Organometallic compounds:
“Organometallic compounds are those compounds that have at least one carbon-metal compound.”
Grignard reagent:
They are named after the Victor Grignard (Nobel prize 1912) who discovered and developed their use as synthetic reagent.
➢ General formula:
RMgX or R-MgX
Where: R=alkyl or aryl group
X=halogens (Cl,Br,I)
➢ Preparation:
•They are prepared in laboratory by the action of alkyl halides on magnesium metal in presence of dry ether.
•Dry ether is pure anhydrous diethyl ether.
R-X + Mg…R-MgX
➢ Laboratory method:
~The apparatus uses round bottom flask which is fitted with a reflux condenser and a dropping funnel.
~Magnesium ribbon cut into small pieces is suspended in dry ether placed in round bottom flask.
~Approximately 1:1 mixture of the alkyl halide and dry ether is placed in dropping funnel.
~Add 2-3 ml of solution from the dropping funnel into round bottom flask and wait till reaction
commences.
~When reaction starts, the ether becomes cloudy an also begins to boil gently. Further,
there are visible signs of attack on magnesium.
~If the reaction doesn’t start, drop one or two crystals of iodine in the flask.
~Once the reaction starts, the addition of solution from the dropping funnel is maintained at such
rate that the ether refluxes gently.
~When the reaction Is complete, a clear solution of the Grignard reagent in ether is obtained.
~This is then treated with IN SITU with various reagents to get desired synthetic products.
➢ Role of ether:
Usually diethyl ether is used as a solvent. It is thought that two ether oxygen atoms complex with magnesium atom, stabilizing the reagent.
➢ Precautions:
1.All reagent should be anhydrous and apparatus oven-dried. Moisture reacts with Grignard reagent
an prevents its formation.
RMgX + water…RH + MgX(OH)
2.There should be no naked flames anywhere near the apparatus. Diethyl ether catches fire very quickly.