Some Commercially Important Alcohols And Phenols

ChemistryAlcohols, Phenols And EthersFor NEET aspirants

The commercially important alcohols and phenols are methanol, ethanol, ethane-1,2-diol, glycerol, phenol and its relatives such as cresols, picric acid and salicylic acid. Methanol is made from CO and H $_{2}$ and is a dangerous poison; ethanol comes from fermentation of molasses or starch and from hydration of ethene. Learning how the commercially important alcohols and phenols are made, graded and used, plus the one safety fact for each, covers the NCERT section fully. A regular one-mark area in NEET.

On this page1Overview2Methanol3Ethanol by fermentation4Grades of ethanol5Glycol, glycerol6Phenols in use7Identify8Examples

Key Formulas - Quick Reference

★ Must learnMethanol: CO + 2 $H_{2}$ $ZnO-Cr_{2} O_{3}, 573 - 673 K, 200 - 300 atm$ $C H_{3} OH$
Molasses: $C_{12} H_{22} O_{11}$ + $H_{2} O$ $invertase$ $C_{6} H_{12} O_{6}$ (glucose) + $C_{6} H_{12} O_{6}$ (fructose)
★ Must learnFermentation: $C_{6} H_{12} O_{6}$ $zymase$ 2 $C_{2} H_{5} OH$ + 2 $C O_{2}$
Starch: diastase gives maltose; maltase gives glucose
★ Must learnRectified spirit: 95.6% ethanol (azeotrope); absolute alcohol: 100%
★ Must learnDenatured spirit: ethanol + $CuS O_{4}$ (colour) + pyridine (smell)
Aspirin: salicylic acid + $(C H_{3} CO)_{2} O$ $\to$ acetylsalicylic acid + $C H_{3} COOH$

1. Four Alcohols Industry Cannot Do Without

Two monohydric alcohols, methanol and ethanol, are made in enormous amounts as solvents, fuels and starting materials. Two polyhydric alcohols, ethylene glycol and glycerol, are valued for their many hydrogen bonds, which make them viscous, high boiling and fully miscible with water.

Figure 1: Four alcohols that industry makes by the million tonnes: one poison, one fuel and beverage, one antifreeze, one moisturiser.

2. Methanol, the Wood Spirit

Methanol was first obtained by destructive distillation of wood, hence the name wood spirit. Today it is made from synthesis gas (carbon monoxide and hydrogen) under high pressure over a zinc oxide-chromium(III) oxide catalyst.

CO + 2 H_{2} ZnO - C r_{2} O_{3}, 573-673 K C H_{3} OH

Figure 2: Methanol from synthesis gas. One CO and two H

_{2}

combine over ZnO-Cr

_{2}

_{3}

at 573-673 K and 200-300 atm; the old name wood spirit comes from its first source.

Methanol is a colourless liquid (b.p. 337 K), used as a solvent for paints and varnishes, for making methanal (formaldehyde), as a fuel and in denatured spirit. It is highly poisonous: drinking small quantities causes blindness and larger quantities death.

Figure 3: Methanol itself is not the real poison. The liver oxidises it to methanal and methanoic acid; a few mL cause blindness and larger amounts death. Ethanol slows this by competing for the same enzyme.

Exam Trick

"Three M's that blind." Methanol, methanal, methanoic acid: the body turns the first into the other two. Ethanol, the antidote, gets to the enzyme first.

Spurious (hooch) liquor tragedies are caused by methanol. The damage is done by methanal and methanoic acid formed in the liver; ethanol is used as an antidote because it keeps the enzyme busy.

Key idea

Methanol: made from CO and

H_{2}

, used as solvent and fuel, and poisonous because the body oxidises it to methanal and methanoic acid.

3. Ethanol by Fermentation

Ethanol has been made by fermentation for thousands of years. Molasses (the syrup left after sugar crystallises from cane juice) or starch from grains and potatoes is fermented by yeast, whose enzymes act one after another. Air must be kept out; otherwise bacteria oxidise the ethanol to ethanoic acid, which is how vinegar forms.

