Oxidative behaviours and relative reactivities of some aliphatic, heterocyclic and aromatic aldehydes towards bis(dihydrogentellurato )cuprate(III) and argentate(III) in alkaline medium

Abstract

The oxidative behaviours and relative reactivities of some aliphatic aldehydes, XCHO (where X = H - , Me - , MeCH₂ -, Me₂CH-), 2-furaldehyde and aromatic aldehydes, YC₆H₄CHO (where Y= -H, -NO₂, -Cl and -OMe) towards bis(dihydrogentellurato)metallate(III) ion (where M^IlI= CU^llI or Ag^III) in alkaline medium have been investigated. The reactions are first order each in [aldehyde] and [M^III(H²TeO⁶)²]^5- - but the rate is independent of [OH^-]. The values of k_obs in the oxidation of some aliphatic aldehydes, 2-furaldehyde and benzaldehyde in aqueous medium follow the order: Me₂CH - > MeCH₂ - > Me - > H - > C₄H₃O - > C₆H₅, -. The electron withdrawing substituents facilitate the rate of oxidation of benzaldehyde whereas electron donating substituents have opposite effect and the values of k_obs, follow the order: - NO₂ > - H > - Cl > - OMe. The pseudofirst order rate constants decrease considerably in benzaldehyde-d₁., than benzaldehyde under comparable condition of experiments. Irrespective of whether the substrate is aliphatic or aromatic, the oxidation reactions occur via free radical mechanism although the substituent and isotope effects in the oxidations of aromatic aldehydes indicate that C - H bond cleavage via proton transfer may also occur. The activation parameters of the reactions have been determined and the mechanism of the reactions is discussed.