The hydrogenation of buta-1,3-diene catalysed by potassium pentacyanocobaltate(II)

Abstract

The hydrogenation of buta-1,3-diene in aqueous solution catalysed by the pentacyanocobaltate(II) ion (Co) has been explained in terms of the formation of the ions, σ-but-1-en-3-ylpentacyanocobaltate(III), (σ₁), σ-but-2-en-1-ylpentacyanocobaltate(III), (σ₂), π-(1-methylallyl)tetracyanocobaltate(III), (π), and hydropentacyanocobaltate(II), (CoH). The relative and absolute rates of formation of but-1-ene and trans- and cis-but-2-ene are consistent with the following mechanism: CoH + C₄H₆ [graphic omitted] σ₁, σ₁ [graphic omitted] π+ CN^–, σ₁ [graphic omitted] σ₂, σ₁+ CoH [graphic omitted] 1-C₄H₈+ 2Co, σ₂+ CoH [graphic omitted] t-2-C₄H₈+ 2Co, σ₂+ CoH [graphic omitted] c-2-C₄H₈+ 2Co, π+ CoH [graphic omitted] 1-C₄H₈+ 2Co, π+ CoH [graphic omitted] t-2-C₄H₈+ 2Co, π+ CoH [graphic omitted] c-2-C₄H₈+ 2Co,. The following rate constants have been evaluated at 25° and at an ionic strength of 0·5: k₁=(4·56 ± 0 04)× 10² l. mole^–1 min.^–1; k₁/k₂=(3·6 ± 0·7)× 10⁵ l. mole^–1; k₃=(9·4 ± 2·3)× 10^–2 min.^–1; k₄= 104 ± 37 l. mole^–1 min.^–1; k₅=(2·4 ± 0·3)× 10^–3 min.^–1; k₇= 3·2 ± 0·2 l. mole^–1 min.^–1; k₉= 10·6 ± 1·6 l. mole^–1 min.^–1; k_9′= 59 ± 8 l. mole^–1 min.^–1. The remaining rate constants have been determined as ratios: k₆=(1·9 ± 0·6)× 10^–3k_–8′ and k_8″=(1·0 ± 0·2)k_8′. The constant k_8′ has been estimated indirectly, to within a factor of two, as 10 l. mole^–1 min.^–1. The validity of the application of the stready-state hypothesis of the concentrations of the butenyl ion intermediates has been verified.