C_{12} H_{22} O_{11} + H_{2} O invertase C_{6} H_{12} O_{6} + C_{6} H_{12} O_{6}

C_{6} H_{12} O_{6} zymase 2 C_{2} H_{5} OH + 2C O_{2}

Figure 4: Fermentation is a chain of enzyme steps from yeast. Every route ends in zymase: C

_{6}

_{12}

_{6}

gives 2C

_{2}

_{5}

OH + 2CO

_{2}

, in the absence of air.

Enzyme	Source	Reaction
diastase	malt (germinated barley)	starch $\to$ maltose
maltase	yeast	maltose $\to$ glucose
invertase	yeast	sucrose $\to$ glucose + fructose
zymase	yeast	glucose or fructose $\to$ ethanol + $C O_{2}$

Exam Trick

"D-M-I-Z." Diastase, Maltase, Invertase, Zymase. Zymase is always the last step; the others only break big sugars down to glucose and fructose.

Quick Recall: tap to check

Which enzyme converts glucose into ethanol?

Zymase (from yeast).

Which enzyme hydrolyses sucrose?

Invertase.

What happens if air enters the fermenter?

Ethanol is oxidised to ethanoic acid (vinegar).

4. Grades of Ethanol and Industrial Ethanol

Distilling the fermented liquid gives rectified spirit, 95.6% ethanol by mass. Further distillation cannot remove the last water because this mixture boils at a constant temperature (an azeotrope). Absolute alcohol is made by distilling rectified spirit with benzene, or by standing it over quicklime. Ethanol blended with petrol is power alcohol; India now sells E-20 petrol containing 20% ethanol.

Figure 5: Rectified spirit stops at 95.6% ethanol (by mass) because it forms a constant-boiling mixture with water; absolute alcohol needs a third substance (benzene or quicklime).

Ethanol sold for industrial use is made unfit for drinking by adding copper sulphate (to give colour) and pyridine (a foul-smelling liquid). This is called denaturation, and the product denatured spirit is exempt from the tax on drinkable alcohol. Most non-beverage ethanol is made by hydration of ethene.

C H_{2} = C H_{2} + H_{2} O H_{3} P O_{4}, 573 K C_{2} H_{5} OH

Figure 6: Three industrial steps. Ethene hydration makes most non-beverage ethanol; benzene breaks the 95.6% azeotrope; copper sulphate (colour) and pyridine (smell) make industrial ethanol undrinkable (denatured).

Rectified spirit95.6% ethanol
constant-boiling mixture with water
by fractional distillation

Absolute alcohol100% ethanol
azeotropic distillation with benzene
or drying with CaO

Key idea

Rectified spirit cannot be distilled to 100% because it is an azeotrope; denaturing adds

CuS O_{4}

and pyridine.

5. Ethylene Glycol and Glycerol

Ethane-1,2-diol (ethylene glycol, b.p. 470 K) is made from ethene through ethylene oxide. Mixed with water it lowers the freezing point, so it is the antifreeze in car radiators; with terephthalic acid it forms the polyester PET used in bottles and fibres. Propane-1,2,3-triol (glycerol, b.p. 563 K) is a by-product of soap making. It is sweet and hygroscopic, used in cosmetics, toothpaste and medicines, and its nitrate ester, nitroglycerine, is both an explosive (dynamite) and a heart drug.

Figure 7: More OH groups, more hydrogen bonds: glycol (b.p. 470 K) and glycerol (b.p. 563 K) are viscous, high-boiling and miscible with water, which is why they are used as antifreeze and moisturiser.

(RCOO)_{3} C_{3} H_{5} + 3NaOH heat C_{3} H_{5} (OH)_{3} + 3RCOONa

Figure 8: Soap making releases glycerol. One mole of fat and three moles of NaOH give one mole of glycerol and three moles of soap.

Glycerol is identified by heating it with potassium hydrogensulphate: it loses two molecules of water to give acrolein (

C H_{2} = CHCHO

), which has a sharp, choking smell.

6. Commercially Important Phenols

Phenol (carbolic acid) was the first antiseptic, used by Lister in surgery. Today most phenol goes into plastics: bakelite with methanal, polycarbonates, and nylon through cyclohexanol. Salicylic acid, made by the Kolbe reaction, is acetylated to aspirin. Cresols are disinfectants, picric acid was a military explosive and is still used in burn dressings, quinol is a photographic developer, and 2-naphthol is used for azo dyes.

