Chemistry 351 Laboratory - Summer 2012 – Professor T. Nalli, Winona State University

Expt #1. Oxidation of 2-Pentanol and Distillation of the Product

Relevant textbook reading - Klein, Chapter 13, Mohrig, Technique 11.1-11.4, 16.3-16.4, 13 (especially 13.4).

Literature References - (a) Stevens, R.V.,Chapman, K. T.,and Weller. H. N., J. Org. Chem. 1980, 45, 2030-2032. (b) Mohrig, J. R.; Nienhuis, D. M; Linck, C. F.; Van Zoeren, C.; Fox, B. G.; Mahaffy, P. G. J. Chem. Educ. 1985, 62, 519-521.

Overview – 2- Pentanol will be oxidized using hypochlorous acid (HOCl) to form 2-pentanone (eq 1). The HOCl is generated in situ from the reaction of acetic acid with aqueous sodium hypochlorite (Clorox). The product will be carefully distilled and characterized by IR, NMR, and refractive index determination.

 

Background - The method we are using for alcohol oxidation is regarded as a relatively "Green" method because it does not employ heavy metal based oxidizing agents (e.g., Cr or Mn) nor does it generate any toxic side products. It is often referred to as the Chapman-Stevens oxidation in honor of the scientists who first reported it.

The mechanism has been proposed to start with a proton transfer to the HOCl oxidant. The resulting H2OCl+ is attacked nucleophilically by the alcohol to generate (after proton transfer) an ROCl intermediate, which then undergoes an E2-like reaction to form the ketone.

In order for the oxidation to reach completion excess HOCl must be used. However, the concentration of hypochlorite in household bleach, while generally stated as 6.0%, varies significantly. Therefore, the presence of excess HOCl cannot be assured by simply using the a predetermined volume calculated based on the bleach concentration. Instead, we will test for the presence of HOCl using KI/starch test paper. This paper is impregnated with iodide ions and starch and will turn black if treated with an oxidizing agent (e.g. HOCl) which oxidizes the iodide ions to iodine molecules (I2), which in turn form a blue-black complex with the starch (eq 2).

Procedures

Running the reaction.

  • Place 0.10 mol of 2-pentanol and a magnetic stir bar in a 500-mL Erlenmeyer flask.
  • Carefully add 25 mL of glacial acetic acid to the pentanol with stirring.
  • Suspend a large separatory funnel over top of the flask using a metal ring.
  • Add 135 mL of bleach to the separatory funnel.
  • Use the separatory funnel to add the bleach drop by drop to the alcohol/acetic acid mixture over a period of about 15 min. Monitor the temperature of the solution during the addition and apply an ice bath and/or slow the addition rate as necessary to keep it in the range of 25-45 °C.
  • When the addition of the bleach is complete test the solution for excess hypochlorous acid and, if necessary, add additional bleach. (See background above.)
  • Stir the reaction mixture for 30 min at room temperature.

Work-up procedures.

  • Destroy excess HOCl by adding 1 mL saturated sodium bisulfite (NaHSO3). Test the reaction for any remaining hypochlorous acid and, if necessary, add additional sodium bisulfite.
  • Add 10 drops of thymol blue (a pH indicator).
  • Add enough 6M NaOH until the solution turns a light blue (indicating a basic pH). (The approximate volume of NaOH solution necessary for this step can be calculated by simply equating the moles of NaOH necessary to the moles of acetic acid started with.
  • Add solid NaCl with stirring until no more will dissolve upon addition.
  • Decant the liquid into a separatory funnel and extract with 20 mL CH2Cl2 taking care to vent frequently. (Review extraction procedures in Mohrig, 11.1-11.4.)
  • Transfer the organic layer to a labeled, dry 125-mL Erlenmeyer flask.
  • Extract the aqueous layer two more times with 20 mL CH2Cl2 each time combining the organic layer with the original organic layer.
  • Dry the organic extracts with Na2SO4.
  • Decant the product solution into a 100 mL round bottom flask and remove most of the DCM solvent by simple distillation. (When the distilling temperature increases significantly beyond that of DCM go to the next step!) (see Technique 13.3 in Mohrig.)
  • Transfer the remaining liquid into a 10-mL round bottom flask and carry out fractional distillation of the product. (see Technique 13.4 in Mohrig.)
  • Start with a small Erlenmeyer or vial as the receiving vessel and collect the distillate that comes over at temperatures significantly lower then the lit bp of 2-pentanone. Once the distillation temperature levels out at a value near the literature value, then switch the receiving vial to a clean, preweighed vial and collect your final product until the temperature drops significantly.

Characterization of Product

Obtain the 1H NMR spectrum using CDCl3 as the solvent.

Obtain an IR spectrum.

Measure the refractive index of the product. Mohrig technique 16.3-16.4.

Assigned Questions

1. Write an equation for how the first step of the work up procedures works to destroy left over HOCl.
2. What was the purpose of the step where NaCl was added?