grignard reaction lab Flashcards
ethereal solvents are used for Grignard reaction because…
The O in the solvent stabilizes the metal complex, making it soluble
why is water excluded from the reaction?
Grignard reagents are strong bases & quenched by water (protic solvent)
why is the dry ice crushed only right before use?
dry ice sublimes quickly, giving less CO2, thus a lower yield
general reaction
Ph-Br –> Mg –> Ph-MgBr –> CO2 –> Ph-COO- + MgBr+ –> H+ –> Ph-COOH
MSDS
PM: I, Diethyl Ether
NFA: I, Dry Ice
Grignard formation reaction
R-X + Mg –> R-MgX
3 main side reactions
- R-MgX + O2 –> R-O2- + MgX+
- R-MgX + CO2 –> R-CO2- + MgX+
- R-MgX + R-X –> R-R + MgX2
How is the third side reaction prevented?
free [R-X] very small relative to [Mg], making X more likely to run into Mg than anything else; [Mg] decreases as rxn proceeds, eventually making X more likely to hit R-MgX much later
3 main sources of water in rxn / how to avoid
- solvent: use anhydrous diethyl ether
- Air: constant a/c & humidity control
- glassware: dried a week before
aprotic solvent
solvent without a proton source
glassware setup
rxn heated to solvent BP until condensation between 1-2/3 of condensor; bromobenzene added via syringe
I2 addition
rxn coaxed by I2 addition, forming MgI2, removing Mg outer layer, exposing inner layer to react w/ Ph-Br
benzoic acid isolation
In sep. funnel: NaOH added to reaction mixture and aq. layer (product) is drained from neutral impurities; HCl added to form white precipitate
IR spectrum functional group
COOH
How is the MW obtained?
Product weighed (g), titrated with known [NaOH] to get a Vol of NaOH; Vol * [NaOH] = mol NaOH = mol benzoic acid (1:1 ratio)
numbers on glass stopper
diameter of widest part / length of stopper
what to do if glass joint freezes
- tap with spatula handle
- steam/hot water bath
- heat over flame
organometallic reagents
C-Metal bond where C is partial negative nucleophile; produce new C-C bonds
aprotic solvents used with Grignard
- diethyl ether (preferred: less expensive, easily removable due to high VP)
- THF (stronger lewis base, better solvating ability)
Grignard / Water reaction
R-MgX + H2O –> R-H + HO-Mg-X
Why are the O2 / CO2 side reactions less problematic?
They are atmospheric and not readily dissolved in diethyl ether due to its high vapor pressure
why is the reaction mixture added slowly to dry ice?
to create the carboxylate salt intermediate that will be protonated to benzoic acid; added slowly so CO2 is in excess, making Ph-MgBr more likely to react w/ CO2 than the Mg salt of benzoic acid
