SYNTHESIS OF HEPT-1-ENE

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.