Dibromofumaric acid

Dibromofumaric Acid (called Dibromofumar-saure in Ger), HOOC.CBr CBr.COOH mw 273.89, O 23.37% crysts (from w), mp 225°, bp - dec 230° readily sol in aic eth was obtd with other products by introducing bromine into an aq soln of acetylenedicarboxylic acid... 

Such a process would always lead to the formation of the cis isomer, whereas it is known that treatment of acetylenedicarboxylic acid HOOGCsGGOOH with bromine gives dibromomaleic and dibromofumaric acids, the cis and trans forms respectively. The formation of the dibromofumaric acid is more favoured from energy considerations, since the molecule is stabilized by the formation of two hydrogen bonds with the bromine atoms (for hydrogen bond formation between the hydrogen of a carboxyl group and a bromine atom, see Chapter 12). 

Halogen addition is characterized by stereospecificity. For example, the addition of bromine to acetylenedicarboxylic acid is accompanied by the predominant formation of dibromofumaric acid, that is, occurs in the trans-position. Therefore, the formation of labile cations with the bridged structure is assumed... 

Elving and co-workers [39, 40] investigated the polarographic behavior of mono- anddibromosubstituted maleic andfumaric acids and their esters. Monobromomaleic acid is reduced to maleic, fumaric, and butadiene-1,2,3,4-tetracarboxylic acid, where the proportions of the products depend on the pH value. Monobromofumaric acid is reduced to fumaric acid. The esters of both acids are reduced to the esters of unsubstituted fumaric acid. Dibromomaleic and dibromofumaric acids and their esters are reduced to acetylene-dicarboxylic acid, and the half-wave potentials of the cis acid and its diethyl ester are less negative (by 0.04-0.18 and 0.10-0.12 V, respectively, depending on the pH value) than the half-wave potentials of the trans compounds. 

Electrolysis of 1,2-dibromofumaric and 1,2-dibromomaleic acid or the diesters produces a 90-100% yield of acetylenedicarboxylic acid or diester 302). For vie-dihalides with no radical or anion stabilizing group in the a position an E2B like elimination mechanism is strongly indicated, /.e., 2e-transfer and double bond formation in a synchronous process (Eq. (148)). 

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