Expt 10 The Grignard Reaction: Synth of Benzoic Acid
We can NOT leave containers/flasks with ether open at any point before, during, or after this experiment. We must always cap the container or use a stopper. Why or Why not?
This is true, - diethyl ether is extremely flammable and can "creep" along bench tops to ignition sources
How is a Grignard reagent formed and what is it's purpose?
- A Grignard reagent is formed by the reaction of an alkyl halide (bromide) with magnesium metal in anhydrous ether − Its purpose is to form new carbon-carbon bonds
What is the purpose of the Grignard reagent in this particular experiment?
- The Grignard reagent in this experiment will be added to carbon dioxide (dry ice) and used in hydrolysis to obtain benzoic acid
Suppose that instead of reacting your Grignard reagent with CO2, you added benzophenone to the prepared Grignard reagent. What product would form? Assuming no loss of Grignard reagent to side reactions and addition of excess benzophenone, how much of that product could you form from 0.70 mL of bromobenzene?
- The product formed would be triphenylmethanol Mass of bromobenzene = density X volume of bromobenzene = 1.495 X 0.7 = 1.0465 g # of mol of bromobenzene = weight of bromobenzene/ molar mass of bromobenzene = 1.0465/ 157.01 = 0.0067 mol Weight of product = # of mol X molar mass of tripheynlmethanol = 0.0067 X 260.33 = 1.7351 g
Why do you think you use extraction techniques instead of chromatography?
- We could use chromatography because benzoic acid and biphenyl have different polarities - Benzoic acid is more polar than biphenyl - The biphenyl is a hydrophobic group so it would dilute first - However, column chromatography is very time consuming, thus it is not efficient for students to do in a lab class where time is limited
How is the Grignard reagent formed in this particular experiment?
- We react magnesium and bromobenzene to create phenyl magnesium (the GR)
Why is the Carbon atom of a Grignard reagent a powerful nucleophile and strong base?
- because Carbon is more electronegative than metals, electrons in the C-Mg bond are attracted more to carbon and away from the metal - thus the Carbon atom has a partial negative charge - because of the ionic nature of this bond, the C atom is a strong nucleophile and base
What class of organic compounds do Grignard reagents belong to?
- orgometallic compounds that contain a carbon bonded to a metal
Why can we turn off the heat once the reaction has started?
- the reaction is exothermic and once started the reaction can progress easily with no more addition of heat
How do we add bromobenzene solution to the reaction?
- using a needle (syringe) to inject the bromobenzene through the rubber cap in small increments over the course of 15 mins
During workup, how do we remove the inorganic compounds(such as magnesium salts)
- we add 6 M HCl that will hydrolyze the benzoate (protonate it to get rid of the charge) and form benzoic acid and react with any remaining magnesium metal - will form two distinct layers - the benzoic acid is soluble in the ether layer - the inorganic compounds are soluble in the aqueous layer (lower layer) - so we remove the bottom layer and discard of it
How do we purify our product?
- we crystallize it by first dissolving it in hot water, then placing it in an ice bath, and collecting it using vacuum filtration
During workup, how do we remove the ether?
- we heat the aqueous extracts for 5 mins then allow to cool - then we precipitate the benzoic acid by adding HCl - we collect the solid using vacuum filtration
During workup, how do we remove the by-products (benzene and biphenyl)?
- we recover the benzoic acid by extracting the ether layer 3X with NaOH solution - the extraction removes benzoic acid from the ether layer by transforming it into water soluble benzoate salt, but leaves the benzene and biphenyl products behind
What are the side reactions in this experiment (byproducts) and how are we avoiding them?
1) As a base, the Grignard reagent will react with any source of proton, so it will react with any water to form benzene - we avoid this by drying the glassware in a 110 C oven for at least 10 minutes before beginning the experiment 2) The Grignard reagent will undergo a coupling reaction with another molecule of bromo benzene to form biphenyl - we can't control this reaction so we remove this byproduct during workup *pic shows biphenyl
If you follow all procedures as instructed in the lab manual and the Grignard reaction does not initiate after you start heating the reaction mixture, what can be done to initiate the reaction?
1) Take some of another lab student's Grignard reaction and inject it into your own with a syringe 2) Crush the magnesium flakes using the glass rod to reveal a fresh metal surface 3) The third method should be performed by the TA - In this method, the TA would add I2 to the reaction mixture
Can the Grignard reagent formed in this experiment be created using non-anhydrous ether? Why or why not?
No, - As a base, Grignard reagents react with all molecules that are more acidic than those found on alkanes or alkenes. Hydrous ether contains water molecules, which is more acidic and would thus react with the Grignard reagent to destroy the reagent and generate the alkane corresponding to the original alkyl halide − The starting material for preparing Grignard reagents can have no acidic protons so hydrous ether would not work
Will a Grignard reagent add to the carbonyl carbon of a carboxylic acid? Why or why not?
No, a Grignard reagent will not add to the carbonyl group of a carboxylic acid because it is a nucleophile and it will accept a proton instead.
How do we know the reaction has started?
Once you start heating the reaction mixture, the production of bubbles from the metal surface and cloudiness in the reaction solution indicate that the reaction has begun
What is the objective of this experiment?
Prepare a Grignard reagent and use addition of this Grignard reagent to synthesize Benzoic Acid from CO2
Why do we use a calcium chloride drying tube in the experiment?
to eliminate atmospheric moisture
