This preparation describes the
production of a tertiary alcohol from a Grignard reagent and a ketone.
The reaction is performed in a
reflux apparatus which has a calcium chloride guard tube inserted in the top of
- 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
- 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
- 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
- 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
- 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
- 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.