2- Chloro-2-methylbutane, and

How would you expect the fraction of elimination toward the methyl groups, as opposed to elimination toward the methylene group, to compare in E1 and E2 reactions of 2-chloro-2-methylbutane and the corresponding deuterium-labeled chloride, 2-chloro-2-methylbutane-3-D2 Give your reasoning. (Review Sections 8-8 and 15-6B.)... 

Draw a reaction profile to illustrate the conversion of 2-methyl-2-butanol and HCl to 2-chloro-2-methylbutane and water (Eqs. 14.18-14.20). Label enthalpy of activation, Afft, for the rate-determining step and the enthalpy,... 

If the order of carbocation stability shown in Figure 3.78 is correct, we should always get the product resulting from formation of the more stable carbocation (Fig. 3.79). We can test this idea. Just allow HCl to add to 2-methyl-2-butene or propene, each an unsymmetrical alkene. If we are right in our ideas about carbocation stability, the products will be 2-chloro-2-methylbutane and isopropyl chloride, respectively. When these experiments are run, those are exactly the products observed (Fig. 3.79). 

Contrasting results have been obtained by other workers in studies of the solvolysis of 4-(trimethylsilyl)-2-chloro-2-methylbutane and its carbon analogue 2-chloro-2,5,5-trimethylhexane in aqueous ethanol and aqueous 2,2,2-trifluoroethanol. With these tertiary systems very little participation by the y-silyl substituent is indicated. 

The submitters purchased 2-chloro-2-methylbutane from Eastman Kodak Company. The checkers prepared the halide as follows. A separatory funnel was charged with 21.5 mL (0.2 mol) of 2-methyl-2-butanol and IQO mL of coned hydrochloric acid. The mixture was shaken vigorously with periodic venting for 10 min. The layers were separated and the 2-chloro-2-methylbutane layer (upper) was washed several times with equal volumes of cold water. The product was dried over calcium chloride and distilled, bp 85 C. 

This procedure has been used to effect the following reductions at ca. 150° bromobenzene to benzene (89%), iodo-benzene to benzene (95%), 1-chlorobutane to n-butane (95%), 2-chloro-2-methylbutane to 2-methylbutane (32%), and isopropyl chloroacetate to isopropyl acetate (63%). 

Rearrangement can also occur after the initial alkylation. The reaction of 2-chloro-2-methylbutane with benzene is an example of this behavior.35 With relatively mild Friedel-Crafts catalysts such as BF3 or FeCl3, the main product is 1. With A1C13, equilibration of 1 and 2 occurs and the equilibrium favors 2. The rearrangement is the result of product equilibration via reversibly formed carbocations. 

Rate constants have been determined for solvolyses of 2-bromo- (or -chloro-) -2-methylbutane and 3-chloro-3-methylpentane in 10 diols at 298.15 K. By combining kinetic data with thermodynamic data, transfer Gibbs energies of the reactants (initial state) and of the activated complex (transition state) were obtained, which allowed the solvent effects on both states to be quantitatively analysed. 

Draw structural formulae for the organic products formed when 2-chloro-2-methylbutane is heated under reflux with (a) aqueous potassium hydroxide and (b) ethanolic potassium hydroxide. 

In a multistep synthesis, the overall percent yield is the product of the fractional yields in each step times 100 and decreases rapidly with the number of steps. For this reason, a low-yield step along the way can mean practical failure for the overall sequence. Usually, the best sequence will be the one with the fewest steps. Exceptions arise when the desired product is obtained as a component of a mixture that is difficult to separate. For example, one could prepare 2-chloro-2-methylbutane in one step by direct chlorination of 2-methyl-butane (Section 4-5A). But because the desired product is very difficult to separate from the other, isomeric monochlorinated products, it is desirable to use a longer sequence that may give a lower yield but avoids the separation problem. Similar separation problems would be encountered in a synthesis that gives a mixture of stereoisomers when only one isomer is desired. Again, the optimal synthesis may involve a longer sequence that would be stereospecific for the desired isomer. 

When 2-chloro-2-methylbutane is treated with a variety of strong bases, the products always seem to contain two isomers (A and B) of formula C5HX0. When sodium hydroxide is used as the base, isomer A predominates. When potassium ferf-butoxidc is used as the base, isomer B predominates. The H and 13C NMR spectra of A and B are given below. 

