reduction

Examples of reduction in the following topics:

• Standard Reduction Potentials

  • Reduction potential (also known as redox potential, oxidation/reduction potential, or Eh) measures the tendency of a chemical species to acquire electrons and thereby be reduced.
  • Reduction potential is measured in volts (V) or millivolts (mV).
  • Each species has its own intrinsic reduction potential.
  • Since the reduction potential measures the intrinsic tendency for a species to undergo reduction, comparing standard reduction potential for two processes can be useful for determining how a reaction will proceed.
  • Recall that a positive reduction potential indicates a thermodynamically favorable reaction relative to the reduction of a proton to hydrogen

• Thermodynamics of Redox Reactions

  • The thermodynamics of redox reactions can be determined using their standard reduction potentials and the Nernst equation.
  • Walther Nernst was a German chemist and physicist who developed an equation in the early 20th century to relate reduction potential, temperature, concentration, and moles of electrons transferred.
  • The Nernst equation allows the reduction potential to be calculated at any temperature and concentration of reactants and products; the standard reaction potential must be measured at 298K and with each solution at 1M.
  • This equation allows the equilibrium constant to be calculated just from the standard reduction potential and the number of electrons transferred in the reaction.
  • Translate between the equilibrium constant/reaction quotient, the standard reduction potential, and the Gibbs free energy change for a given redox reaction

• Reduction

  • The metal hydride reductions and organometallic additions to aldehydes and ketones, described above, both decrease the carbonyl carbon's oxidation state, and may be classified as reductions.
  • Other useful reductions of carbonyl compounds, either to alcohols or to hydrocarbons, may take place by different mechanisms.
  • The first diagram below shows how this reduction may be used to convert cyclopentanone to cyclopentane.
  • A possible mechanism for the Clemmensen reduction is displayed in the second diagram below.
  • These derivatives may be isolated and purified before continuing the reduction.

• Reduction

  • At this point it will be useful to consider three kinds of reductions:
  • The exceptional reactivity of acyl halides, on the other hand, facilitates their reduction under mild conditions, by using a poisoned palladium catalyst similar to that used for the partial reduction of alkynes to alkenes.
  • Examples of these reductions are provided in the following diagram.
  • In most nitrile reductions ammonia is added to inhibit the formation of a 2º-amine by-product.
  • Since relatively few methods exist for the reduction of carboxylic acid derivatives to aldehydes, it would be useful to modify the reactivity and solubility of LAH to permit partial reductions of this kind to be achieved.

• Nucleophilic Addition Reactions & Reduction

  • Electron addition to a functional group is by definition a reduction, and we noted earlier that alkynes are reduced by solutions of sodium in liquid ammonia to trans-alkenes.
  • To understand how this reduction occurs we first need to identify two distinct reactions of sodium with liquid ammonia (boiling point -78 ºC).
  • Returning to the reducing capability of the blue electron solutions, we can write a plausible mechanism for the reduction of alkynes to trans-alkenes, as shown below.

• Reductions & Oxidations of Carboxylic Acids

  • Reduction to a 1º-alcohol takes place rapidly on treatment with the powerful metal hydride reagent, lithium aluminum hydride, as shown by the following equation.
  • These reductions take place by the addition of hydride to the carbonyl carbon, in the same manner noted earlier for aldehydes and ketones.
  • The resulting salt of a carbonyl hydrate then breaks down to an aldehyde that undergoes further reduction.
  • Partial reduction of carboxylic acids directly to aldehydes is not possible, but such conversions have been achieved in two steps by way of certain carboxyl derivatives.
  • Reduction of a 1-degree Alcohol when treated with lithium aluminum hydride.

• Balancing Redox Equations

  • Reduction is the gain of electrons—or the decrease in oxidation state—by a molecule, atom, or ion.
  • Write the oxidation and reduction half-reactions for the species that is reduced or oxidized.
  • Reduction: $6H^+ + BiO_3^- \rightarrow Bi^{3+} + 3H_2O$ (Bi goes from a +5 to a +3)
  • Reduction: 3 e− + 2 H2O + MnO4− → MnO2 + 4 OH−
  • An alternative method for balancing reduction/oxidation (redox) reactions.

• Preparation of 2º & 3º-Amines

  • (ii) Reduction of alkyl imines and dialkyl iminium salts.
  • (iii) Reduction of amide derivatives of 1º & 2º-amines.
  • Examples #2 & #3 make use of the carbonyl reductive amination reaction (method #4 in the preceding table.
  • These stable compounds may be isolated, identified and stored prior to the final reduction.
  • A useful variant of the reductive amination method uses formic acid or formate salts as reductants.

• Addition to Carbonyl Double Bonds

  • Selective reduction of 4-tert-butylcyclohexanone (I) to a 10:1 mixture of trans- and cis-4-tert-butylcyclohexanol by LiAlH4 is an example of diastereoselectivity, reflecting a preference for hydride attack at the more hindered axial face of the carbonyl group.
  • A similar influence of steric hindrance is seen in reductions of the bicyclo[2.2.1]heptanones II and III (shown below).
  • The hindrance is reversed in compound III, and the products from LiAlH4 reduction reflect this difference.

• Predicting if a Metal Will Dissolve in Acid

  • The activity series has long been used to predict the direction of oxidation-reduction reactions.
  • Therefore, the half-cell potential for the Zn/Zn2+ electrode always refers to the reduction reaction:
  • These values can be determined using standard reduction potentials, which can often be looked up.
  • Set up the oxidation and reduction half-reactions with their cell potential:
  • Predict whether a metal will dissolve in acid, given its reduction potential