Methyl iodide is probably the most common methylating agent used in chemistry to this day. Unfortunately it is so good at it’s job that it also likes methylating your DNA, which has been found to be unhealthy in the long run. It is not advisable to keep larger quantities in stock, since it is unstable to light and easily evaporates. I used all of my methyl iodide for the methylation of pyranine, which is a fluorescent dye sometimes found in neon text markers.
| Substance | M [g/mol] | n [mmol] | m [g] | V [ml] | Eq. |
|---|---|---|---|---|---|
| Red Phosphorus | 30.97 | 330 | 10.20 | – | 1.0 |
| Methanol | 32.04 | 1000 | 32.00 | 40.6 | 3.0 |
| Iodine | 253.80 | 500 | 126.9 | – | 1.5 |
Experimental:
To a 100 ml round bottom flask was added methanol (32.0 g, 40.6 ml, 1.00 mol, 3 Eq.) and red phosphorus (10.2 g, 0.33 mol, 1 Eq.) and a 100 ml pressure equalizing addition funnel plugged with cotton1 and filled with iodine (126.9 g, 0.50 mol, 1.5 Eq.) was fitted to the flask. A dimroth condenser with a calcium chloride drying bottle was attached to the top of the addition funnel and the stopcock was opened. Continuous reflux of the reaction mixture was performed using a heating mantle set to approximately 40W power until no more iodine remained inside the addition funnel, at which point the heating power was reduced to 20W for an additional hour.2 The mixture was allowed to cool, and the apparatus was rearranged for fractional distillation. The fraction with a boiling range of 42 – 46 °C was collected, washed twice each with water and brine containing a small amount of sodium thiosulfate and dried over magnesium sulfate. Methyl iodide (97.94 g, 0.69 mol, 69%) was obtained as a clear liquid and stored in an amber glass bottle over copper wire. Refractive index: ND20 = 1.5320 (lit. 1.5320, 1.5304).
Adapted from: Becker, H. G., Berger, W. & Domschke, G. (1998). Organikum: Organisch chemisches Grundpraktikum (20. Aufl.). Wiley-VCH.
