Download Homi Bhabha Centre For Science Education [HBCSE-TIFR], Indian National Chemistry Olympiad [INChO] Past Papers and Sample Question Papers Online
Radicals are commonly known to be highly unstable due to their reactivity. However, in 1960, the synthesis and isolation of a stable radical, 2,2,6,6-tetramethylpiperidine-1- oxyl (TEMPO) changed this notion. Stable radicals are also called persistent radicals. TEMPO, a persistent radical is a red solid compound which has applications in organic synthesis and chemical analysis.
Part A: The redox equilibria of TEMPO 1.1 Draw the resonance structure of TEMPO (including the lone pairs) other than the one shown above. The redox reactions of TEMPO are known to be affected by the pH of the medium. TEMPO undergoes one electron oxidation to yield species A. TEMPO also undergoes reduction (accompanied by protonation) to give B. E(ATEMPO) = 0.745 V (w.r.t. NHE) and E(TEMPO B) = 0.610 V (w.r.t. NHE).
1.2 Draw the structures of A and B.
1.3 Write the half-cell reaction and Nernst equation for the conversion of TEMPO to B (refer 1.2). Derive the relation between the reduction potential (E) of TEMPO and pH. The stability of different redox species at specific potential and pH can be represented on a 2 dimensional plot as shown here for the TEMPO system. Such a plot is known as Pourbaix diagram. In this diagram, the regions between the lines represent conditions at which one particular species is thermodynamically more stable than all other species. The lines represent equilibrium between two species (in this case at equal concentrations) under standard condition (T = 298.15 K). A horizontal line (line 1) separates species related by electron transfer process only. The slant lines (line 2 and 3) separate species related by both electron and proton transfers.
1.4 Draw the structure of C and write the equilibrium process between the species (with the structures) separated by line 4 at pH = 7.4.
1.5 Derive the expression for the ratio slope (line 3) slope (line 2) starting from the corresponding Nernst equations and determine its value using the parameters involved. A disproportionation reaction is one in which two identical units of a species react together to give a reduced and an oxidized species. (Reverse of this reaction is called comproportionation reaction). At pH < 3, TEMPO undergoes disproportionation reaction, yielding A and C.
1.6 Write balanced equation for the disproportionation reaction and determine the equilibrium constant for the reaction at standard temperature? Show the main steps of calculations.
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