Titration of Alkyl Lithium (RLi) Using Butylated Hydroxy Toluene (BHT)

Jayesh G. Bokria

The alkyl lithiums (RLi) that we purchase have a listed molarity, but that number may not be very reliable, especially if that bottle has been punctured before. For reactions that involve RLi, it is critical that we know its exact concentration.

Equipment: Round bottom flask with stir-bar, 1 mL syringes, needles

Chemicals:
(1) Standardized BHT Solution in Anhy. Toulene (Titrant)
(2) Unknown Alkyl Lithium
(3) 1 % wt 1,10-phenanthroline in anhy. benzene or toluene (Indicator)

Procedure:
1. Preparation of Standard BHT Solution (titrant):
a. Purification of BHT: Dissolve BHT in minimum amount of hexanes at room temperature. Slowly raise the temperature to 40 ^OC (DO NOT OVER HEAT) and dissolve some more BHT. Cover the flask with foil and let it sit in hood for 2-3 h. You should get clear and transparent chunks of BHT. Filter, wash with ice-cold hexanes. Crush the crystals and dry under vacuum.
b. Standard Solution Prep.: Quickly weigh the pure and dry BHT into a volumetric flask and cannulate anhy. toluene. Typically, a 0.2500 M solution is prepared. It is not important to have exactly 0.2500 M solution, however it is very important to know the exact concentration of BHT.

2. Titration of Alkyl Lithium:
a. Clean and dry a 1- or 2-neck 25-50 mL flask using standard schlenk techniques, and keep it under inert atmosphere. Also, purge all your syringes and needles.
b. Add 1 mL of the indicator solution, via a syringe.
c. Using a syringe, add a known volume (typically 0.1 - 0.2 mL) of alkyl lithium. The color should become DEEP ORANGE.
d. Add the titrant (BHT Solution) drop-wise via a syringe until one drop changes the color from orange to pale yellow. This is your end point.
e. Repeat steps (c) through (d) at least FOUR times. While adding the alkyl lithium, one drop should change the color back to orange.

Calculations:
Use the following equation to calculate the molarity of your alkyl lithium: