REFERENCES:
- Hornback, Organic Chemistry, pp646-651
- Journal of Chemical Education, January 1999, p76
- Roberts. Gilbert, Rodewald and Wingrove, Modern Experimental Organic Chemistry, pp 548-549
INTRODUCTION:
The Grignard reaction if an example of a class of reactions in which the carbon skeleton of a molecule is modified through the formation of a new carbon-carbon bond. This is in contrast to your previous experient in which you performed a functional group interconversion (FGI) with out any change in the basic carbon chain of the original reactant. The Grignard reaction is typical of a large class of "Organometallic" reactions in that the Grignard reagent acts as a source of nucleophilic carbon which forms a new carbon-carbon bond by attacking a relatively electron deficient or electropositive carbon of a second reactant. Typically this second substance is an aldehyde, ketone or organic ester, but the very reactive Grignard reagent will attack any of a large variety of acidic or electropositive reagents. In this sense the organometallic reagents like the Grignard reagent or the Organolithium reagent are similar to any other nucleophilic substances. Before beginning this experiments you should thoroughly review these ideas as they are presented in your lecture text.
EXPERIMENTAL:
Notes:
1. All glassware and the magnesium used in a Grignard synthesis must be scrupulously dry. Even apparently dry glassware can contain moist air and a surprisingly large amount of water adhered to the wall of the glass equipment. This can greatly lower you yield or even prevent the reaction from starting. Place your glassware to be used in the drying oven at the beginning of the laboratory period and before beginning the experiment gently heat all the surfaces of the glassware with a burner for about 5 minutes. ( Be sure that no ether is present during this flaming of your glassware.)
2. Diethyl ether is extremely volatile and its vapors of flammable and explosive. No flames can be present when any ether is being using the the rooms.
3. Due to the occurrence of side reactions in this procedure the reactants for the Grignard procecure will not be used in strictly stoichiometric amounts.
Procedure:
Into a flame dried 250 ml round-bottom flask set up with a magnetic stirring bar, place 0.12 mole of oven-dried magnesium turning and about 10 ml of anhydrous reagent grade diethyl ether from a freshly opened can. Place a dried Claisen head equiped with a dried condenser in its center hole on the flask. On the side arm of the Claisen head put a dried 125 ml separatory funnel containing 0.10 mole of your alkyl halide, and cap both the condenser and the separatory funnel with either drying tubes or dry nitrogen gas (See your insructor). Add about 1 ml of the alkyl halide from the separatory funnel to the ether/ magnesium mixture in the round-bottom flask, and wait several minutes while carefully observing the mixture for signs of reaction. If after five minutes no reaction seems to be occurring consult your instructor for help in initiating the reaction. When the reaction has started immediately add 30 ml of more anhydrous ether directly to the the reaction flask and 20 ml of anhydrous ether to the alkyl halide in the separatory funnel, and beginning the magnetic stirring of the reaction mixture. This reaction is EXOTHERMIC and at this point the ether solvent will begin to reflux vigorously. You must have cold (preferably ice cold) water running through your reflux condenser. Add the alkyl-ether mixture from the separatory funnel slowly at a rate that just maintains a gentle reflux. After the addition is complete, stir the reaction mixture for an additional 15 to 20 minutes as it slowly reaches room temperature.
Place 0.090 mole of a freshly distilled sample of your assigned aldehyde and 25 ml of anhydrous ether in the separator funnel on your Claisen head. Begin a dropwise addition of this solution to the freshly prepared Grignard reagent in the 250 ml round-bottom flask while vigorously stirring the mixture. Use a ice water bath to control the temperature if the reaction begins to get out of control. After all of the aldehyde has been added reflux the mixture for 20 minute. You can stop the Grignard procedure at this point, but if you decided to continue you must go on through the completion of the extraction to the point where the product is over a drying agent.
After addition of the aldehyde cool the mixture in an ice bath and add10mls of dry acetone and 10mls of dry ether to the separatory funnel. This mixture should be swirled briefly and added dropwise to the cold Grignard solution over a period of 30 minutes. Next carefully add 25 ml of deionized water to the reaction mixture; a gelatinous mixture of magnesium salts should be formed. Slowly add enough cold 3M sulfuric acid to completely dissolve the magnesium salts. At this point there should be two liquid phases (organic and aqueous) in the flask,
but little or no solid residue.Transfer the reaction mixture to an appropriate size separatory funnel, and separate the layers putting aside the organic phase (Careful) while you continue to work with the aqueous phase. Extract the aqueous phase with two 25 ml portions of technical ether from the larg solvent can and combine all of these ether extracts with the original ether phase from the reactionl. Wash the combined ether layers with two 25 ml portions of a saturated sodium carbonate solution and once with 25ml of a saturated sodium chloride solution. Place the ether solution over anhydrous sodium or magnesium sulfate to dry until the next laboratory period.
Decant the ether solution from the drying agent and remove the ether by simple distillation unless the boiling point of your product alcohol dictates that a fraction distillation would be more appropriate. When the ether is largely gone transfer the remain oil (product) to an appropriate sized (small) flask and continue the distillation to collect your product. Remember that you should collect the product over a fairly small boiling range (about 5 degrees).
Before preceding with the analysis of your product determine the yield, and then record the appropriate spectral and chromatographic data:
GLPC
IR
NMR(300Mhz)
MS
nD