[caphyboy]

     Hi,every one！
   Ｎow I am going to use Grignard Reagent .But I donnot know the concentration.Can anyone who has used the Grignard Reagent tell me how to determin the concentration,please?
     THanks!

[Custos]

The concentration of a Grignard reagent can be estimated using the titration procedure of Watson and Eastham (J. Organometal. Chem., 9, 165, 1967).

In a typical titration, performed under a nitrogen atmosphere, a 5.0 ml aliquot of the Grignard reagent is added to a solution of about 2 mg of o-phenanthroline in 10 ml of anhydrous benzene. The resulting purple solution is titrated with a standard solution (1.00 M) of sec-butyl alcohol in xylene until the purple color of the o-phenanthroline-Grignard reagent charge transfer complex was just discharged. In this procedure the number of millimoles of sec-butyl alcohol added is equal to the number of millimoles of alkylmagnesium chloride present in the aliquot of Grignard reagent.

[kiwi]

that'll do it, but one modification that is easier than making an accurate solution of dry s-butanol is to make one of a commercially available solid dry alcohol, eg. menthol.

[ziqquratu]

The best method I know of for determining the concentration of Grignards or organolithium reagents is to titrate using diphenylacetic acid.

Simply, you accurately weigh out the diphenylacetic acid, dissolve in a littlre dry THF or ether, and then titrate with the organometallic. When you add just a tiny bit over one equivalent of organometallic reagent, the solution turns bright yellow. Based on the original amount of diphenylacetic acid and the amount of organometallic solution added, you can easily determine the concentration.

How it works is simple. The base deprotonates the carboxylic acid first (duh). Once all of the carboxylic acid is neutralised, any extra base reacts with the next most acidic protons, which are the protons adjacent to the carbonyl. The carbanion which is formed is intensely yellow.