4. Acquisition

Time requirement: 5 min

Sample: Racemic 2-butanol (0.75 mmol) is dissolved in 2 ml of CDCl₃ and dry pyridine (0.75 mmol) is added (excess pyridine does not influence the ³¹P NMR determination). To the stirred solution, 0.25 mmol PCl₃ dissolved in 2 ml CDCl₃ is added. The mixture is stirred for 10 min at room temperature and subsequently transfered, without the necessity of any workup or further purification, into an NMR tube.

The instrument is set to ³¹P detection with composite pulse proton decoupling. You have to set:

decoupler attenuation and 90° pulse duration for CPD
td	16 k
sw	20 ppm
o1	middle of ³¹P NMR spectrum
o2	middle of ¹H NMR spectrum
p1	30° ³¹P transmitter pulse
d1	2 s
ns	32

5. Processing

Use standard 1D processing as described in Experiment 3.2 using exponential line-broadening with lb = 3 Hz. Reference against external 85% H₃PO₄ with δ_P = 0 using the Ξ-scale (see Chapter 9, Introduction). Apply baseline correction on the spectrum for good integration.

6. Result

The figure shows the ³¹P NMR spectrum obtained on an Avance DRX-400 spectrometer with a multinuclear probe-head. The two smaller signals at δ_P = 6.5 and δ_P = 5.75 stem from the meso pair, the components of which are diastereomeric to each other due to the pseudoasymmetric center at phosphorus. The larger signal at δ_P = 6.2 arises from the enantiomeric d,l pair. The literature claims that the integrals should give a 1:1 ratio of meso and d,l forms for the racemic 2-butanol; note the deviations in the figure.

7. Comments

In the reaction, trialkylphosphites are probably formed first, and these are cleaved under the reaction conditions to give phosphonates. An enantiomerically pure alcohol with configuration S will give only SS phosphonate, and the signals of the meso compounds will be missing.

For the ee-determination of thiols, methylphosphonic dichloride or in general alkylphosphonic dichlorides are recommended as reagents in the literature. Thiophosphites P(SR)₃ did not give well-resolved ³¹P NMR signals for the diastereo-isomers. The advantage of these reagents is that only two equivalents of ROH or RSH are necessary.