Lesson 5: Hydrocarbon Derivatives - Alcohols and Phenols

• In the past organic compounds were grouped by physical properties such as smell and taste, or by a few chemical properties.
• With the progress of chemical analysis it became clear that the properties of a compound are controlled mainly by a particular group of atoms in its molecule, the functional group.

Functional group: an atom or group of atoms, joined in a particular way, that forms part of the molecule and decides the characteristic chemical properties of the whole molecule.

Compounds with the same functional group belong to one homologous series and react similarly.

• Both contain one or more hydroxyl (–OH) groups.
• If the –OH is attached to an alkyl group (R–) the compound is an alcohol; if it is attached directly to a carbon of a benzene ring (aryl group, Ar–) it is a phenol.
• Both may be viewed as alkyl/aryl derivatives of water (one H of H₂O replaced by R or Ar) or as hydroxyl derivatives of hydrocarbons.

• Common system: the word "alcohol" is added after the name of the alkyl group: CH₃OH methyl alcohol, C₂H₅OH ethyl alcohol.
• The prefix iso is used when the chain ends in a carbon bearing two methyl groups and one hydrogen: isopropyl alcohol (CH₃)₂CH–OH.

IUPAC system: take the longest chain containing the carbon that carries –OH, change -e of the alkane to -ol, and number from the end nearest the –OH; put the number of the carbon holding –OH before the name:

• CH₃OH methanol; C₂H₅OH ethanol; CH₃CH₂CH₂OH 1-propanol (n-propyl alcohol); CH₃CH(OH)CH₃ 2-propanol (isopropyl alcohol).
• The three alcohol isomers derived from pentane: 1-pentanol, 2-pentanol, 3-pentanol (and there are others from branched chains: for example 2,2-dimethyl-1-propanol).
• Other examples: (CH₃)₂CHCH₂OH 2-methyl-1-propanol; 2,2-dimethyl-1-butanol is CH₃CH₂C(CH₃)₂CH₂OH; isopentyl alcohol (CH₃)₂CHCH₂CH₂OH (3-methyl-1-butanol).

(a) According to the number of –OH groups

• Monohydric: one –OH: CH₃OH, C₂H₅OH.
• Dihydric: two –OH: ethylene glycol HOCH₂CH₂OH (1,2-ethanediol).
• Trihydric: three –OH: glycerol (glycerine) HOCH₂–CH(OH)–CH₂OH (1,2,3-propanetriol).
• Polyhydric: more than three –OH: sorbitol (six –OH groups).

(b) According to the type of carbinol carbon (the carbon carrying –OH), for monohydric alcohols

• Primary (1°): carbinol carbon is attached to one other carbon (and two H): R–CH₂OH, e.g. ethanol, 1-propanol.
• Secondary (2°): carbinol carbon is attached to two carbons (and one H): R₂CHOH, e.g. 2-propanol, 2-butanol.
• Tertiary (3°): carbinol carbon is attached to three carbons (no H): R₃C–OH, e.g. 2-methyl-2-propanol (tert-butyl alcohol). (Methanol, the exception, has the carbinol carbon attached to no carbon and is counted with the primary alcohols.)

Ethanol, C₂H₅OH, is the oldest organic compound prepared in industry; the ancient Egyptians made it by fermenting sugars and starches thousands of years ago.

• 1.
• Alcoholic fermentation. About 20 % of the world's ethanol is made by fermenting starch and sugar materials (sugar cane, sugar beet, maize).
• In Egypt it is made from molasses (the sugar-rich residue of sugar refining, containing sucrose), for instance at the Egyptian Sugar and Distillation Company (Hawamdeya).
• Yeast is added:

• C₁₂H₂₂O₁₁ (sucrose) + H₂O —(invertase enzyme of yeast)→ C₆H₁₂O₆ (glucose) + C₆H₁₂O₆ (fructose)
• C₆H₁₂O₆ —(zymase enzyme)→ 2C₂H₅OH + 2CO₂ The resulting dilute alcohol (about 10–15 %) is concentrated by fractional distillation (about 95–96 % ethanol, a constant-boiling mixture with water).

2. Catalytic hydration of ethene. The main industrial method in petroleum-producing countries: ethene obtained by cracking petroleum fractions is hydrated with steam in the presence of dilute sulphuric or phosphoric acid: CH₂=CH₂ + H₂O —(H₃PO₄ or dil. H₂SO₄, heat, pressure)→ CH₃CH₂OH Ethanol is therefore called a petrochemical (made from petroleum). Note: ethene is the only alkene that gives a primary alcohol by hydration; other alkenes follow Markovnikov's rule and give secondary or tertiary alcohols: CH₃CH=CH₂ + H₂O → CH₃CH(OH)CH₃ (2-propanol, secondary) (CH₃)₂C=CH–CH₃ + H₂O → (CH₃)₂C(OH)–CH₂CH₃ (2-methyl-2-butanol, tertiary)

