Chemistry, Laboratory

New Leaving Cert Chemistry Experiment

The powers that be have decided that the oxidation reaction using Cr(VI) is now too dangerous for our school students to carry out, and have proposed to replace this reaction with the oxidation of phenylmethanol (benzyl alcohol) to benzoic acid. Some details about this reaction are below.*

phenylmethanol to benzoic acid scheme

Oxidation of Phenylmethanol

Procedure

Phenylmethanol (1 mL) is added to a 100 mL conical flask.  A solution of potassium permanganate (25 mL, 0.2 M) and sodium carbonate (0.5 g) are added and the solution heated at about 60 °C for 20 minutes on a water bath. The solution turns brown on heating. After cooling slightly, a few drops of conc. HCl are added until the solution was acidified; i.e. until no more fizzing is apparent. The solution clarifies leaving a brown residue of manganese dioxide. Finally, sodium sulfite (a few drops of saturated solution) is added until the solution cleared. Benzoic acid precipitates as the solution cooled. This is filtered off using a Hirsch funnel and weighed when dry (0.6g, 50%).

Sequence of reaction from left to right: (i) addition of permanganate and sodium carbonate; (ii) solution cooled slightly after twenty minutes; (iii) addition of conc. HCl; (iv) precipitation of product; (v) isolation and weighing.
Sequence of reaction from left to right: (i) addition of permanganate and sodium carbonate; (ii) solution cooled slightly after twenty minutes; (iii) addition of conc. HCl; (iv) precipitation of product; (v) isolation and weighing.

About the reaction

The reaction involves oxidation with MnO4. The mechanism is not well understood, but the permanganate provides the oxygen atoms necessary for the oxidation. The oxidation state of Mn changes over the course of the reaction. The initial solution of permanganate will be dark purple. As it oxidises the phenylmethanol, it itself will be reduced to Mn4+, precipitating out as MnO2 – the brown powder is visible as the reaction progresses. In order to complete the reaction under necessary alkaline conditions, sodium carbonate is added. The Mn4+ is again reduced to soluble Mn2+, using sodium sulfite, which allows for all solid product to be easily isolated by filtration, and safe disposal of filtrate. Benzoic acid is sparingly soluble in water, and as the reaction cools, it will precipitate out.

Purpose of each reagent

  • KMnO4: This is the oxidising agent, which will oxidise phenylmethanol to benzoic acid. In the process, it is itself reduced.
  • Na2CO3: This increases the pH of solution, so that it is alkaline. This is necessary for the reaction to proceed as indicated.
  • HCl: This is used to acidify the solution. In doing so it neutralises the sodium carbonate. HCl also protonates the benzoate anion, forming benzoic acid, so that it will precipitate out of solution when the reaction cools.
  • Na2SO3: This is used to reduce the MnO2 precipitate so that it can be solubilised.

Analysis

The density of phenylmethanol is 1.05 g/mL. Therefore mass used in reaction is 1.05 g. As the molecular mass of phenylmethanol is 108.1 g/mol, this means that 9.7 x 10-3 mol of reactant were used, and hence 9.7 x 10-3 mol of product (122.1 g/mol) expected. This corresponds to a theoretical yield of 1.19 g. Using the above conditions, a yield of 0.6 g (50% was obtained).

Melting point of benzoic acid: 122 °C.

*Thanks to Gráinne Hargaden, Claire Mc Donnell, Maria Sheehan, Marie Walsh.