Methanol and Ethanol: Industrial Uses and Toxicity

Methanol and ethanol are, by a wide margin, the two most industrially important alcohols, but they

differ sharply in one critical respect: how the human body handles them once ingested.

Methanol. Methanol ($\text{CH}_3\text{OH}$) is manufactured on a huge industrial scale by the

catalytic hydrogenation of carbon monoxide over a zinc oxide/chromium oxide (or, in the modern

low-pressure process, copper-based) catalyst:

Historically it was obtained by the destructive distillation of wood (hence its old name, 'wood

spirit'). It is used on a large scale as an industrial solvent, as a denaturant added to ethanol to

make it unfit for drinking without affecting its usefulness as an industrial solvent, as the

starting material for manufacturing formaldehyde (methanal, $\text{HCHO}$, itself the feedstock for

several plastics and resins) and formic acid, and increasingly as a feedstock for biodiesel

production via transesterification of vegetable oils.

Methanol's toxicity. Unlike ethanol, methanol is genuinely dangerous if ingested, even in

comparatively small quantities, and this danger is a direct consequence of how the body's own

enzymes process it. The liver enzyme alcohol dehydrogenase -- the same enzyme that normally

metabolises ethanol harmlessly -- oxidises methanol first to formaldehyde and then, by a

second oxidation, to formic acid. Formic acid is directly toxic: it causes a severe metabolic

acidosis (a dangerous drop in blood pH) and is specifically toxic to the optic nerve and retina,

which is why methanol poisoning characteristically causes blindness, in addition to other systemic

effects, and can be fatal even from amounts as small as $10$–$15\ \text{mL}$ in some cases. This is

precisely why methanol-contaminated ('spurious') liquor is such a serious public-health hazard, and

why methanol should never be substituted for ethanol in any beverage.

Ethanol. Ethanol ($\text{CH}_3\text{CH}_2\text{OH}$) is obtained on an industrial and

traditional scale by fermentation of sugars: yeast's own enzyme complex, zymase, converts

glucose (from sugarcane, grain, or other fermentable feedstocks) directly into ethanol and carbon

dioxide,

and, industrially, also by the acid-catalysed hydration of ethene (Section 16.3). Ethanol is used

as a solvent for paints, varnishes, perfumes and pharmaceuticals; as the alcohol of alcoholic …