1.4- Dibromobutane: Butane, 1,4-dibromo

Dibromobutane from 1 4 butanediol). In a 500 ml. threenecked flask fltted with a stirrer, reflux condenser and dropping funnel, place 154 g. (105 ml.) of 48 per cent, hydrobromic acid. Cool the flask in an ice bath. Add slowly, with stirring, 130 g. (71 ml.) of concentrated sulphuric acid. To the resulting ice-cold solution add 30 g. of redistilled 1 4-butanediol dropwise. Leave the reaction mixture to stand for 24 hours heat for 3 hours on a steam bath. The reaction mixture separates into two layers. Separate the lower layer, wash it successively with water, 10 per cent, sodium carbonate solution and water, and then dry with anhydrous magnesium sulphate. Distil and collect the 1 4-dibromo-butane at 83-84°/12 mm. The yield is 55 g. 

Halogenation of double bonds is stereospecific. A reaction is stereospecific when a particular stereoisomeric form of the starting material gives a specific stereoisomeric form of the product. For example, the halogenation of cis- and tra 5-2-butene produces a racemic mixture of 2,3-dibromo-butane and mc5o-2,3-dibromobutane, respectively. 

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... 

Consider the reactions of cis- and 2-butene with bromine shown below. When trans-2-h ite.r t adds bromine, the product is the meso compound, (2i ,3S)-2,3-dibromo-butane. When cw-2-butene adds bromine, the product is a racemic mixture of (2/ ,37 )-2,3-dibromobutane and (25,3S)-2,3-dibromobutane ... 

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