Benzoic Acid Lab Flashcards
Why must the glassware used in a Grignard reaction be scrupulously dry and not exposed to air?
“In reaction involving Grignard reagents, it is important to exclude water and air, which rapidly destroy the reagent by protonolysis or oxidation. Since most Grignard reactions are conducted in anhydrous diethyl ether, side-reactions with air are limited by the protective blanket provided by solvent vapors.
How can you make sure glassware is completely dry?
Wash it with acetone and put it in the oven at a low temperature so the acetone evaporates
Why do you add the bromobenzene in portions to the Mg/ether solution?
In order to avoid side reactions
What is one side product of the benzoic acid formation?
biphenyl (phenyl-MgBr attacks bromobenzene)`
What is in the beaker after you’ve added dry ice (CO2) to the grignard reagent/ether solution?
1) ether
2) Anion-benzoic acid salt (+MgBr is the cation)
What must be added after adding dry ice in order to synthesize benzoic acid? Why?
6M hydrochloric acid must be added in order to hydrolize the anion-benzoic acid salt
What is in the beaker after you’ve added 6M hydrochloric acid to the ether/anion-benzoic acid salt?
1) Benzoic acid
2) MgBr(OH)
3) Ether
How do you get rid of remaining magnesium chips when doing a grignard reaction?
Any remaining magnesium chips will react with acid to evolve hydrogen.
What should you do if you have solid present after you’ve added 6M hydrochloric acid to the ether/anion-benzoic acid salt?
Add more ether. Benzoic acid is soluble in ether, whereas the inorganic compounds are soluble in the acid soltion.
After one has collected the ether layer which contains benzoic acid, what is the next step? Why?
- Add NaOH to the separatory funnel and shake it.
- This extraction removes benzoic acid from the ether layer by converting it to the water-soluble sodium benzoate. By adding the NaOH solution, you’re also creating the aqueous layer.
After NaOH has been added to the ether layer containing benzoic acid, what stays in the ether layer?
The by-product, biphenyl, stays in the ether layer along with some remaining benzoic acid.
Why would one perform 3 NaOH extractions with three 4mL portions instead of one 12mL portion?
The extraction is more efficient when done in multiple parts.
When forming the Grignard reagent, it is usually necessary to add a tiny
iodine crystal, why?
The iodine crystal is added to initiate the reaction.
Why do you heat the basic benzoic acid extract?
It is heated in order to remove any either that may be dissolved in the aqueous phase.
Ether is soluble in water to the extent of 7%.
How do you precipitate benzoic acid from the basic alkaline solution?
1) add some 6M hydrochloric acid
2) cool it in an ice bath
3) collect the solid by vacuum filtration on a Hirsch funnel.
4) wash the solid with several portions of cold wear
