Francois Auguste Victor Grignard
(1871 - 1935)
Nobel Prize for Chemistry 1912
(with Paul Sabatier)

        This preparation describes the production of a tertiary alcohol from a Grignard reagent and a ketone.

        The Grignard reagent is butylmagnesium bromide, CH3CH2CH2CH2MgBr, which is reacted with propanone. The structure of Grignard reagents is, perhaps surprisingly, not wholly clear. That it is complexed in some way with the solvent is known, as is its behaviour as a nucleophile, effectively CH3CH2CH2CH2- .

        The reaction is performed in a reflux apparatus which has a calcium chloride guard tube inserted in the top of the condenser.

Hazards: the major hazard is the explosive nature of ethoxyethane vapour, followed closely by its anaesthetic effect. This preparation should only be done in an efficient fume cupboard, with no naked flames anywhere in the laboratory. Ethoxyethane vapour is very dense and can creep long distances along bench-tops and floors. Any attempt at this preparation must be based on the full text of the recipe (from Vogel's 'Handbook of Preparative Organic Chemistry') and a risk assessment appropriate to the institution must be made.

  • Place a 250 cm3 three-necked round-bottomed flask, a double-surface water condenser, a tap funnel and a guard tube containing anhydrous calcium chloride in an oven at 120oC for an hour or so.
  • What is a double-surface condenser, and why is it needed?
  • A double-surface condenser has a water jacket both on the outside and through the centre of the condensing tube. It is needed where, as here, the condensed liquid is very volatile.
  • What is the purpose of the guard tube?
  • The guard tube prevents water vapour from entering the apparatus from the atmosphere.
  • Why is the apparatus and the reagents dried?
  • Grignard reagents react easily with water. Any moisture present will prevent the reaction from occurring.
  • Allow the apparatus to cool, and assemble it as a refluxing apparatus with the guard tube in the top of the condenser,  a tap funnel in the second neck of the flask and a mechanical stirrer in the third neck. The apparatus should be clamped so as to leave room for the use of a cooling bath of iced water around the flask if necessary.
  • Why might the apparatus need to be cooled during the reaction?
  • The reaction is exothermic.
  • Place in the flask 6.2g of magnesium turnings (those for the preparation of Grignard reagents) and 100 cm3 of dry ether. Place 27 cm3 of dried 1-bromobutane in the tap funnel.
  • Justify the quantities used.
  • There is 0.25 mol of magnesium and 0.25 mol of 1-bromobutane (density 1.28 g cm-3), this being the 1:1 ratio in which the reactants react.
  • What is used to dry the ethoxyethane?
  • Sodium metal.
  • What is used to dry the 1-bromobutane?
  • Anhydrous potassium carbonate.
  • Run in about half of the 1-bromobutane and stir briefly. When the mixture has settled, drop in one or two small crystals of iodine so that they rest on the surface of the magnesium - do not stir. The reaction should begin within a few minutes; the reaction mixture should be allowed to reflux gently with cooling as necessary using an ice-water bath. During this time add the remaining 1-bromobutane in small portions.
  • What is the purpose of the iodine crystals?
  • The iodine initiates the reaction. (It is often said that it is catalytic, but it cannot be recovered from the reaction mixture at the end - it is chemically changed, so is not really catalytic.)
  • When the preparation of the Grignard reagent is complete the liquid in the flask will be cloudy and of a greyish colour. Place in the tap funnel a solution of 18.5cm3 of dried propanone in 19 cm3 of dried ethoxyethane. Stir the mixture in the flask rapidly, and add the propanone solution slowly, cooling the flask in ice if necessary. This reaction is vigorous. When all the propanone has been added the mixture should be allowed to stand overnight.
  • Why is the propanone dried?
  • Water (with which propanone is miscible in all proportions) would decompose the Grignard reagent to give an alkane.
  • Suggest a suitable drying agent for propanone.
  • Anhydrous potassium carbonate or anhydrous calcium chloride.
  • Decompose the product by pouring the reaction mixture on to 150g of crushed ice, stirring well. Dissolve the precipitated magnesium compounds by the addition  of 10% aqueous hydrochloric acid with stirring until the precipitate has disappeared. Transfer the mixture to a separating funnel, and run off and keep the lower aqueous layer; transfer the ethoxyethane layer to a suitable flask.
  • What is the precipitated magnesium compound?
  • Magnesium hydroxide.
  • Which ions are present in the aqueous solution after treatment with hydrochloric acid?
  • Magnesium ions, and bromide and chloride ions.
  • Return the aqueous layer to the funnel, and wash it with 15cm3 of ethoxyethane. Keep the aqueous layer as before, combining the ethoxyethane solution with that separated earlier. Repeat the washing of the aqueous layer with 15cm3 of ethoxyethane twice more, combining all the washings with the original solution.
  • Why is the aqueous layer washed with ethoxyethane?
  • The product alcohol is significantly soluble in water, so the washing with fresh ethoxyethane removes some of the dissolved alcohol.
  • Why is the aqueous layer washed three times with 15 cm3 of ethoxyethane rather than with one 45 cm3 portion?
  • The alcohol in the aqueous layer is in equilibrium with that in the ethoxyethane layer. It can be shown that three washings are more efficient at removing the alcohol rather than one washing with the same total volume of ethoxyethane. This is a general principle in solvent extraction.
  • Add some anhydrous potassium carbonate to the ethoxyethane solution, and leave to stand until it is clear.
  • What is the purpose of adding potassium carbonate?
  • It is a drying agent.
  • What is the significance of the liquid becoming clear?
  • Wet organic liquids that are not water miscible are cloudy; the clearness shows that the ethoxyethane solution is now dry.
  • Transfer the ethoxyethane solution to a clean distilling flask, and distil off the ethoxyethane using a warm water bath.
  • Why is a warm water bath used to heat the flask?
  • Ethoxyethane vapour is explosive in air, so no naked flames must ever be used anywhere in a room where ethoxyethane is being used.
  • When all the ethoxyethane has been removed, fractionally distil the remaining liquid using an electric heating mantle, collecting the fraction that boils between 137-141oC.
  • Why is the range between 137-141oC chosen?
  • The product alcohol boils at 139oC so distils over in this range.
  • Why is an electric heating mantle used?
  • A boiling water bath would be too cool, and although the ethoxyethane has been distilled off there is still enough vapour around to make the use of flames dangerous.

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JRG Beavon 2002