STEP 1: In a 1-litre
three-necked flask prepare the Grignard reagent, butylmagnesium bromide, from
12.2 g (0.5 mol) of dry magnesium turnings, a small crystal of iodine, 68.5 g (53 ml,
0.5 mol) of butyl bromide and 260 ml of anhydrous ether.
STEP 2: Equip a 500-m1 three-necked flask with
a sealed stirrer unit, a 100-m1 separatory funnel and a double surface condenser.
Force the solution of the Grignard reagent with the aid of pure, dry
nitrogen and a tube containing a plug of purified glass wool (1) into the 500-m1
flask through the top of the double surface condenser.
STEP 3: Charge the
separatory funnel with a solution of 50 g (35 ml, 0.42 mol) of allyl bromide in 25 ml of anhydrous ether; place calcium chloride drying tubes
into the top of the double surface condenser and of the dropping funnel. Immerse the flask containing the
Grignard reagent in cold water, stir vigorously, and add dip allyl bromide at such a
rate that the ether boils gently; cool momentarily in ice if the reaction
becomes too vigorous.
STEP 4: It is important that the allyl bromide reacts when
added, as indicated by gentle boiling of the solution (2).
STEP 5: When all the allyl
bromide has been introduced, continue stirring for 45 minutes while refluxing
gently by immersing the flask in a bath of warm water.
STEP 6: Allow to cool (3).
STEP 7: Pour
the reaction mixture cautiously on to excess of crushed ice contained
in a large beaker.
STEP 8: Break up the solid magnesium complex and decompose it
with ice and dilute sulphuric acid or concentrated ammonium sulphate solution.
STEP 9: Separate the ether layer, wash it with ammoniacal ammonium sulphate
solution to remove any dissolved magnesium salts and dry over magnesium
sulphate.
STEP 10: Distil the dry ethereal solution through a fractionating
column: after the ether has passed over, collect the hept-l-ene at 93-95 °C. The
yield is 29 g (71%).
Notes to keep in mind:
1. Solid magnesium must be absent to avoid the formation
of biallyl via allyl
magnesium
bromide; the insertion of a short plug of glass wool effectively removes any finely divided magnesium or alternatively use a tube terminating in a glass frit.
2. If reaction
does not occur when a little allyl bromide is first introduced, further addition must
be discontinued until the reaction has commenced. Remove 2-3 ml of the Grignard
solution with a dropper pipette, add about 0.5 ml of allyl bromide and warm gently to
start the reaction; after this has reacted well, add the solution to the main portion
of the Grignard reagent.
3. A slight excess
of Grignard reagent should be present at this stage. Test for the presence of
the reagent as follows. Remove 0.5 ml of the clear liquid with a dropper pipette and
add 0.5 ml of a 1 per cent solution of Michler's ketone [4,4'-bis(dimethylamino)benzophenone]
in benzene, followed by 1 ml of water and 3-4 drops of 0.01 M iodine in
glacial acetic acid; shake. A greenish-blue colour results if a Grignard
reagent is present. In
the absence of iodine, the colour fades. A dye of the diphenylmethane type is
produced.
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