Below is an example the alkylation of cyclopentanone with 2-chloro-2-methylbutane. The ketone was converted to the trimethylsilyl enol ether with triethylamine and trimethylsilylchloride we discussed this step on p. 538 (Chapter 21). Titanium tetrachloride in dry dichloromethane promotes the alkylation step. 

Gilbert and Martin have described an excellent kinetics experiment which illustrates solvolysis of 2-chloro-2-methylbutane via an SnI mechanism (10). The reaction is carried out in a green solvent mixture of water and 2-propanol. In addition, we have scaled down to the use of only 118 microliters of 2-chloro-2-methylbutane per student, and carry out our titrations using reusable plastic syringes instead of burets (77). This lab exercise provides experimental support for the proposed SnI mechanism and illustrates the effect of solvent polarity (increasing percentages of water in the solvent mix) on reaction rate. 

In one case the product stream was analysed for chlorinated compounds (ZSM-5, 400°C, WHSV=24) as shown in Table 1. The total selectivity to chlorinated reaction products (chloroisobutane, 2-chloro-2-methylbutane) is 4% and stays... 

The best alkylating agents for silyl enol ethers are tertiary alkyl halides they form stable carbocations in the presence of Lewis acids such as TiCl4 or SnCl4. Most fortunately, this is just the type of compound that is unsuitable for reaction with lithium enolates or enamines, as elimination results rather than alkylation a nice piece of complementary selectivity. Below is an example the alkylation of cyclopentanone with 2-chloro-2-methylbutane. The ketone was converted to the trimethylsilyl enol ether with triethylamine and trimethylsilylchloride we discussed this step on p. 466 (Chapter 20). Titanium tetrachloride in dry dichloromethane promotes the alkylation step. 

This result confirmed the earlier work of Whitmore, F. C. Johnston, F. /. Am. Chem. Soc. 1933, 55,5020, who had found that the addition of HCl to 3-methyl-l-butene without solvent, in a sealed reaction tube for 7 weeks, gave both 2-chloro-3-methylbutane and 2-chloro-2-methylbutane. This result contradicted the suggestion of earlier investigators that the 3° alkyl halide was formed by rearrangement of the 2° alkyl halide formed from the addition reaction. Hammond, G. S. Collins, C. H. /. Am. Chem. Soc. 1960, 82, 4323 found that addition of HCl to 1,2-dimethylcyclopentene produced Tchloro-frans-l,2-dimethylcyclopentane as the major (perhaps only) addition product, but it isomerized to l-chloro-c/s-l,2-dimethylcyclopentane. 

As noted previously, some chemical reactions produce one or more products that are gases. The reaction of 2-chloro-2-methylbutane (24) and KOH, for example, gives 2-methyl-2-butene (25) and HCl gas, and a gas may escape... 

Analysis Determine the mass of 2-chloro-2-methylbutane isolated and calculate the percent yield. Perform the alcoholic silver nitrate and sodium iodide/acetone classification tests on your product (Sec. 25.9). Obtain IR and NMR spectra of your starting material and product and compare them with those of authentic samples (Figs. 14.5-14.8). 

Using a filter-tip pipet, transfer the 2-chloro-2-methylbutane to a centrifuge tube, and dry the liquid over several microspatula-tips full of anhydrous sodium sulfate." Swirl the tube occasionally for 10-15 min until the product is dry. Add several small portions of anhydrous sodium sulfate if the liquid is cloudy. Using a Pasteur or a filter-tip pipet, transfer the crude product to a tared sample vial. 

The IR spectrum and NMR spectral data for 2-chloro-2-methylbutane, 2-methyl-2-butanol, 2-methyTl-butene, and 2-methyl-2-methylbutene are provided in Figures 14.7,14.8,14.5, 14.6, and 10.4r-10.7, respectively. 

For primary alcohols with extensive -branching, such as 2,2-dimethyl-l-propanol (neopentyl alcohol), it is difficult, if not impossible, for reaction to occur by direct displacement of H2O from the primary carbon. Furthermore, formation of a 1 ° car-bocation is also difficult, if not impossible. Instead, primary alcohols with extensive 8-branching react by a mechanism involving formation of a 3° carbocation intermediate by simultaneous loss of H2O and migration of an allgrl group, as illustrated by the conversion of 2,2-dimethyl-l-propanol to 2-chloro-2-methylbutane. Because the rate-determining step of this transformation involves only one reactant, namely the protonated alcohol, it is classified as an Sj. 1 reaction. 

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