CH 220C Grignard Reaction
what are the three potential side rxns w/ grignard reagents? which one are we doing? which is reactive? which is a big problem? **write out the 3 reaction equations**
(1) R-MgX + O2 --> R-O2 + MgX >>>(peroxide is very reactive) (2) R-MgX + CO2 --> R-CO2 + MgX >>> (we are tyring to do) (3) R-MgX + R-X --> R-R + MgX2 >>>(problem!)
what is addn FA for iodine for ingestion?
(Iodine = I'm not fine) give milk then starch soln
what is added FA for iodine?
(Iodine = I'm not fine) keep warm and at rest
what is fire hazard, P, and FA for iodine?
(Iodine = fine) H: negligible fire hazard, may ignite/explode on contact w/ combustible materials P: avoid contact w/ comustible materials FA: appropriate fire extinguishing agents
what is fire H, P, and FA for magnesium?
(maggie is on fire) H: severe fire hazard P: no sparks, no flames, no contact w/ hot surfaces FA: regular dry chemical, sand, lime, soda ash
what chemical has a negligible fire hazard? what is P and FA?
(negligible = Don't bother) Dry Ice P: N/A FA: appropriate fire extinguishing agents
What chemical has a slight fire hazard? what is P and FA?
(slight = barely = benzoic acid) P: avoid heat, sparks, flames FA: chemical powder, CO2, water, foam
what makes the reaction so hard in lab?
WATER
what are we synthesizing before adding our electrophile? what are we producing it with?
a grignard reagent Mg (solid)
what should you do before greasing the glass joints?
add Mg and iodine so the solids don't get stuck to the grease, preventing a proper seal
Inhalation of magnesium MSDS sheet
- inhalation of magnesium leads to burning sensation, coughing, wheezing, headache - use near local ventilation, exhaust, and if inhaled remove from exposure & get medical attention
what kind of solvent is essential for this reaction? why?
aprotic solvent because the grignard reagent is a strong base and a good nucleophile, so it will be quenched/protonated by anything with protons (like water) before it can be added to the desired electrohpile
why is the side rxn of the grignard reagent with bromobenzene a problem?
because we can't prevent them from being in the soln at the same time, so this rxn WILL occur
what are we synthesizing in lab?
benzoic acid
what is the reagent?
bromobenzene
a new _______-______ _______ is formed during a grignard reaction
carbon-carbon bond
what is the course of the colors during the grignard synthesis?
clear --> white/cloudy --> dark brown
what solution do you wash the product w/ when filtering w/ the buchner funnel?
cold water
what is the only FA for ingestion for diethyl ether?
contact physician immediately (DE = don't even keep head lower than hips)
what does sodium hydroxide do?
deprotonates benzoic acid (makes it aqueous) so it can be separated
reaction will fail if glassware is not _____
dry
what does the drying tube do?
filters water out of the air before it enters the apparatus
what does the HCl do?
reutralizes residual NaOH, reforms benzoic acid
what is sublimation?
solid --> gas
what are two characteristics of grignard reagents?
strong base, good nucelophile
what are the three sources of water in the reaction?
1) the solvent - so we use anhydrous solvent 2) the atmosphere 3) the glassware
how do you take off the needle cap correctly?
1. Bend the cap a little first to crack the seal 2. Cap should slide off (can also push on benchtop)
which indentation in the aluminum block should you use?
the one that fits the round bottom flask
why are ethereal solvents important?
the oxygen of the ethereal solvent stabilizes the metal complex
what are the first 8 steps of the procedure? (through the addition of bromobenzene)
1. add 0.5g Mg powder and 1/2 crystals of iodine to a clean, dry 50mL round-bottom flask, and stir bar 2. add 10mL of ether to the round-bottom flask 3. lightly grease the joints btwn flask and claisen adapter and claisen and condenser. Connect the water hoses to condenser w/ metal clamps (water in bottom, water out top) 4. add rubber septum to the straight side of the claisen adapter 5. turn on water in condenser, very slowly just so water flows through 6. prepare a solution of 5mL ether and 2.5mL bromobenzene 7. add the bromobenzene soln dropwise through the septum using a syringe 8. once all bromobenzene soln has been added, gently reflux for 30 min (stir)
which 2 joints do you grease? why? how?
1. btwn the round-bottom flask and the claisen adapter 2. between the claisen adapter and the condenser so that the glassware doesn't seize/freeze together place a small amount of grease on the male joint w/ your finger, rotate so grease spreads evenly, then secure with a keck clamp
describe the 4 steps of the procedure for MW determination by titration
1. place .2g benzoic acid into an erlenmeyer flask w/ 25mL methanol 2. once solid has dissolved, add a drop/two of phenolpthalein soln 3. using burette, titrate soln w/ .1M NaOH till pink endpoint 4. note V of NaOH necessary to titrate benzoic acid, calculate MW benzoic acid
describe the pieces of glassware used to synthesize the grignard reagent **be able to sketch this apparatus**
1. round-bottom flask attached to claisen adapter 2. claisen adapter attached to rubber septa and condenser 3. condenser attached to water hoses and the drying tube
what are the three sources of H2O and how to deal w/ it
1. solvent: choose anhydrous solvent 2. air: humidity maintained in lab, drying tube where air can get in 3. glassware: flame-dry w/ bunsen burner, heat gun, drying oven, etc. but we wash/dry all glassware week before
what three things should you remember about the water hoses?
