grignard reaction lab

Question 1

ethereal solvents are used for Grignard reaction because…

Answer 1

The O in the solvent stabilizes the metal complex, making it soluble

Question 2

why is water excluded from the reaction?

Answer 2

Grignard reagents are strong bases & quenched by water (protic solvent)

Question 3

why is the dry ice crushed only right before use?

Answer 3

dry ice sublimes quickly, giving less CO2, thus a lower yield

Question 4

general reaction

Answer 4

Ph-Br –> Mg –> Ph-MgBr –> CO2 –> Ph-COO- + MgBr+ –> H+ –> Ph-COOH

Question 5

MSDS

Answer 5

PM: I, Diethyl Ether
NFA: I, Dry Ice

Question 6

Grignard formation reaction

Answer 6

R-X + Mg –> R-MgX

Question 7

3 main side reactions

Answer 7

1. R-MgX + O2 –> R-O2- + MgX+
2. R-MgX + CO2 –> R-CO2- + MgX+
3. R-MgX + R-X –> R-R + MgX2

Question 8

How is the third side reaction prevented?

Answer 8

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

Question 9

3 main sources of water in rxn / how to avoid

Answer 9

1. solvent: use anhydrous diethyl ether
2. Air: constant a/c & humidity control
3. glassware: dried a week before

Question 10

aprotic solvent

Answer 10

solvent without a proton source

Question 11

glassware setup

Answer 11

rxn heated to solvent BP until condensation between 1-2/3 of condensor; bromobenzene added via syringe

Question 12

I2 addition

Answer 12

rxn coaxed by I2 addition, forming MgI2, removing Mg outer layer, exposing inner layer to react w/ Ph-Br

Question 13

benzoic acid isolation

Answer 13

In sep. funnel: NaOH added to reaction mixture and aq. layer (product) is drained from neutral impurities; HCl added to form white precipitate

Question 14

IR spectrum functional group

Answer 14

COOH

Question 15

How is the MW obtained?

Answer 15

Product weighed (g), titrated with known [NaOH] to get a Vol of NaOH; Vol * [NaOH] = mol NaOH = mol benzoic acid (1:1 ratio)

Question 16

numbers on glass stopper

Answer 16

diameter of widest part / length of stopper

Question 17

what to do if glass joint freezes

Answer 17

1. tap with spatula handle
2. steam/hot water bath
3. heat over flame

Question 18

organometallic reagents

Answer 18

C-Metal bond where C is partial negative nucleophile; produce new C-C bonds

Question 19

aprotic solvents used with Grignard

Answer 19

1. diethyl ether (preferred: less expensive, easily removable due to high VP)
2. THF (stronger lewis base, better solvating ability)

Question 20

Grignard / Water reaction

Answer 20

R-MgX + H2O –> R-H + HO-Mg-X

Question 21

Why are the O2 / CO2 side reactions less problematic?

Answer 21

They are atmospheric and not readily dissolved in diethyl ether due to its high vapor pressure

Question 22

why is the reaction mixture added slowly to dry ice?

Answer 22

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