3. General method: hydrolysis of alkyl halides. Heating an alkyl halide with aqueous alkali (KOH or NaOH) replaces the halogen with –OH: R–X + KOH(aq) → R–OH + KX

• CH₃CH₂Br + KOH → CH₃CH₂OH + KBr (primary alcohol)
• CH₃CHBrCH₃ + KOH → CH₃CH(OH)CH₃ + KBr (secondary)
• (CH₃)₃C–Br + KOH → (CH₃)₃C–OH + KBr (tertiary) Ease of hydrolysis: R–Cl < R–Br < R–I (the weaker the C–X bond, the easier). So alkyl iodides are hydrolysed most easily. Which halide for which alcohol? methanol ← CH₃X; 2-butanol ← 2-halobutane; 2-methyl-2-pentanol ← 2-halo-2-methylpentane.

• Pure ethanol (96 %) is highly taxed to discourage its use in alcoholic drinks (harmful to health and society).
• Ethanol for fuel, solvent and industry is made cheaper by adding poisonous or ill-smelling substances that are very hard to remove: methanol (causes blindness and madness), pyridine (foul smell) and a coloured dye.
• This is denatured alcohol (converted alcohol, red spirit; about 85 % ethanol, 5 % methanol, 1 % other additive plus colour/odour, rest water).
• It is unfit and illegal to drink.

• Alcohols are neutral, colourless compounds.
• The first members are light liquids, completely miscible with water; the middle members are oily; the higher ones are waxy solids.
• Compared with alkanes of similar molecular mass, alcohols have high boiling points and (the lower members) high solubility in water.
• Both effects are due to the polar –OH group, which forms hydrogen bonds between alcohol molecules themselves and between alcohol and water molecules.
• Boiling points: ethanol 78.5 °C; ethylene glycol 197 °C.
• As the number of –OH groups rises, the number of hydrogen bonds rises, and so do solubility and boiling point.
• (Order of boiling point: ethanol < ethylene glycol < glycerol < sorbitol.)

Four types of reaction are distinguished:

A. Reactions of the hydrogen of the –OH group

1. Reaction with active metals (Na, K): ethanol is neutral to litmus, but it shows weak acid character by reacting with sodium, releasing hydrogen (which burns with a pop) and forming sodium ethoxide: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑ (The O–H bond is polar: the electron pair is displaced toward oxygen so the H can be replaced by metal.) Sodium ethoxide is hydrolysed by water, reforming ethanol: C₂H₅ONa + H₂O → C₂H₅OH + NaOH. Methanol: 2CH₃OH + 2Na → 2CH₃ONa + H₂.
2. Ester formation (with carboxylic acids): alcohols react with acids to form esters and water. Labelling with heavy oxygen ¹⁸O showed the water comes from –OH of the acid and H of the alcohol: CH₃COOH + C₂H₅¹⁸OH ⇌ CH₃CO¹⁸OC₂H₅ + H₂O (¹⁸O ends in the ester, not in the water). The reaction is reversible, so concentrated H₂SO₄ is added (catalyst and dehydrating agent).

B. Reaction of the –OH group (substitution) With halogen acids (HX) the –OH is replaced by X; e.g. ethanol with concentrated HCl in presence of ZnCl₂ (catalyst): C₂H₅OH + HCl —(ZnCl₂)→ C₂H₅Cl + H₂O Conversion back: C₂H₅Cl + KOH(aq) → C₂H₅OH + KCl.

C. Reactions of the carbinol group — oxidation Alcohols are oxidised by acidified K₂Cr₂O₇ (orange → green) or KMnO₄ (violet colour disappears). The oxidation removes hydrogen from the carbinol carbon, first giving an unstable gem-diol (two –OH on the same carbon) which loses water.

• Primary alcohol → aldehyde → carboxylic acid (two steps): CH₃CH₂OH —[O]→ CH₃CHO (ethanal) —[O]→ CH₃COOH (ethanoic acid; vinegar odour) Experiment: heat 3 mL ethanol with an equal volume of acidified potassium dichromate in a water bath for about 10 minutes — the colour changes from orange to green and the smell of vinegar appears.
• Secondary alcohol → ketone (one step): CH₃CH(OH)CH₃ —[O]→ CH₃COCH₃ (propanone, acetone).
• Tertiary alcohol: no hydrogen on the carbinol carbon, so it is not oxidised under these conditions.

D. Reaction of the whole molecule — dehydration with conc. H₂SO₄

• At 180 °C (excess acid): one molecule of water is removed from one molecule of alcohol → alkene: C₂H₅OH → CH₂=CH₂ + H₂O.
• At 140 °C (excess alcohol): one water molecule is lost from every two alcohol molecules → ether: 2C₂H₅OH → C₂H₅–O–C₂H₅ (diethyl ether) + H₂O.

