Download 32 GRIGNARD REACTION Alkyl halides can react with magnesium

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GRIGNARD REACTION
Alkyl halides can react with magnesium to give a very versatile synthetic reagent, the
Grignard reagent, for which Victor Grignard received the Nobel Prize in 1912. The Grignard
reagent, RMgX, is one of the most useful reagents known. Although the carbon magnesium
bond is covalent, it is highly polar with a significant amount of the electron density associated
with the R group; therefore, the reagent is highly reactive. The Grignard reagent can act as a
base and react with any acidic hydrogens that maybe present in the reaction mixture or it can act
as a nucleophile and react with an electrophilic reagent. Thus, it can react with water or an
alcohol to give a hydrocarbon (acid/base reaction), with carbon dioxide to give a carboxylic acid
(nucelophilic/electrophilic reaction) and with many other organic compounds to give a variety of
products. It is primarily used to react with aldehydes and ketones to give secondary and tertiary
alcohols, respectively, or with CO2 provide carboxylic acids after an acid workup.
In today's experiment a carboxylic acid is prepared from a Grignard reagent. Diethyl ether
is one of the best solvents for the Grignard reaction and is thought to solvate the Grignard
reagent.
BUT GREAT CAUTION MUST BE TAKEN BECAUSE OF THE DANGER OF
SERIOUS FIRES. THEREFORE NO OPEN FLAMES OR HOT PLATES ARE TO BE
USED DURING THIS EXPERIMENT.
The Grignard reagent reacts with the CO2 to give a carboxylic acid (after treatment with
HCl) containing an additional carbon atom. Specifically, in today's lab, you will be preparing
benzoic acid.
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Experimental Procedure
ETHER IS VERY FLAMMABLE - NO FLAMES OR HOT PLATES ALLOWED.
BROMOBENZENE IS A POWERFUL SKINIRRITANT - WASH HANDS AFTER
HANDLING.
All your apparatus must be ABSOLUTELY DRY for this experiment, since Grignard
reagent reacts with water to give hydrocarbons (benzene in this case).
Mount a 50 mL round bottom flask with a condenser. Do not run water through your
condenser. Place 0.7 g Mg turnings in the flask. Add 10 mL of dry ether, followed by 0.5 mL of
bromobenzene. Your TA will then come around and add a few crystals of iodine to your
reaction mixture.
Place the flask in a beaker of warm water (hot water from the tap is warm enough) for up
to 5 minutes to see if a reaction occurs (indicated by spontaneous boiling of the ether and/or a
colour change). Remove the beaker of warm water occasionally to make sure the boiling
continues spontaneously. If there is still no spontaneous bubbling or no colour change consult
your demonstrator. If your reaction has begun, take a solution of 2.5 mL of bromobenzene in
10 mL of ether and add 1 mL at a time through the top of the condenser. Wait until the bubbling
has stopped before adding the next portion. If the bromobenzene is added too fast the yield will
go down. (If the bromobenzene concentration in the reaction mixture is too high it reacts with
the Grignard reagent to form biphenyl.) Warm the reaction during this addition of reagents,
using only warm water from the tap. When the spontaneous reaction ceases the reaction is
essentially over.
Take 10 g of crushed dry ice (solid CO2), wipe off any excess moisture with a towel, and
place in a 250 mL beaker. Add 5 mL of anhydrous ether and pour the Grignard reagent with
vigorous stirring over the dry ice.
Then add 30 mL of dilute hydrochloric acid in small
quantities with vigorous stirring. The resulting benzoic acid is insoluble in aqueous HCl and
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should precipitate out at this point. If no solid is seen, then perhaps too much ether has been left.
In this case, heat your beaker gently on the STEAM BATH IN THE FUMEHOOD until solid
precipitates out. Cool and filter by suction to obtain the crude benzoic acid. Wash with
2 portions of 5 mL of cold water. Dissolve this crude acid in a solution of 5 g of sodium
bicarbonate in 30 mL of water. This step is very slow and gentle heating may be necessary for
the majority of the solid to go into solution. Filter off any insoluble impurities, cool and
reprecipitate the benzoic acid by adding concentrated hydrochloric acid drop wise with stirring
until the solution is acidic to litmus (WARNING EFFERVESCENCE).
Note that this
neutralization is slow and a positive acid test may be given before the neutralization is actually
complete. For this reason allow plenty of time for this step. Filter off the precipitate and
recrystallize the benzoic acid by adding 30 mL of water, heat to dissolve and allow to cool.
Finally, filter the benzoic acid by suction and wash with a little cold water. Determine the
melting point and yield of the dry products.
Reference:Organic Chemistry (6th edn., 2004) by J. McMurry; Sections 10.8, 17.6, 19.8, 20.6,
21.4, 21.6.