1-Bromo-l-phenylethane

A second piece of evidence in support of the E2 mechanism is provided by a phenomenon known as the deuterium isotope effect. For reasons that we won t go into, a carbon-hydrogen bond is weaker by about 5 kj/mol (1.2 kcal/mol) than the corresponding carbon-rfaiiferiwm bond. Thus, a C-H bond is more easily broken than an equivalent C-D bond, and the rate of C-H bond cleavage is faster. For instance, the base-induced elimination of HBv from l-bromo-2-phenylethane proceeds 7.11 times as fast as the corresponding... 

The dehydrohalogenation of 1- or 2-haloalkanes, in particular of l-bromo-2-phenylethane, has been studied in considerable detail [1-9]. Less active haloalkanes react only in the presence of specific quaternary ammonium salts and frequently require stoichiometric amounts of the catalyst, particularly when Triton B is used [ 1, 2]. Elimination follows zero order kinetics [7] and can take place in the absence of base, for example, styrene, equivalent in concentration to that of the added catalyst, is obtained when 1-bromo-2-phenylethane is heated at 100°C with tetra-n-butyl-ammonium bromide [8], The reaction is reversible and 1-bromo-l-phenylethane is detected at 145°C [8]. From this evidence it is postulated that the elimination follows a reverse transfer mechanism (see Chapter 1) [5]. The liquidrliquid two-phase p-elimination from 1-bromo-2-phenylethanes is low yielding and extremely slow, compared with the PEG-catalysed reaction [4]. In contrast, solid potassium hydroxide and tetra-n-butylammonium bromide in f-butanol effects a 73% conversion in 24 hours or, in the absence of a solvent, over 4 hours [3] extended reaction times lead to polymerization of the resulting styrene. 

When the rate constant (%) for elimination of HBr from l-bromo-2-phenylethane is compared with the rate constant ( d) for elimination of DBr from 2-bromo-... 

PROBLEM 5.3 Which of the following compounds is chiral a. 1-bromo-l-phenylethane b. l-bromo-2-phenylethane... 

The effects of /3-cyclodextrin (/3-CD) on the rates of dehydrobromination of l-bromo-2-phenylethane and its ring-substituted derivatives in basic solution (HO /H20-dioxane) have been interpreted in terms of the formation of an inclusion complex whose structure depends on the substrate.The effect of /3-CD (which retards the reaction rate in most cases) is to increase the Hammett p value from 1.3 to 2.3, indicating an increase in the carbanion character of the transition state. 

Obtained by reaction of l-bromo-2-phenylethane with quinacetophenone in the presence of potassium carbonate in refluxing acetone for 24 h (21%) [3204]. 

For example, the rates of dehydrobromination of l-bromo-2-phenylethane and l-bromo-2,2-dideuterio-2-phenylethane differ by a factor of seven. 

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