Synthesis of pinacol and pinacolone

Objective

To synthesize α-glycol (pinacol) from ketone by radical condensation and to dehydrate α-glycol to form pinacolone.

Background

Glycols can be synthesized by numerous methods, and this product can be used as starting olefin or unsaturated carbonyl compounds. For example, from olefins the synthesis can be carried out by adding halogen, by adding hypohalogenous acid (XOH), by epoxidation with peracid and subsequent hydrolysis of the epoxide formed, or by adding OsO₄ and/or MnO₄^– . Moreover, pinacol synthesis is induced by a reductive coupling of acetone. This ketone is treated with an active metal (Na or Mg), resulting in a transfer of an electron to the carbonyl producing a cetyl radical, which dimerizes, leading to C−C bond formation and yielding the diol. With aromatic ketones the synthesis can also be carried out photochemically. Thus, the pinacolinic transposition is formally a dehydration between two molecules ketone catalyzed by an acid medium.

Experimental procedure

A) Preparation of pinacol (2,3-dimethyl-butane-2,3-diol)

In a dry 100 ml round-bottom flask, place 3 g of magnesium and 0.5 g mercuric chloride HgCl₂. Add 10 ml of anhydrous acetone and stir until the reaction begins. Adapt the flask with a reflux condenser and gradually add another 40 ml of acetone. Stir for 15 to 20 min, and add another 25 ml of acetone. After 30 to 45 min, heat the mixture in a water bath to maintain a good reflux rate. After 1 h of reflux, end the reaction. Remove the flask and connect the condenser via an adapter to a vacuum line, while continuing to heat the flask, thereby removing the excess acetone (rotary evaporator can alternatively be used), and magnesium pinacolate is as a powder. Reconnect the reflux condenser and add 100 ml of water containing 5 g of trisodium phosphate through the condenser (mildly heating the reaction). Reflux the flask for 15 min. Vacuum filter the hot mixture and cool the filtrate in an ice bath with stirring. A precipitate will appear. When the crystallization is complete, vacuum filter and wash the resulting crystals with 25 ml of ice water or with 25 ml of petroleum ether.

B) Preparation of pinacolone (2,3-dimethyl-butan-2-one)

In a 100 ml Erlenmeyer flask, place 15 ml of water and add slowly and carefully 10 ml of H₂SO₄ (conc.). Then add 6 g pinacol previously prepared and dissolved, transfer to a flask, and distill. Allow the distillate to stand and then dry over anhydrous sodium sulfate Na₂SO₄ and gravity filter.

Physico-chemical properties

This table collects data for the molecular weight (M_w), melting point (M.p.) boiling point (B.p.) and density of the reactives and compounds used in this laboratory experiment.

GHS pictograms

Hazard pictograms form part of the international Globally Harmonized System of Classification and Labelling of Chemicals (GHS) and are collected in the followinf Table for the chemical compounds used in this experiment.

Name	GHS
Acetone
H2SO4
HgCl2
Magnesium
Na3PO4
Petroleum ether
Pinacol
Pinacol hexahydrate	See MSDS
Pinacolone
p-Xylene

International Chemical Identifier

The IUPAC InChI key identifiers for the main compounds used in this experiment are provided to facilitate the nomenclature and formulation of chemical compounds and the search for information on the Internet for these compounds.

References

• Isac-García, J.; Dobado, J. A.; Calvo-Flores, F. G.; and Martínez-García, H. (2015). Experimental Organic Chemistry Laboratory Manual. Elsevier Science & Technology. ISBN: 978-0-12-803893-2
• Vogel, A.I., Furniss, B.S., Hannaford, A.J., Tatchell, A.R., and Smith, P.W.G. (1989). Vogel’s Textbook of Practical Organic Chemistry (Vogel’s Textbook series). Longman. ISBN: 9780470214145