Commercially Important Alcohols & Distinguishing Tests

Commercially Important Alcohols — Methanol

Methanol, CH$_3$OH (methyl alcohol, "wood spirit") was originally made by destructive distillation of wood. Today it is manufactured industrially by passing carbon monoxide and hydrogen over a ZnO-Cr$_2$O$_3$ catalyst at high temperature and pressure:

CO + 2 H$_2$ →(ZnO/Cr$_2$O$_3$, 200-300 atm, ~573-673 K)→ CH$_3$OH

Uses: as a solvent for paints, varnishes and shellac; as antifreeze; as a feedstock for making formaldehyde (HCHO) and other chemicals; and as a fuel.

Toxicity: methanol is highly poisonous. Even small amounts taken internally can cause blindness, and larger quantities cause death, because the body oxidises it to toxic formaldehyde and formic acid.

Key Point: methanol = wood spirit, made from CO + 2H$_2$ over ZnO/Cr$_2$O$_3$; a solvent, antifreeze and fuel; very toxic — causes blindness and death.

Commercially Important Alcohols — Ethanol

Ethanol, C$_2$H$_5$OH (ethyl alcohol) is the alcohol of alcoholic drinks. Industrially it is made by the fermentation of sugars. Sugar from molasses or starch is acted on by enzymes: invertase converts sucrose to glucose and fructose, and zymase (from yeast) then converts these to ethanol and CO$_2$:

C$_6$H$_{12}$O$_6$ →(zymase)→ 2 C$_2$H$_5$OH + 2 CO$_2$

The fermented liquid is then concentrated by fractional distillation (giving rectified spirit, about 95% ethanol).

Uses: as a solvent, in medicines (tinctures), in cosmetics, as a fuel and fuel additive, and in alcoholic beverages.

Key Point: ethanol is made by fermentation of sugars (enzymes invertase then zymase); used as a solvent, in medicines and as a fuel.

Denatured and Power Alcohol

Denatured (methylated) spirit: commercial ethanol is made unfit for drinking by adding small amounts of poisonous substances such as methanol, pyridine and a blue/violet dye (copper sulphate). This is done so that industrial alcohol can be sold without the heavy excise duty charged on drinkable alcohol, while preventing its misuse as a beverage.

Power alcohol: ethanol blended with petrol (e.g. a 10-20% ethanol-petrol mixture) is called power alcohol and is used as a motor fuel. It improves combustion, conserves petroleum, and is a renewable, cleaner-burning additive (the basis of modern "ethanol-blended petrol").

Key Point: denatured spirit = ethanol + poisons (methanol/pyridine/dye), not drinkable; power alcohol = ethanol-petrol blend used as motor fuel.

Distinguishing 1°, 2° and 3° Alcohols — the Lucas Test

The Lucas test uses Lucas reagent = concentrated HCl + anhydrous ZnCl$_2$. The alcohol is shaken with the reagent at room temperature; the alkyl chloride formed is insoluble and appears as turbidity (cloudiness). The time taken to turn cloudy depends on the class of alcohol (rate of carbocation formation: 3° > 2° > 1°):

• Tertiary (3°) alcohol: turbidity appears immediately.
• Secondary (2°) alcohol: turbidity appears in about 5 minutes.
• Primary (1°) alcohol: no turbidity at room temperature (reacts only on heating).

Key Point: Lucas reagent (conc. HCl + ZnCl$_2$): 3° → immediate cloudiness, 2° → cloudy in ~5 min, 1° → no reaction at room temperature. The order follows carbocation stability.

Victor Meyer Test and the FeCl$_3$ Test

Victor Meyer test distinguishes 1°, 2° and 3° alcohols by the colour of the nitrolic acid product with NaOH:

1. The alcohol is converted to the iodide (with P/I$_2$), then to the nitroalkane (with AgNO$_2$), and finally treated with nitrous acid (HNO$_2$) and then NaOH.

• 1° alcohol → blood-red colour
• 2° alcohol → blue colour
• 3° alcohol → colourless (no colour)

Neutral FeCl$_3$ test (phenol vs alcohol): phenols give a characteristic violet/purple colour with neutral ferric chloride (forming a coloured iron-phenoxide complex), whereas alcohols give no colour. This is the standard way to tell a phenol from an alcohol.

NaHCO$_3$ test: carboxylic acids give effervescence (CO$_2$) with NaHCO$_3$; phenols and alcohols do not — this separates a carboxylic acid from a phenol.

Key Point: Victor Meyer colours — 1° red, 2° blue, 3° colourless. Neutral FeCl$_3$ gives a violet colour with phenols (not with alcohols). NaHCO$_3$ effervesces only with carboxylic acids.

Solved Examples

Example 1: Manufacture of methanol

Write the industrial preparation of methanol from water gas.

Solution: A mixture of CO + 2H$_2$ is passed over a ZnO-Cr$_2$O$_3$ catalyst at about 200-300 atm and 573-673 K: CO + 2H$_2$ → CH$_3$OH.

Example 2: Why is methanol dangerous?

Why is methanol called a poison?

Solution: In the body methanol is oxidised to formaldehyde and formic acid, which damage the optic nerve. Small amounts cause blindness and larger amounts cause death.

Example 3: Fermentation of sugar

Write the reaction by which ethanol is produced from glucose.

Solution: The enzyme zymase (from yeast) converts glucose: C$_6$H$_{12}$O$_6$ → 2 C$_2$H$_5$OH + 2 CO$_2$. (Sucrose is first hydrolysed by invertase to glucose and fructose.)

Example 4: Lucas test result

A student adds Lucas reagent to three alcohols. One turns cloudy at once, one after 5 minutes, and one stays clear. Identify them.

Solution: Immediate turbidity = tertiary (3°) alcohol; turbidity in ~5 min = secondary (2°) alcohol; no turbidity at room temperature = primary (1°) alcohol.

Example 5: Distinguish phenol from ethanol

How would you distinguish phenol from ethanol in the lab?

Solution: Add neutral FeCl$_3$: phenol gives a violet/purple colour, ethanol gives no colour. (Phenol also turns blue litmus faintly red and reacts with NaOH; ethanol does not.)

Example 6: Denatured spirit

What is denatured alcohol and why is it made?

Solution: It is ethanol made unfit to drink by adding poisons such as methanol, pyridine and a dye. It allows industrial alcohol to be used without excise duty while preventing its misuse as a beverage.

Example 7: Victor Meyer colours

What colours does the Victor Meyer test give for 1°, 2° and 3° alcohols?

Solution: 1° → blood-red, 2° → blue, 3° → colourless.

Example 8: Power alcohol

What is power alcohol and why is it used?

Solution: Power alcohol is a blend of ethanol with petrol used as a motor fuel. It improves combustion, conserves petroleum, and provides a cleaner, partly renewable fuel.

Example 9: Distinguish phenol from benzoic acid

How can phenol be distinguished from benzoic acid?

Solution: Benzoic acid gives effervescence with NaHCO$_3$ (releases CO$_2$); phenol does not. Also, phenol gives a violet colour with neutral FeCl$_3$ while benzoic acid does not.

Example 10: Distinguish 1° from 3° alcohol quickly

Which single test rapidly distinguishes a tertiary alcohol from a primary alcohol?

Solution: The Lucas test: a 3° alcohol gives immediate turbidity, while a 1° alcohol gives no turbidity at room temperature.