Threo-2,3-dibromobutane

Threo-2,3-dibromobutane CHjCHBr-CHBrCHs 84.9 3 355.2 12.6 Derived from in ref. 1996NIST... 

Scheme 7.42. A representation of a, 3-dehalogenation from erytho- and threo-2,3-dibromobutanes on treatment with iodide anion (I ) in ethanol (CH3CH2OH).

Elimination of halogen from a,P-dihalides in a variety of circumstances has been observed and can be used to prepare alkenes and alkynes from vicinyl dihalides. The process is usually of limited utility since the dihalides themselves are often prepared from the unsaturated compounds. As shown in Scheme 7.42, both erythro (meso) and threo ( )-2,3-dibromobutane diastereomers undergo stereospecific elimination of bromine to their respective (E)- (or trans) and (Z)- (or cis) 2-butenes on treatment with iodide anion (T) in ethanol (ethyl alcohol, CH3CH2OH). 

The first important evidence for the existence of this mechanism was the demonstration that retention of configuration can occur if the substrate is suitable. It was shown that the threo dl pair of 3-bromo-2-butanol when treated with HBr gave (f/-2,3-dibromobutane, while the erythro pair gave the meso isomer ... 

The terms erythro and threo are generally used only with molecules that do not have symmetric ends. In symmetric molecules such as 2,3-dibromobutane and tartaric acid, the terms meso and (d,l) are preferred because these terms indicate the diastere-omer and tell whether or not it has an enantiomer. Figure 23-4 shows the proper use of the terms erythro and threo for dissymmetric molecules, as well as the terms meso and (.d,l) for symmetric molecules. 

The bromonium ion in Figure 9.2 was proposed by Roberts and Kimball to explain the stereospecifidty of the addition to alkenes. Independent evidence for the existence of bromonium ions was provided by Winstein and Lucas, who found that ( )-e/yf/iro-3-bromo-2-butanol was converted to mcso-2,3-dibromobutane upon reaction with fuming HBr (equation 9.1), but the ( )-threo reactant formed the ( )-2,3-dibromo-butane. These authors suggested that the bromine provides anchimeric assistance to the departure of the protonated OFI group and forms a bromonium ion which, upon nucleophilic attack by bromide ion, gives a... 

Write mechanisms with full electron pushing that explain the following two reactions. Here we show that d- or /-erythro-3-bromo-2-butanol forms meso-2,3-dibromobutane upon treatment with HBr. However, d- or /-threo-3-bromo-2-butanol forms the corresponding chiral 2,3-dibromobutanes. Are these reactions stereospecific ... 

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