titration of n-BuLi

Titration of BuLi (http://www.pushingarrows.com/Lab/page12/page12.html)

- Equipments and reagents needed:
dry isobutyl alcoholanhydrous ether1,10-phenanthroline (indicator)2- or 3-necks 25 ml round bottom flask (with magnetic stirrer bar)dry syringes – 5.0 ml, 2.0 ml, 1.0mlice bath(hexane to wash out needle and syringe contaminated with BuLi)- Procedures:
To a dry* 2- or 3-necks 25 ml round bottom flask with magnetic stirrer bar was added 5.0 ml of anhydrous ether under an inert atmosphere.
A crystal of indicator 1,10-phenanthroline was added and after it dissolved completely, the mixture was cooled to 0°C for about 10 minutes.
The maximum amount of dry isobutyl alcohol that is needed to quench the BuLi could be calculated as follows:

For example, if the previous titration was determined to be 3.1M then 2.0ml of this BuLi solution should contain

3.1M x 2.0ml / 1000ml = 6.2x10-3 mol of BuLi.

Since it takes 1 equivalent of isobutyl alcohol (MW 74.12; density 0.803 g/ml) to quench the BuLi, therefore

6.2x10-3 mol x 72.14g/mol / 0.803 g/ml = 0.57 ml of isobutyl alcohol was potentially needed.

2.0ml of BuLi solution was then added under an inert atmosphere and it was titrated by slow addition of approx. 1.0ml of dry isobutyl alcohol.

The colour change should be very apparent.

n-BuLi : orange-brown to bright yellow
t-BuLi : purple to bright yellow
Using the amount of isobutyl alcohol added (initial volume – final volume), the molarity of the BuLi solution can be calculated as follows:

For example, if 0.41 ml of isobutyl alcohol was used,

0.41 ml x 0.803 g/ml / 74.12 g/mol = 4.44x10-3 mol of isobutyl alcohol was used = 4.44x10-3 mol of BuLi present

The strength of the BuLi solution is therefore

4.44x10-3 mol / (2.0 ml / 1000ml) = 2.22M
(2.22M vs 3.1M makes a big difference!)
* oven dried then cooled under a stream of inert gas, or flame dried under reduced pressure then cooled under a stream of inert gas.