Kinetic Study of the Bromine‐Catalyzed Isomerization of Maleic Acid in Aqueous Ce(IV)‐Br − ‐H 2 SO 4 Medium

The presence of ceric and bromide ions catalyzes the isomerization of maleic acid (MA) to fumaric acid (FA) in aqueous sulfuric acid. A kinetic study of this bromine‐catalyzed reaction was carried out. The reaction between ceric ion and maleic acid is first order with respect to Ce(IV). For [Ce(IV)] 0 =5.0×10 −4 M , [H 2 SO 4 ] 0 =1.2 M , μ=2.0 M (adjusted by NaClO 4 ), and [MA] 0 =(0.5–1.0) M , the observed pseudo‐first‐order rate constant ( k 03 ) at 25° is k 03 =7.622×10 −5 [MA] 0 /(1+0.205[MA] 0 ). The reaction between ceric and bromide ions is first order with respect to Ce(IV). For [Ce(IV)] 0 =5.0×10 −4 M , [H 2 SO 4 ] 0 =1.2 M , μ=2.0 M , and [Br − ] 0 =(0.025–0.150) M , the pseudo‐first‐order rate constant ( k 02 ) at 25° is k 02 = (4.313±0.095)x10 −2 [Br − ] 2 +(2.060±0.119)x10 −3 [Br − ]. The reaction of Ce(IV) with maleic acid and bromide ion is also first order with respect to Ce(IV). For [Ce(IV)] 0 =5.0×10 −4 M , [MA] 0 =0.75 M , [H 2 SO 4 ] 0 =1.2 M , μ=2.0 M , and [Br − ] 0 = (0.025–0.150) M , the pseudo‐first‐order rate constant ( k 03 ) at 25° is k 03 = (5.286±0.045)x10 −2 [Br − ] 2 +(3.568±0.056)x10 −3 [Br − ]. For [Ce(IV)] 0 =5.0 × 10 −4 M , [Br − ] 0 =0.050 M , [H 2 SO 4 ] 0 =1.2 M , μ=2.0 M , and [MA] 0 =(0.15–1.0) M at 25°, k 03 =(2.108×10 −4 +2.127×10 −4 [MA] 0 )/(1+0.205[MA] 0 ). A mechanism is proposed to rationalize the results. The effect of temperature on the reaction rate was also studied. The energy barrier of Ce(IV)—Br − reaction is much less than that of Ce(IV)—MA reaction. Maleic and fumaric acids have very different mass spectra. The mass spectrum of fumaric acid exhibits a strong metastable peak at m / e 66.5.