Alcohols and phenols

Alcohols are organic compounds in which one or more hydrogen atoms of a hydrocarbon are replaced by hydroxyl groups –OH, the hydroxyl group being attached to an sp³ carbon (if it is attached directly to a benzene ring, the compound is a phenol). Ethanol is the best-known organic solvent, a fuel and a raw material for medicines; methanol is used to make methanal and other chemicals; ethane-1,2-diol is antifreeze; and glycerol is used in cosmetics, medicine and explosives. Exam questions constantly involve the reactions of alcohols with sodium, dehydration, oxidation and the test for polyhydric alcohols. This topic bridges hydrocarbons and carbonyl compounds.

§1. Classification, naming and isomerism

• By the number of hydroxyl groups: monohydric (methanol CH₃OH, ethanol C₂H₅OH), dihydric (ethane-1,2-diol HOCH₂–CH₂OH), trihydric (glycerol, propane-1,2,3-triol HOCH₂–CH(OH)–CH₂OH).
• By the type of carbon bearing the OH: primary (R–CH₂OH), secondary (R₂CHOH), tertiary (R₃COH).

IUPAC names add -ol to the alkane stem and give the position of the OH: propan-1-ol, propan-2-ol, 2-methylpropan-2-ol.
Isomerism: chain, position of the hydroxyl group, and functional-group isomerism with ethers (C₂H₆O: ethanol and methoxymethane). For example, C₃H₈O fits three substances: propan-1-ol, propan-2-ol (both alcohols) and methoxyethane CH₃–O–C₂H₅. The number of isomers grows quickly with chain length: there are four alcohols C₄H₉OH and eight C₅H₁₁OH.

Worked example

7.4 g of a saturated monohydric alcohol reacted with excess sodium, releasing 1.12 L of hydrogen (STP). Find the formula of the alcohol.

1. 2ROH + 2Na → 2RONa + H₂: n(H₂) = 1.12/22.4 = 0.05 mol → n(alcohol) = 0.1 mol.
2. M = 7.4/0.1 = 74 g/mol.
3. 14n + 18 = 74 → n = 4: C₄H₉OH (butanol).

§2. Physical properties and hydrogen bonding

Alcohol molecules form O–H···O hydrogen bonds with one another. That is why alcohols boil much higher than hydrocarbons and ethers of similar molar mass: ethanol boils at 78 °C, while methoxymethane of the same molar mass is a gas at room temperature. The lower alcohols (C₁–C₃) mix with water in all proportions because they also hydrogen-bond to water; solubility falls as the hydrocarbon part grows. Polyhydric alcohols are viscous, sweet-tasting liquids. The lower monohydric alcohols are colourless, volatile liquids with a characteristic smell; from about C₁₂ alcohols are solids and practically insoluble in water. For alcohols with the same number of carbons, the boiling point rises with the number of OH groups: ethane-1,2-diol boils at 197 °C.
Methanol is highly poisonous: even a small amount can cause blindness and death. Ethanol is also harmful; long-term use damages the liver and nervous system.

§3. Chemical properties

• With reactive metals (via the hydroxyl hydrogen): 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑ (sodium ethoxide). Alcohols are extremely weak acids, weaker even than water, so they do not react with alkalis (unlike phenol).

The weak acidity of alcohols is also shown by hydrolysis of alkoxides: sodium ethoxide is completely decomposed by water into the alcohol and the alkali: C₂H₅ONa + H₂O → C₂H₅OH + NaOH. As the alkyl group gets larger, the reaction with sodium slows: methanol reacts quickly, butanol gently.

• With hydrogen halides (the OH group is replaced): C₂H₅OH + HBr → C₂H₅Br + H₂O.
• Dehydration (conc. H₂SO₄ or Al₂O₃):

• Oxidation (hot CuO, or acidified K₂Cr₂O₇):

• Esterification: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O.
• Combustion: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O — a blue, almost smokeless flame.

Worked example

4.6 g of ethanol reacted with excess sodium. What volume of hydrogen (STP) is released?

1. n(C₂H₅OH) = 4.6/46 = 0.1 mol.
2. 2C₂H₅OH → H₂: n(H₂) = 0.05 mol.
3. V = 0.05·22.4 = 1.12 L.

§4. Polyhydric alcohols

Ethane-1,2-diol is a poisonous, sweet-tasting liquid; its aqueous solution freezes at low temperature, so it is used as car antifreeze, and it is a monomer for PET polyester. Glycerol (propane-1,2,3-triol) is obtained by hydrolysis of fats; it is non-toxic and hygroscopic and is used in cosmetics, leather tanning and food.
Several neighbouring hydroxyl groups influence one another and slightly increase acidity, so polyhydric alcohols form a complex with copper(II) hydroxide:

Monohydric alcohols do not give this test. Glycerol reacts with nitric acid (in the presence of conc. H₂SO₄) to give nitroglycerine: C₃H₅(OH)₃ + 3HNO₃ → C₃H₅(ONO₂)₃ + 3H₂O — a powerful explosive and a heart medicine.

§5. Preparation

Ethanol from fermentation is used in food and medicine, synthetic ethanol for technical purposes. Ethane-1,2-diol is made industrially by hydrating epoxyethane, and in the laboratory by mild oxidation of ethene with aqueous KMnO₄ (the Wagner reaction). Ethanol is widely used as a fuel additive (bioethanol), as an antiseptic (70 % solution) and as a solvent in perfumes; it is converted into ethanoic acid, ethoxyethane and butadiene.

Worked example

What mass of ethanol forms when 90 g of glucose is completely fermented?

1. n(C₆H₁₂O₆) = 90/180 = 0.5 mol.
2. C₆H₁₂O₆ → 2C₂H₅OH: n(C₂H₅OH) = 1 mol.
3. m = 1·46 = 46 g.

§6. An official test item

1 mol of a compound whose benzene ring carries –COOH, a phenolic –OH and –CH₂CH₂OH groups was treated with excess NaOH. Find the molar mass of the product.

Worked example

1. The compound is HOOC–C₆H₃(OH)–CH₂CH₂OH = C₉H₁₀O₄, M = 182.
2. Only the carboxyl group and the phenolic hydroxyl react with NaOH; the alcoholic hydroxyl does not.
3. Two H atoms are replaced by Na: M = 182 − 2 + 46 = 226 g/mol.

Check yourself

1. What volume of H₂ (STP) is released when 9.2 g of ethanol reacts with excess Na?

2. What forms when propan-2-ol is oxidised by copper(II) oxide?

3. How many alcohols have the formula C₄H₁₀O?

4. How can glycerol be distinguished from ethanol?

5. What forms when ethanol is heated with conc. H₂SO₄ at 130 °C?

6. Which compound is a functional-group isomer of ethanol?

7. How many moles of oxygen are needed to burn 0.2 mol of ethanol completely?

8. Which alcohol is not oxidised by CuO: primary, secondary or tertiary?

Common mistakes

• Writing that alcohols react with alkalis.
• Oxidising a tertiary alcohol with CuO to an aldehyde.
• Mixing up dehydration temperatures: alkene above 140 °C, ether below.
• Getting 1 mol of H₂ from 1 mol of alcohol with Na: it is 0.5 mol.
• Counting phenol as an alcohol: in phenol the OH is attached directly to the benzene ring.