Figure 9: Phenols in daily life: plastics, disinfectants, medicines, dyes and photography all start from the Ar-OH group.

C_{7} H_{6} O_{3} + C_{4} H_{6} O_{3} H^{+} C_{9} H_{8} O_{4} + C_{2} H_{4} O_{2}

Figure 10: Two big uses of phenol. Acetylating the OH of salicylic acid gives aspirin; phenol condensed with methanal gives the thermosetting plastic bakelite.

JEE Advanced

Aspirin is an acetylated phenol, not an acetylated carboxylic acid: the acetyl group goes on the phenolic oxygen, and the free COOH group is why aspirin is acidic and can irritate the stomach. In the body it is hydrolysed back to salicylic acid, the active painkiller.

7. Telling Them Apart

Simple tests identify each compound. The iodoform test is the classic way to tell ethanol from methanol: ethanol has the $C H_{3} CH (OH) -$ group, methanol does not.

Figure 11: Flowchart: three one-step tests separate the compounds on this page. The iodoform test is the classic way to tell ethanol from methanol.

Quick Recall: tap to check

Test to distinguish ethanol from methanol?

Iodoform: ethanol gives a yellow precipitate, methanol does not.

Why is methanol called wood spirit?

It was first obtained by destructive distillation of wood.

Two denaturants of ethanol?

Copper sulphate and pyridine.

Figure 12: Mind map: how each compound is made, what it is used for, and the one safety fact you must know.

8. Solved Examples

Solved Example 1

Drinking methanol causes blindness because it is
(A) converted to ethanol
(B) oxidised to methanal and methanoic acid
(C) a strong base
(D) insoluble in blood

Solution:

Answer: (B). Liver alcohol dehydrogenase oxidises it to methanal, then methanoic acid, which damage the optic nerve and cause acidosis.

Solved Example 2

Ethanol is denatured by adding
(A) methanol and water
(B) copper sulphate and pyridine
(C) benzene
(D) sodium chloride

Solution:

Answer: (B). Copper sulphate gives a colour and pyridine a foul smell (NCERT). Benzene is used to make absolute alcohol, not to denature it.

Solved Example 3

What mass of ethanol and of carbon dioxide can 1.00 kg of glucose give on complete fermentation?

Solution:

$C_{6} H_{12} O_{6} \to 2 C_{2} H_{5} OH + 2C O_{2}$ . $M$ : glucose 180.2, ethanol 46.07, $C O_{2}$ 44.01 g mol $^{- 1}$ . Ethanol $= 1000 \times 2 \times 46.07/180.2 = 511$ g; $C O_{2}$ $= 489$ g. The masses add up to 1000 g, as they must.

Solved Example 4

What volume of carbon dioxide at STP (273 K, 1 bar) is given by the fermentation of 90.0 g of glucose?

Solution:

90.0 g is 0.500 mol of glucose, which gives 1.00 mol of $C O_{2}$ . At STP one mole occupies 22.7 L, so the volume is 22.7 L (22.4 L if STP is taken as 1 atm).

Solved Example 5

Calculate the mass of aspirin that can be made from 13.8 g of salicylic acid.

Solution:

$M$ (salicylic acid, $C_{7} H_{6} O_{3}$ ) = 138.1 and $M$ (aspirin, $C_{9} H_{8} O_{4}$ ) = 180.2 g mol $^{- 1}$ , one to one. 13.8 g is 0.100 mol, giving 18.0 g of aspirin.

Solved Example 6

Why cannot 100% ethanol be obtained by fractional distillation of a fermented liquid?

Solution:

At 95.6% ethanol the vapour has the same composition as the liquid (an azeotrope that boils at a constant 351.1 K), so distillation can concentrate it no further. A third substance such as benzene, or a drying agent such as quicklime, removes the remaining water.

Solved Example 7

How much glycerol is released when 1.00 kg of glyceryl tristearate,

(C_{17} H_{35} COO)_{3} C_{3} H_{5}

, is saponified?

Solution:

$M$ (tristearin) = 891.5 and $M$ (glycerol) = 92.09 g mol $^{- 1}$ ; one mole of fat gives one mole of glycerol. Mass $= 1000 \times 92.09/891.5 = 103$ g.

