in binary 1,4-dioxane/water (A), ethanol/water (B), acetone/water (C), and methanol/water (D) mixtures at 25 °C cf. Eq. (5-13) in Section 5.3.1.

Reaction type 3 in Table 5-25 is best represented by the SnI solvolysis of 2-chloro-2-methylpropane cf. Eq. (5-13) in Section 5.3.1. Considering the heterolysis of the C—Cl bond, one would expect the activation volume to be positive because of the C—Cl stretching during the activation process. However, a negative activation volume of AF = —22.2 cm mol has been found for this solvolysis at 30 °C in ethanol/ water (80 20 cL/L), indicating a strong volume contraction due to solvation of the dipolar activated complex (electrostriction) [756]. Typical activation volumes for halo-alkane solvolyses in pro tic solvents are in the range of —15... —30 cm mol ... 

Since reaction rates can be strongly affected by solvent polarity cf. Chapter 5), the introduction of solvent scales using suitable solvent-sensitive chemical reactions was obvious [33, 34]. One of the most ambitious attempts to correlate reaction rates with empirical parameters of solvent polarity has been that of Winstein and his co-workers [35-37]. They found that the SnI solvolysis of 2-chloro-2-methylpropane (t-butyl chloride, t-BuCl) is strongly accelerated by polar, especially protic solvents cf. Eq. (5-13) in Section 5.3.1. Grunwald and Winstein [35] defined a solvent ionizing power parameter Y using Eq. (7-13),... 

The composition of the reaction mixture formed in an Sjyjl reaction affects neither the rate of the reaction nor the value of the rate constant because the products all arise from a common intermediate, namely a carbocation, that is formed in the rate-determining step (Eq. 14.2). Thus, the four products that are likely to be formed in the solvolysis of 2-chloro-2-methylpropane in aqueous 2-propanol may individually be formed at different rates because of the differing values of the rate constants, and of the concentrations of the reagents that react with the... 

Solvolysis studies of alkyl halides in mixed solvents are common (see, e.g., Buncel and Millington, 1965). Grunwald and Winstein (1948) introduced the parameter Y, a measure of ionizing power based on the rate of solvolysis of 2-chloro-2-methylpropane (t-butyl chloride). It is a measure of the ability of the solvent to stabilize the ions that are generated in the unimolecular, ratedetermining first step ... 

The solvolysis of 2-chloro-2-methylpropane in water occurs by the mechanism... 

Experiment 3.1. Experimental determination of the rate constant and reaction order for the solvolysis of 2-chloro-2-methylpropane in ethanol-water mixture [2-4]... 

Figure 3.2 Dependence of the initial rate of solvolysis of 2-chloro-2-methylpropane with initial concentration of the solute in ethanol-water mixture. The slope of the hnear plot gives the rate constant k = 1.4 X lO sec at 20 °C.

We return to the solvolysis of 2-chloro-2-methylpropane in water discussed in Chapter 3 (Section 3.1). The rate-determining step of this reaction is the ionisation of the haloalkane, such that the reaction follows first-order kinetics. Thus, we can write... 

The confirmation of the reaction order is obtained by noting that for a first-order process the rate is directly proportional to the concentration. This dependence can be shown nsing the initial velocity for varions well-defined concentrations. The initial rate for each concentration is obtained from the tangent at time f = 0 of the experimental concentration-time cnrve. For the solvolysis of 2-chloro-2-methylpropane in water, the initial velocities obtained for different concentrations show a linear dependence on the initial concentration that passes through the origin, with a slope k, confirming that the reaction is first order (see Figure 3.2). 

We can see from the above equation that for a first-order process the half-life does not depend upon the initial concentration. This is seen with the data on the solvolysis of 2-chloro-2-methylpropane in water, where from Table 3.1, the first half-life is about 210 sec, giving a rate constant = 3.3X10 sec , while for the second half-life, corresponding to the time necessary for the concentration of the reactant to be reduced from [A]q/2 to [A](/4, the value is virtually identical. Similarly, the half-lives for aU steps of this reaction are the same within experimental error. This provides an excellent technique for confirming the reaction order, and, as a working definition, it is normally accepted that if the... 

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