Commercially Important Alcohols: Methanol & Ethanol

Why Only Two Alcohols Matter

NCERT opens Section 7.5 with a plain statement: methanol and ethanol are the two commercially important alcohols. The reason is scale. Methanol, $\ce{CH3OH}$, is the simplest alcohol and the starting feedstock for an enormous downstream chemistry — formaldehyde, acetic acid, methyl tert-butyl ether (MTBE) and a host of solvents. Ethanol, $\ce{C2H5OH}$, is the alcohol of beverages, antiseptics, perfumery and, increasingly, of motor fuel.

For NEET, the value of this subtopic is its density of factual hooks: a named catalyst, two named enzymes, a boiling-point pair, a toxicity mechanism and a vocabulary of "spirits." Each is a clean single-statement question. The sections below take methanol first, then ethanol, then place the two side by side.

Methanol: Wood Spirit to Catalytic Synthesis

Methanol carries the old name wood spirit (or wood alcohol) because it was originally obtained by the destructive distillation of wood — heating wood in the absence of air. That route is now of historical interest only. Today, as NCERT states, most methanol is produced by the catalytic hydrogenation of carbon monoxide at high pressure and temperature in the presence of a $\ce{ZnO}$–$\ce{Cr2O3}$ catalyst:

$$\ce{CO + 2H2 ->[\text{ZnO–Cr2O3}][\text{high } P,\ T] CH3OH}$$

The synthesis gas mixture of $\ce{CO}$ and $\ce{H2}$ is itself derived from coal or natural gas. The reaction is exothermic and proceeds with a decrease in the number of gas molecules (three moles of reactant gas to one of product), so high pressure favours methanol — a direct application of Le Chatelier reasoning that examiners enjoy linking back to equilibrium.

Methanol: Properties & Uses

Methanol is a colourless, volatile liquid that boils at 337 K (about 64–65 °C) and is completely miscible with water through hydrogen bonding. Like all small alcohols its boiling point is far above that of the comparable hydrocarbon and ether because of intermolecular hydrogen bonding — a point developed in properties of alcohols.

Industrially it earns its place as a solvent in paints and varnishes and, above all, as a feedstock. NCERT singles out the manufacture of formaldehyde (methanal); NIOS adds that methanol is also a starting material for acetic acid and for MTBE, $\ce{CH3OC(CH3)3}$, a petrol additive. A compact way to remember its three industrial faces is: solvent, antifreeze and feedstock.

The Toxicity of Methanol

The most heavily examined fact about methanol is that it is intensely poisonous. NCERT states it directly: ingestion of even small quantities can cause blindness, and larger quantities cause death. The chemistry behind this is the same oxidation sequence that all primary alcohols undergo in the body, catalysed by the enzyme alcohol dehydrogenase:

$$\ce{CH3OH ->[\text{[O]}] HCHO ->[\text{[O]}] HCOOH}$$

Methanol is oxidised first to methanal (formaldehyde) and then to methanoic (formic) acid. These metabolites attack the optic nerve and trigger metabolic acidosis — hence blindness and, at higher doses, death. The danger is amplified because adulterated or denatured liquor sometimes contains methanol mixed with ethanol.

For comparison, the biological oxidation of ethanol gives ethanal and then ethanoic acid; ethanal acting on the central nervous system is what produces intoxication, while large quantities depress respiration and can be fatal in their own right. The difference between the two alcohols is therefore not that one is metabolised and one is not — both are — but that methanol's metabolites are specifically neurotoxic.

Ethanol by Fermentation

Ethanol is obtained commercially by fermentation, the oldest of all chemical manufacturing processes. The sugar present in molasses, sugarcane or fruits such as grapes is the raw material. NCERT describes a two-enzyme sequence. First the disaccharide is hydrolysed to monosaccharides by the enzyme invertase:

$$\ce{C12H22O11 ->[\text{invertase}][\text{H2O}] C6H12O6 + C6H12O6}$$

Both products — glucose and fructose — share the formula $\ce{C6H12O6}$. These hexoses then undergo fermentation in the presence of zymase, an enzyme found in yeast, to yield ethanol and carbon dioxide:

$$\ce{C6H12O6 ->[\text{zymase}] 2C2H5OH + 2CO2}$$

Fermentation proceeds under anaerobic conditions (in the absence of air), and the carbon dioxide released is what makes a fermenting vat bubble. NCERT adds two practical limits worth remembering: the action of zymase is inhibited once the alcohol concentration exceeds about 14 percent, and if air is admitted, atmospheric oxygen oxidises ethanol to ethanoic acid, souring the product.

NCERT also gives the winemaking illustration: as grapes ripen, sugar content rises and yeast grows on the outer skin, so crushing the grapes brings sugar and enzyme into contact and fermentation begins on its own.

Ethanol by Hydration of Ethene

Fermentation is ancient, but it is no longer the whole story. NCERT notes that large quantities of ethanol are now obtained by the hydration of ethene — the same acid-catalysed hydration covered under preparation of alcohols. Ethene reacts with water in the presence of an acid catalyst to add $\ce{H}$ and $\ce{OH}$ across the double bond:

$$\ce{CH2=CH2 + H2O ->[\text{H3PO4 / } H+][\text{high } P,\ T] CH3CH2OH}$$

NIOS describes the closely related industrial variant that proceeds through ethyl hydrogen sulphate: ethene is absorbed in concentrated sulphuric acid to give $\ce{C2H5OSO3H}$, which is then hydrolysed to ethanol. Either way the petrochemical route, drawing on ethene from cracking, gives ethanol uncontaminated by the fusel-oil by-products of fermentation, which matters for solvent-grade and reagent-grade product.

Grades of Ethanol: Rectified, Absolute, Denatured

Fermented liquor is dilute, and ethanol is sold in several well-defined grades. Distillation cannot reach pure ethanol directly because ethanol and water form a constant-boiling azeotrope at roughly 95% ethanol — a fact that explains the vocabulary of "spirits" used in chemistry and industry.

NCERT describes denaturation explicitly: commercial alcohol is made unfit for drinking by mixing in some copper sulphate (to give it a colour) and pyridine (a foul-smelling liquid). The resulting product — denatured or methylated spirit — escapes the heavy excise duty levied on potable alcohol while remaining usable as a solvent and fuel. NIOS makes the same point: ethanol used for scientific and industrial purposes is made purposely toxic to discourage consumption.

Power Alcohol & Ethanol-Blended Fuel

When ethanol is blended with petrol so that the mixture can serve as a motor fuel, the product is called power alcohol. NIOS records the application directly: ethanol can be used as a fuel and as a fuel additive. Three advantages drive its adoption. Ethanol raises the octane rating of the blend, so it acts as an anti-knock agent; it contains oxygen and therefore burns more cleanly, lowering carbon monoxide and unburnt hydrocarbon emissions; and because it is made from biomass such as molasses, it is a renewable resource that reduces dependence on imported crude.

National programmes that mix a fixed proportion of ethanol into petrol — for instance a 10% or 20% blend — are described as ethanol-blended fuel programmes. The chemistry connects neatly to fermentation: the same molasses that yields potable and industrial spirit also feeds the fuel-ethanol supply, making this one of the few NEET facts that ties laboratory chemistry to national energy policy.

Methanol vs Ethanol: The Comparison

Because the two alcohols appear together in NCERT, examiners frequently ask students to distinguish them on a single point. The table below collects the contrasts that carry marks.