Solved Example 8

A liquid gives a yellow precipitate with

I_{2}

and NaOH and no colour with neutral

FeC l_{3}

. It is one of methanol, ethanol, phenol. Identify it.

Solution:

Ethanol. No violet colour rules out phenol; the iodoform test needs a $C H_{3} CH (OH) -$ group, which ethanol has and methanol does not.

Practice Questions

Name the catalyst and conditions for manufacturing methanol.Answer: ZnO- $C r_{2} O_{3}$ , 573-673 K, 200-300 atm.
Which enzyme converts maltose to glucose?Answer: maltase.
What is power alcohol?Answer: ethanol blended with petrol and used as motor fuel.
Why is ethylene glycol used in car radiators?Answer: it lowers the freezing point of water (antifreeze).
Name the product of heating glycerol with $KHS O_{4}$ .Answer: acrolein, $C H_{2}$ =CHCHO.
Give one use each of picric acid and quinol.Answer: explosive/burn dressing; photographic developer.
Why is ethanol given as an antidote in methanol poisoning?Answer: it competes for alcohol dehydrogenase, slowing methanal formation.

Common Mistakes to Avoid

Watch out

Calling methanol grain alcohol. Methanol is wood spirit; ethanol is grain alcohol.
Writing 100% ethanol from distillation. Rectified spirit stops at 95.6% because it is an azeotrope.
Using benzene as a denaturant. Denaturants are copper sulphate and pyridine; benzene is used for absolute alcohol.
Letting air in during fermentation. Air oxidises ethanol to ethanoic acid (vinegar).
Mixing up the enzymes. Invertase acts on sucrose, diastase on starch, maltase on maltose, zymase on glucose.
Writing aspirin as an ester of the COOH group. The acetyl group is on the phenolic oxygen.
Thinking methanol itself blinds. Its oxidation products, methanal and methanoic acid, do the damage.
Using the iodoform test to detect methanol. Methanol gives no iodoform; ethanol does.

Frequently Asked Questions

How is methanol manufactured commercially?

Methanol is made by passing carbon monoxide and hydrogen (synthesis gas) over a zinc oxide and chromium(III) oxide catalyst at 573-673 K and 200-300 atm. It was first obtained by destructive distillation of wood, which is why it is called wood spirit.

Why is methanol poisonous?

In the liver, alcohol dehydrogenase oxidises methanol to methanal and then to methanoic acid. These damage the optic nerve and cause acidosis, so small amounts cause blindness and larger amounts death. Ethanol is used as an antidote because it competes for the same enzyme.

How is ethanol obtained by fermentation?

Yeast enzymes break down sugars step by step. Invertase converts sucrose from molasses into glucose and fructose, and zymase converts these into ethanol and carbon dioxide. Starch is first broken down by diastase and maltase. Air must be excluded to avoid vinegar formation.

What is denatured alcohol?

Denatured alcohol is ethanol made unfit for drinking by adding copper sulphate, which gives it a colour, and pyridine, which gives it a foul smell. It is used as an industrial solvent and fuel and is exempt from the tax on drinkable alcohol.

What is the difference between rectified spirit and absolute alcohol?

Rectified spirit is 95.6 per cent ethanol, a constant-boiling mixture with water that ordinary distillation cannot concentrate further. Absolute alcohol is 100 per cent ethanol, made by distilling rectified spirit with benzene or by drying it over quicklime.

What are the uses of phenol?

Phenol is used to make bakelite with methanal, nylon through cyclohexanol, polycarbonate plastics, aspirin via salicylic acid, dyes and picric acid. It was also the first surgical antiseptic, called carbolic acid, and its methyl derivatives, the cresols, are disinfectants.

What does NEET ask about commercially important alcohols?

NEET usually asks the denaturing agents of ethanol, the enzymes of fermentation such as invertase and zymase, the catalyst for methanol synthesis, or why methanol causes blindness. These four facts cover almost every question from this NCERT section.

How is this topic linked to JEE Main questions?

JEE Main uses these compounds in calculations and tests: mass of ethanol or carbon dioxide from a given mass of glucose, yield of aspirin from salicylic acid, or distinguishing methanol from ethanol by the iodoform test. Balanced equations and molar masses are the key.

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