QuantitativeAnalysis of a Mixture by NMR Spectroscopy Mixtures oforganic compc,undi can Ire assayed hy NMR spectroscupy i f d~stinguishableSignals from each component can be integrated separately. Addlrion to the rnlxture of a measured quantity of a ref~rencewmpound, i.e., an internal standard, converts the integration from a relative to an absolute assay. For convenience the internal standard should be chemically inert and easily removed after completion of the assay. This usually means low-boiling, although for accurate weighing the substance should not evaporate too rapidly. The standard should also have a simple NMR spectrum that does not overlap with signals from the mixture. Examples are trichloroethene (b.p. 87OC; 6 6.5, s, lH), dibromomethane (h.p. 97°C; 6 4.9, s, 2H), 1,2-dichloroethane (b.p. 84'C; 6 3.7,s, 4H), and methyl 2.2-dimethylpropanoate (b.p. 10IDC;6 3.7, s, 3H, and 1.2, s, 9H). While this technique is valuable in a research context, i t can also provide the basis for an instructive undergraduate experiment. For the following example, the student is provided with a mixture prepared by dissolving 1,2,4,5-tetramethylbenzene (2.0 g) and fluorene (10.0 g) in tetrachloromethane containing 1%tetramethylsilane, to give 100 ml of solution. Only the identities of the components are revealed. For the assay, a portion (5.00 ml) of the solution is transferred by pipet to a clean, dry, Quickfit flask (10 or 25 ml), which is then stoppered and weighed on a milligram balance. Dihromomethane (500 t o 700 me) is added and the weieht increase accuratelv recorded. The solution is swirled t o ensure comolete mixing. -. and a asmple';~transferred to a clean NMK tube. The spe.ctrum and integral are then recorded in the normal way. The cmnpmitim ofthe nnginal 5-ml portion of rhe solution can be calculated by selecting a clearly defined signal cauaed hg N, protons from a nmpmwnt molecule, and the signal caused by N, prutons from the reference molecule, and applying the equation Moles of component in 5 ml portion N - Integral (component signal) x -2 Moles uf reference in 5 ml portion Integral (reference signal) N, For an added weight of 650 mg dihnmomethane and using the tetramethylbenzene and fluorene signals at 6 2.2 (s, 12H) m d 6 3.8 (s, 2H) respectively, the theoretical integral ratios are 1.25:1.5:1.0, and the assay I100 mg tetramethylhenzene and 500 mg flouren") is usually accurate tu about IOC0 or better, depending on the weighing accuracy. Tlm Wallace Univmity of Salford Salford M5 4WT. United Kingdom
1074
Journal of Chemical Education
