Methanol

Methanol

Methanol or Methyl alcohol is also known as carbinol. Methanol is a colorless, inflammable liquid, b.p. 64^oC and is poisonous. It is miscible with water in all proportions, and is also miscible with most organic solvents. It burns with a faintly luminous flame, and its vapour forms explosive mixtures with air or oxygen when ignited. It combines with calcium chloride to form CaCl₂.4CH₃OH, and hence cannot be dried this way (cf. ethanol).

Preparation of methanol (carbinol)

Methyl alcohol or methanol is prepared industrially by several methods. The earliest method was the destructive distillation of wood, whereby tar and an aqueous fraction known as pyroligneous acid are obtained. Pyroligneous acid contains methanol, acetone and acetic acid, and all three compounds may be obtained by suitable treatment. It was this method which gave rise to the name ‘wood spirit’ for methanol. The modern methods are synthetic.

Synthesis gas is passed at a pressure of 200 atmospheres over catalysts containing the oxides of copper, zinc and chromium at 300^oC.

CO   +   2H₂    →    CH₃OH

If the proper precautions are taken, the yield of methanol is almost 100 per cent, and its purity is above 99 per cent. By changing the catalyst and the ratio of carbon monoxide to hydrogen, methanol and a variety of higher alcohols are produced.

By the catalytic oxidation of methane. A mixture of methane and oxygen (ratio by volume of 9:1) at a pressure of 100 atmospheres is passed through a copper tube at 200^oC:

Structure of Methanol

Analysis and molecular-weight determinations show that the molecular formula of methanol is CH₄O. Assuming that carbon is quadrivalent, oxygen bivalent and hydrogen univalent, only the structure is possible:

This is supported by all the chemical reactions of methanol, e.g., (i) only one hydrogen in methanol is replicable by sodium; this suggests that one hydrogen atom is in a different state of combination from the other three. (ii) Methanol is formed from methyl chloride by hydrolysis with sodium hydroxide. Methyl chloride can have only the structure CH₃Cl. It is reasonable to suppose that the methyl group in methyl chloride is unchanged by the action of dilute alkali, and that the reaction takes place by the replacement of the chlorine atom by a hydroxyl group.

Use of Methanol

Methanol is used as a solvent for paints, varnishes, shellac, celluloid cements, etc. in the manufacture of dyes, perfumes, formaldehyde, etc. It is also used for making methylated spirit and automobile antifreeze mixtures.