Kinetics and mechanism of the reaction of mercury(II) with a water-soluble octabromoporphyrin

✍ Scribed by N. Nahar; M. Tabata

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 141 KB
• Volume: 02
• Category: Article
• ISSN: 1088-4246
• DOI: 10.1002/(sici)1099-1409(199807/10)2:4/5<397::aid-jpp100>3.0.co;2-w

✦ Synopsis

The reaction of mercury(II) hydroxide with 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin, [Formula: see text]( H ~2~P^4−^), to form the mercury(II) porphyrin [( TPPSBr ~8~) Hg ]^4−^( HgP ^4−^) was investigated in the pH range 6.2-8.5. The observed rate constant was first-order with respect to the mercury(II) concentration and decreased with increasing pH from pH 6.2 to 7.5 and then increased from pH 7.5 to 8.5. The rate expression was written as d [ HgP ^4−^]/dt = (k~HP~K~−1~[ H ^+^]^−1^ + k ~H ~2~ P~ + k ~H ~3~P~ K ^1^[ H ^+^])(1 + K~1~[ H ^+^] + K~−1~[ H ^+^]^−1^)^−1^[ Hg ( OH )~2~][ H ~2~ P ^4−^]. The k~HP~, k~H~2~P~ and k~H~3~P~ values were found to be (1.33 ± 0.02) × 10^8^, (5.50 ± 0.08) × 10^6^ and (1.40 ± 0.08) × 10^8^ M ^−1^ s^−1^ respectively, with K~1~ = [ H ~3~ P ^3−^][ H ~2~ P ^4−^]^−1^[ H ^+^]^−1^ = 10^4.83 ± 0.04^ and K~−1~ = [ HP ^5−^][ H ^+^][ H ~2~ P ^4−^]^−1^ = 10^10.02 ± 0.02^. The activation parameters were [Formula: see text] and ΔS^‡^ ~HP~ = 226 ± 22 J K ^−1^ mol ^−1^ for the k ~HP~ path, [Formula: see text] and [Formula: see text] for the k ~H ~2~ P~ path and [Formula: see text] and [Formula: see text] for the k ~H ~3~ P~ path. The kinetic results show the high reactivity of mercury(II) hydroxide towards the protonated porphyrin.