Titration of n Butylmagnesium Bromide and n-Butyllithium.
Reference ： Watson, S. C.; Eastham, J. F. J. Organomet. Chem. 1967, 9, 165-167.
This experiment entails the preparation of a Grignard reagent, nBuMgBr from 1-bromobutane and elemental magnesium. It is essentially identical to the first portion of the Grignard reaction you would have performed in Chemistry 352L. The second part of the experiment entails titration of a Grignard reagent and of a commercial bottle of an alkyllithium reagent. These procedures are very important in organometallic chemistry where knowing the amount of your reactive compound is critical to the success of an experiment or synthesis.
Preparation of the Grignard Reagent. Assemble an apparatus for preparation of the Grignard reagent as shown. Add 1.5 grams of Mg to the flask and attach a nitrogen bubbler to thevacuum adapter. Start a slow flow of nitrogen through the system (the bubbler should bubble gently when the stopcock on the separatory funnel is closed, but not when it is open). With the stopcock open, dry the flask using a torch by brushing the flame lightly over the lower portion of the flask (Do not overheat. All you want is to drive the water from the flask and Mg). Once the flask is cool to the touch, close the stopcock and add 25 mL of anhydrous ether to the separatory funnel followed by 6 mL of 1-bromobutane. Swirl to mix. Open the stopcock and run in about 5 mL of the solution (just enough to nearly cover the magnesium chips). Warm the bottom of the flask with your hand. If a reaction does not start within a few minutes, remove the separatory funnel and gently break up several pieces of the magnesium using a stirring rod (Be careful not to punch through the bottom of the flask. It only takes a little pressure to break up the turns). If you still do not see a reaction (cloudiness) ask the instructor. Once the reaction has started and begins to boil, Add a stir bar, place the separatory funnel back on the Claisen head and turn on the stirrer. Open the stopcock such that solution drips into the flask at a rate of one drop every second or two to maintain a steady, but gentle reflux.
While the reaction is proceeding, set up for the titration of butyllithium.
The apparatus for the titration is assembled as below with a 25 mL round bottom flask. The flask is dried using a torch flame under nitrogen or argon flow with a mineral oil bubbler. Once cool, a septum is placed loosely over the open end of the laisen head and an ice bath is placed under the round bottom flask. A clean, dry stirbar is added to the round bottom flask along with 1-2 all crystals (~2-3 mg) of 1,10 phenanthroline hydrate. The septum is then secured in place by folding the edges over the glass joint.
Dry THF (10 mL) is added using a syringe, followed by 2-butanol (0.500 mL, measured as accurately as possible using
your 1 mL syringe). Allow the solution to cool for 5 minutes. As accurately as possible, draw 5.00 mL of n-butyllithium
in hexanes from the reagent bottle using your 10 mL syringe. (Caution: n-butyllithium is extremely corrosive and can ignite spontaneously in air. Treat it with respect.) Add the butyllithium to the solution dropwise, noting when the first rust-red color appears and persists (this is your first endpoint). Once you have persistent color, add the remainder of the butyllithium to the flask. Immediately rinse your syringe twice with hexane and expel the washings into an open beaker in the hood before cleaning the syringe out with water and acetone (Otherwise, your needle will plug from the formation of lithium hydroxides and carbonates and will be destroyed). Next, back titrate the butyllithium in the flask with 2-butanol until the rust red color is just extinguished (this is your second endpoint). This should be done immediately as the indicator is not stable and will degrade over time.
Your reaction mixture can be allowed to warm to room temperature and be dumped in the organic waste (be sure to retrieve your stirbar). Calculate the concentration of the butyllithium solution from both the forward and back titrations and compare these values to the stated value on the bottle. Assume one mole of butyllithium reacts with one mole of 2-butanol to form the
corresponding lithium alkoxide.
Repeat the procedure for the solution of the Grignard reagent you just made, noting the difference in color between the two complexes. Calculate the concentration and yield of your Grignard reagent, assuming the total volume of your reaction was 31 mL.
Obtain the Watson and Eastham reference and answer the following questions:
Though it contains water of hydration, why does the 1,10 phenanthroline hydrate not affect the outcome of the titration?
How does the phenanthroline act as an indicator (e.g. what is responsible for the color)?
An alternate procedure for determining the concentration of butyllithium is to titrate a solution of diphenylacetic acid, which also produces a colored solution. Give a reaction showing how this is possible (see Kofron, W. G.; Baclawski, L. M. J. Org. Chem. 1976, 41, 1879-1880).
(Note: The solution of TMPMgCl-LiCl is titrated by dissolving 2 mmol benzoic acid in 5 mL anhydrous tetrahydrofuran containing a all amount of 4-(phenylazo)-diphenylamine. The solution of base is added to the solution while cooling to 0 oC. When the solution is consistently a pink color the end of the titration is reached. The molarity of the solution is determined by the dividing the volume of base (mL) used by the mmol of benzoic acid added.)
