Grignard Experiment

How do you make the grignard reagent chemically?

alkyl or aryl halide with magnesium in anhydrous ether

If reaction does not start, what does the TA add?

dibromoethane or iodine (I2)

Caution in this experiment the diethyl ether is ____.

extremely flammable

What compounds could you expect in the pet ether extract? How would it compare to the crude extract on TLC?

side rxn products: R-R, MgX2; unused reagents; It would have a similar product since there would be similar impurities but the crude would have more impurities and compounds on its TLC plate.

what does the drying with Na2SO4 do?

takes out any left over water

What is the procedure for grignard prep?

1. assemble apparatus and dry bottom to top 2. Attach drying tubes with CaCl2 and cotton 3. dry graduated cylinder and Erlenmeyer flask and cover with foil 4. Fill sep funnel with bromobenzene and diethyl ether; add mg pieces to RBF 5. Add just enough solution to Mg to cover it 6. Use hot water to warm RBF and add solution slowly; reflux 30 minutes

What is the procedure for triphenylmethanol prep?

1. cool RBF for 5 minutes 2. add methyl benzoate and diethyl ether to sep funnel 3. slowly add to RMgX reflux for 30 minutes with hot water bath 4. In an Erlenmeyer flask, add crushed ice, H2SO4, and the reaction mixture 5. swirl and dissolve lumps 6. Transfer to sep funnel and remove aqueous layer (bc triphenylmethanol is more nonpolar and prefers the ether layer) 7. Wash the ether layer with sat'd NaCl; Will create ions with impurities to remove from ether layer; 8. Dry ether layer with NaSO4 and filter through a cotton plug in glass funnel into a preweighed distilling flask. Removes any water or aqueous or solid impurities 9. Remove ether layer with rotovap. ( left with triphenylmethanol with side products and impurities. It contains numerous non-polar impurities including unreacted starting materials and biphenyl side products. These can be mostly removed by trituration with petroleum ether.) 10. Triturate residue with cold pet. ether (This type of purification procedure in which an impure solid is washed with a solvent in which it is insoluble is referred to as "trituration"). 11. Decant pet. ether 12. Recrystallize from iso-propyl alcohol 13. TLC crude/pure/pet ether extract with pet ether as eluent 14. Mass/MP/yield

How would you set up the drying tubes? and in what part would you use them?

1. cotton on the bottom bulb, CaCl2, cotton on the top of bulb. 2. Grignard prep and triphenylmethanol prep

How do you keep the environment dry in the experiment?

1. dry the apparatus bottom to top with heat gun 2. place drying tubes at top of apparatus with CaCl2 with cotton 3. Drying both the grad cylinder and Erlenmyer flask to keep reagents as dry as possible with heat gun 4. Covering the ether immediately after using and covering the flask you pour it into to minimize contact with water in the air. 5. Do not wash equipment with H2O only acetone

Explain the reason for the sequence of addition of the ether solution employed at the beginning of the formation of the Grignard reagent in the experimental procedure above.

It helps bromination of the Grignard reagent because it helps solvate and stabilize the reagent as it forms. Halides are readily solvable in ether and becasue ethers are non acidic they do not protonate the grignard reagent onced formed like water or alcohol. This incresees the amound of halide used it increases the reaction yield. There is little halide left then to proceed with the side reacion especially when its rate depends on the halide concentration.

Is pet ether a strong eluent? how far would it carry a polar molecule?

No, so it would not carry a polar compound like alcohol very far.

What is the equation to make a grignard reagent?

R-X ---Mg/ether---> R-MgX

The rate of coupling depends on

the square of the concentration of the halide.

How will you know a reaction occurred?

the surface of the metal becomes discolored, a cloudy haze may appear in the liquid phase, ebullition is evident or refluzing of the solvent may be observed.

In the experiment, phenylmagnesium bromide reacts with ___ ____.

methyl benzoate

Draw and name the equipment used to create the apparatus for Grignard.

on paper

What is the mechanism to create the triphenylmethanol compound?

on paper

What does the H2SO4 do?

provides the proton needed in the last step of the grignard rxn

how would you calculate percent yield?

pure/crude *100

what are the two kinds of carbonyl acceptor structures in addition to benzoate esters can be used in reaction with phenylmagnesium bromide to afford triphenylmethanol?

Benzophone; Benzoic Anhydride

An unavoidable side reaction of alkyl halides with active metals which lowers the yield of Grignard reagents is called _______.

Coupling 2 RX + Mg ---------> R-R + MgX2

Why is the grignard reaction important?

it creates c-c bonds

what does the washing with NaCl do?

it takes out any impurities