Experiment 14 - Grignard Reaction

Overview of a Grignard reaction

- Grignard reagents (ex. very polar, carbon metal bond) are used to introduce C-C bonds in molecules. - the Grignard reagent reacts as a carbon nucleophile. - Grignard reagents w/ aldehydes give a secondary alcohol, but with ketones give a tertiary alcohol.

Objectives of the experiment

- be able to handle moisture-sensitive reagents. - synthesize an alcohol from the grignard reaction.

What is the point of the work up step?

- generally, this step is involves a series of manipulations required to isolate and purify the product(s) of a chemical reaction. these manipulations may include: - separating the reaction mixture into organic and aqueous layers by liquid-liquid extraction. - removal of solvents by evaporation. - quenching a reaction to deactivate any unreacted reagents. - in this lab, we used a work-up step to protonate the negatively charged oxygen (alkoxide) to give the alcohol.

What was the limiting reagent and the reagent in excess?

- limiting reagent: ketone or aldehyde - reagent in excess: Grignard reagent (ex. phenyl magnesium bromide)

Why is it important for water to be excluded when running a Grignard reaction?

- the Grignard reagent is very reactive with water, and if it does react w/ water or moisture the experiment will be damaged.

Quick steps of the mechanism

- the grignard reagent first attacks a carbonyl compound (aldehyde, ketone, and ester) to form an alkoxide ion. - acidification of solution: we add an acid to neutralize the O and make the alcohol.

What can one do to avoid pressure build-up during the extraction step?

- ventilate the system (remove the cap off the separatory funnel) every five minutes or so.

Safety Precautions

- water & moisture is detrimental to the reaction; all glassware must be clean and pre-dried in an oven. - ether is extremely flammable & should be kept away from heat sources such as hotplates; it is also very volatile, thus it should be kept in well-ventilated areas. - grease all joints in apparatus; mate them tightly to prevent diethyl ether from escaping. - the rxn of Grignard reagents is exothermic; the temperature of the rxn must be controlled with an ice bath; make sure the ether does not boil too vigorously.

What is the electrophile in in the reaction?

the aldehyde or ketone compound.

Which carbon in the Grignard reagent acts as a nucleophile and why?

the carbon at the side of the benzene ring (that is attached to the "MgBr" in our lab) is the nucleophile, due to the very polar carbon metal bond.

Why is diethyl ether used during the extraction step?

this solvent will form two layers in contact with aqueous solutions if it is used in sufficient quantities; this is because the solvent is immiscible in water. Also, because it is less dense than the aqueous layer, it'll appear as the top layer.