Lab H- Grignard Reaction: Synthesis of Triphenylmethanol
anhydrous MgSO4 should be disposed of in?
a container marked anhydrous. MgSO4 waste
what is it easiest to form a Grignard reagent from? most difficult?
alkyl or aryl iodides followed by bromides chlorides are most difficult to use
what are the possible by-products of the reaction? how can it be avoided?
an impurity, biphenyl dimer, and magnesium bromide adding Ph-Br slowly so that it reacts with the Mg turnings and is not present in a large enough concentration to react with any previously formed Grignard reagent; using dilute solution conditions only after the initial reaction has been activated; using the least heat as possible
what is the drying agent used? why? how does it dry? what is done to begin the doing process before this?
anhydrous MgSO4 will react with water to form the hexahydrate which is a clumpy solid; it binds with the water present in the organic solvent; the solution is dried with brine
what is the solvent used in this Grignard reaction and why?
anhydrous diethyl ether; Grignard reagents can react with O2 but the vapors of this solvent prevent O2 from reaching the mixture and also the ether molecules coordinate with and help stabilize the Grignard reagent
why is it important that we have a magnesium surface not coated in MgO?
because Grignard reagents can react with oxygen
the RB flask containing any reacted Mg should be
brought to 186 for cleanup
how do we quench our reaction? how does this stop the reaction?
by pouring it onto ice-cold dilute sulfuric acid; it provides a proton to the alkoxy salt to give triphenylmethanol, will react with any unreacted magnesium metal, and confer the magnesium salts that are insoluble into soluble salts
what indicates the initiation of a reaction and Grignard formation?
color changes and the autorefluxing of the reaction
what serves to reduce the rate of dimer formation during the reaction? however, what does this make more difficult and how did we fix it?
diluting the reaction first before continuing to add the remaining bromobenzene from the sep funnel; these dilute conditions make a Grignard reaction difficult to initiate because it reduces the concentration of bromobenzene near the magnesium surface; by adding additional ether to dilute the reaction after it has initiated
why are Grignard reagents synthesized in anhydrous ether?
serves as a solvent; its electron pairs coordinate with the magnesium metal and help to solubilize the reagent; its high vapor pressure will effectively exclude oxygen from the reaction
what are the names for the connections of the condensor?
source: bottom return: top
how is bromobenzene added in this lab? why? what must we make sure before going on to the next step?
the addition of the bromobenzene at a rate that will maintain a gentle reflux (a steady drip) to prevent explosions that the reaction has been initiated
why do we use a 3-way piece (claisen adaptor)?
the bent arm allows the slow drip of the bromobenzene and the straight arm allows a direct reflux into the condenser; the bottom junction sits in the RB flask and converts one neck into two
the Grignard reflux occurs at what temp?
the boiling point of the solvent
what is the purpose of creating an organometallic compound?
the bond is relatively ionic, reversing the polarity of the carbon changing it from being partially positive to partially negative, and thus making it a strong nucleophile
how does a condenser work in a reflux setup?
the cold water runs from the bottom to the top because what is being vaporized, re-condenses and goes back into the flask
what is a final reaction that can occur throughout the Grignard reaction?
the formation of a dimer, biphenyl
what is the first step of a Grignard reaction?
the insertion of Mg into a carbon-halogen bond to form an organometallic compound
what are the steps of the Grignard mechanism?
1. Formation of the Grignard reagent 2. Nucleophilic Attack of GR (phenyl magnesium bromide) 3. pi bond of C=O reforms and loss of CH3OMgBr 4. Nucleophilic Attack by 2nd equivalent of GR 5. protonation of alkoxide with diluted H2SO4
what is diethyl ether's boiling point?
35º c
the aqueous waste from the extractions should be discarded where?
Grignard aqueous waste bin
what can happen if there is a lot of substrate present? how do we avoid this outcome in the lab?
the reaction can begin to vaporize more solvent than the reflux condenser can condense, and thus build up more pressure than can escape, causing a possible explosion by initiating the reaction and then gradually adding the rest of the substrate
what makes a Grignard reaction autocatalytic ?
the reaction is highly exothermic thus when it is initiated it generates a lot of heat and keeps the reaction going
after the reaction has been quenched, what needs to be done? how is this done?
the reaction needs to be purified; the magnesium salts of the reaction mixture are extracted and the mixture is then washed with water; the organic layer is then washed with NaHCO3 to neutralize excess acid and then with brine to begin the drying process
what are the drying tubes used for in this Grignard reaction? what do they contain and how does this contribute to the reaction?
they help to equalize pressure and prevent atmospheric moisture from entering the system via the condenser and yet allow the reaction flask to be open to the atmosphere so that gas pressure does not build up. contain anhydrous CaSO4 that have an indicator (CoCl) that is blue when dry and pink when it has absorbed moisture
what is the use of the boiling stones/stir bar during the reflux?
they prevent bumping
why do Grignard reagents require so many certain experimental precautions? because of this what precautions must be taken?
they serve as carbanions and strong bases that will react more quickly with proton sources than with their intended carbonyl targets; they are incredibly reactive the synthesis is necessary to be carried out in anhydrous conditions
what is the purpose of a Grignard reaction?
to form new carbon carbon bonds
why do we do a reflux?
to make sure we don't lose solvent to evaporation and since we are working with flammable material we don't want to heat it directly
why do we do a simple distillation?
to purify and isolate the final product and to remove diethyl ether
what do we do in this lab to ensure anhydrous conditions?
using anhydrous ether and indicating drierite tubes and cleaning the glassware a week before so that it can dry
how do you activate the magnesium?
you grind it to release the oxide coating and remove the oxide layer
when do we no longer have to worry about having anhydrous conditions?
once the Grignard reagent has reacted and we have produced the magnesium salt of an alcohol and thus the reaction with the air-sensitive Grignard is now complete
in our Grignard reaction, what are the intermediates formed?
phenyl magnesium bromide -> unstable tetrahedral intermediate -> benzophenone -> intermediate alkoxide
how is the triphenylmethanol isolated?
hexanes is added to the purified reaction mixture because the product is insoluble in it and during a simple distillation the product crystallizes out and this is then collected by doing a vacuum filtration and wash away the impurities
why are bromides more frequently used?
in order to balance the cost of the starting material with the ease of the reaction (basically its cheap and pretty effective)
ether distillate collected in the receiver container should be disposed of in?
into a container for labeled ether and hexane waste
filtrate after vacuum filtration should be disposed of into
into the specially labeled container for labeled ether and hexane waste
what is the function of iodine?
it functions as an indicator because its color fades as Mg is activated functions as an activator because it chemically cleans the oxide layer on the surface of the Mg turnings to expose fresh Mg surface in order to react with bromobenzene
the formation of the Grignard reagent is autocatalytic, meaning?
it is difficult to initiate the reaction however, once it is initiated it generates a lot of heat and begins to proceed very rapidly
what happens with the left over Mg metal?
it reacts with acid to evolve H2 gas