1. use the metal clips 2. water in at bottom, water out at top 3. keep pressure as low as you can so the hoses don't come off
describe the remaining 5 steps of the procedure
16. add 10mL sodium hydroxide soln, shake, vent, drain bottom aqueous layer into a clean beaker 17. acidify the basic soln with 6M HCl until pH ~4, a white precipitate will form 18. cool beaker in ice bath, set up buchner 19. filter solid product, wash w/ cold water 20. save product till next week for mass and MP
what solution do you put in the flask when making phenyl magnesium bromide?
5 mL of ether and 2.5 mL bromobenzene (add to round bottom flask w 0.5g of Mg powder & iodine crystals)
what is a good % yield of benzoic acid?
60%
what is H, P, and FA for fire for bromobenzene and diethyl ether?
H: (DE- severe fire hazard) (B- moderate fire hazard), may ignite @distant sources P: no flames, no sparks, no contacts with hot surfaces FA: alcohol resistant foam, CO2, regular dry chemical, (B = water) (Bees need water)
what is the equation for the reaction of iodide with Mg?
I2 + Mg --> MgI
what is the reaction of grignard reagent with water? **draw this reaction/mechanism out**
R-X + H2O --> RH + Mg(OH)Br
what is the general reaction for the synthesis of a grignard reagent? **draw out the partial +/- charges**
R-X + Mg ---(solvent = ether)--> R-MgX
how do you know when the grignard reagent has formed?
the soln color turns from chalky-white to dark brown
How does the grease prevent the glass joints from freezing together?
they fill air pockets so there's no place for the ether to move into
what is FA for eye exposure for all?
thoroughly wash eyes with water for several minutes, remove contacts, seek medical attention immediately
why should you put the rubber septum in the Claisen adapter ASAP after adding Mg, ether, and iodine?
to avoid exposure to atmospheric moisture
what are the 2 layers of the second wash?
top = ether bottom = water + benzoic acid
what is in the bottom and top layers after the first separatory funnel separation?
top = ether + benzoic acid bottom = aqueous waste
at what point does [R-MgX] surpass [Mg]?
towards the end of the experiment (so it's still relatively minimized
what does reflux mean?
vapour continuously condenses and drips back into the reaction flask
how do we prevent the unwanted side rxn?
we minimize it by keeping [Mg] high and [R-X] low so R-MgX is more likely to react w/ Mg than R-X we do this by adding R-X dropwise over the course of the experiment
when do most people get impaled by needles
when taking off cap
what will you need to sign in/out (3 things)?
needle, syringe, septum
should you try to recap your syringe before disposing?
no
how do you remove excess carbon dioxide?
once dry ice + ether added to the reaction flask, let the excess dry ice sublime when you set the round-bottom flask down
what does the sulfuric acid do?
protonates the carboxylate/ hydrolyzes the bond between the Oxygen and MgBr (protonates the benzoate)
how should you carry your needle to the sharps container?
put it in a beaker
what is the general reaction of this experiment? what is the solvent? **be able to draw out each compound**
reaction is in diethyl ether bromobenzene + Mg --> phenyl magnesium bromide + CO2 (s) --> bromomagnesium benzoate + H2SO4 (H30+) --> benzoic acid + Mg(OH)Br (MgBr + H2O)
what is FA for all for skin exposure?
remove contaminated jewelry/clothes, thouroughly wash skin w/ water, get medical attention (warm water for dry ice!)
describe the next 7 steps of the procedure (through first draining of funnel)
9. the soln should turn to chalky-white to dark brown, once the rxn turns dark brown, move on 10. get 5 or 6 pieces of dry ice, crush it w/ aluminum block, then add about 10g to a 125mL erlenmeyer flask 11. pour the rxn mixture into the larger flask and swirl to mix, rinse round-bottom flask w/ 5mL diethyl ether and add it to flask, swirl 5 min, set down, let excess dry ice sublime 12. while dry ice sublimes, prepare soln of 5mL sulfuric acid and ~5mL water 13. add sulfuric acid soln to rxn mixture slowly, there will be bubbling, add ether if not yet 25mL 14. transfer soln to separatory funnel, you should see 2 clear layers 15. shake the separatory funnel, vent, then drain the bottom aqueous layer (waste)
Why should you only get the dry ice and crush it just before using it?
Because dry ice sublimes quickly at room T, so doing so would mean less CO2 would be added = lower yield.
Why must the reagents, solvents, and apparatus be dry?
Because gringard reagents are are strong bases and will be quenched in the presence of any water
what are the partial negativities of a grignard reagent that allow you to make a new C-C bond?
C is very -, M is very +
what electrophile are we using?
CO2
what is FA for inhalation for all?
remove from exposure immediately, get medical attention
why does the soln reflux for 30 minutes?
forming a grignard reagent is highly exothermic, so the refluxing (heating, then recondensing and falling back into the flask) prevents the evaporation of the ether
what is FA for most chemicals for ingestion?
get medical attention immediately, keep head lower than hips if vomiting
what should you do if your plunger for the needle has any problems at all?
get rid of it immediately and get a new one
what does aprotic mean?
has no protons to donate
what are the 2 possible mutagens?
ioDInE DIEthyl ether
what is the purpose of iodide?
iodide reacts w/ the outer layer of the magnesium (the more oxidized part), freeing up the more reactive inner layer of the magnesium
why is diethyl ether (ether) a good solvent? **be able to draw this**
it is aprotic and the oxygen atoms stabilize the metal complex
why should you be careful using ether?
low BP, highly flammable, anaesthetic