• Ethanol: solvent for oils, fats, resins; making drugs, paints, polishes, perfumes; antiseptic/disinfectant (sterilising skin, mouthwash); fuel, mixed with gasoline in some countries (e.g. Brazil); main component of denatured alcohol; filling thermometers for very low temperatures (down to about −50 °C, because its freezing point is very low, −110 °C). Alcoholic drinks damage health (liver cirrhosis, cancer of the stomach and oesophagus).
• Ethylene glycol (dihydric): antifreeze in car radiators in cold countries; hydraulic brake fluids (viscous); printing inks; making polyethylene glycol and the polyester fibre Dacron, photographic film, recording tapes.
• Glycerol (trihydric): moisturiser in creams and cosmetics; softens textiles; manufacture of the explosive nitroglycerine (glyceryl trinitrate): C₃H₅(OH)₃ + 3HNO₃ —(conc. H₂SO₄)→ C₃H₅(ONO₂)₃ + 3H₂O Nitroglycerine in very small, safe doses is also used to widen arteries in heart treatment.

Phenols are hydroxy-aromatic compounds in which one or more –OH groups are attached directly to carbon atoms of a benzene ring: phenol C₆H₅OH (hydroxybenzene); 1,2,3-trihydroxybenzene (pyrogallol); 1,2,4-trihydroxybenzene.

Phenol (carbolic acid), C₆H₅OH has great industrial importance as the starting material of plastics (bakelite), dyes, disinfectants, salicylic acid derivatives (aspirin) and picric acid.

1. From coal tar by fractional distillation.
2. Hydrolysis of chlorobenzene with sodium hydroxide at high temperature and very high pressure (about 300 atm), giving sodium phenoxide which is acidified: C₆H₅Cl + 2NaOH (≈ 300 °C, 300 atm) → C₆H₅ONa + NaCl + H₂O C₆H₅ONa + HCl → C₆H₅OH + NaCl

• A colourless, corrosive (caustic) solid with a characteristic odour, m.p. 43 °C.
• It is sparingly soluble in cold water; solubility rises with temperature and it is completely miscible with water above 65 °C.

1. Acidity — phenol compared with alcohols. Both react with sodium metal releasing hydrogen: 2C₆H₅OH + 2Na → 2C₆H₅ONa + H₂↑ ; 2ROH + 2Na → 2RONa + H₂↑. But only phenol reacts with sodium hydroxide solution (alcohols do not): C₆H₅OH + NaOH → C₆H₅ONa + H₂O (sodium phenoxide). Hence phenol is called carbolic acid. The benzene ring withdraws electrons from the oxygen of the O–H group, which makes the O–H bond more polar, longer and weaker, and so the proton leaves more easily. (Phenol is still a weak acid – weaker than carboxylic acids and even carbonic acid.)

• 2.
• Reaction with halogen acids. In alcohols the –OH can be replaced by X with HX.
• In phenol the C–O bond is shorter and stronger (the oxygen lone pair interacts with the ring), so the –OH cannot be replaced by halogen acids; phenol does not react with HCl.

3. Nitration. Phenol with concentrated nitric acid in the presence of concentrated sulphuric acid gives 2,4,6-trinitrophenol (picric acid): C₆H₅OH + 3HNO₃ —(conc. H₂SO₄)→ C₆H₂(NO₂)₃OH + 3H₂O Picric acid is an explosive and is used in treating burns; it stains the skin yellow for several days (until the outer skin layer is renewed).

4. Condensation with formaldehyde → Bakelite. In acidic or basic medium phenol reacts with methanal (formaldehyde, HCHO): one molecule of methanal joins two molecules of phenol with loss of water; the units keep joining by condensation polymerisation to form a three-dimensional network polymer, Bakelite. Condensation polymers are formed from two kinds of monomers with elimination of a small molecule such as H₂O. Bakelite is a dark-brown thermosetting plastic (it does not soften again on heating), heat-resistant and a good electrical insulator — used for electrical fittings, switches, handles and ash trays.

1. Few drops of iron(III) chloride solution → a violet colour.
2. Bromine water → a white precipitate (2,4,6-tribromophenol).

(Ethanol, by contrast, gives no colour with FeCl₃ and no precipitate with bromine water; its detection uses the acidified dichromate test of Lesson 10.)

🔍 In-Depth Explanations, Deductions & Multi-Book Insights:

Chemistry Thanaweya Amma (2026/2027) Lesson 5 - Hydrocarbon Derivatives - Alcohols and Phenols

Detailed Academic Lesson Overview: Alcohols and phenols contain the hydroxyl group (-OH). Alcohols are aliphatic derivatives classified as primary (1°), secondary (2°), or tertiary (3°) based on carbinol carbon connectivity. Phenols are aromatic derivatives where -OH is bonded directly to the benzene ring.

🌳 Hierarchy Classification & Conceptual Roadmap

Alcohols and Phenols Chemistry

├── 1. Classification of Alcohols

│ ├── Monohydric: 1° (Ethanol), 2° (Isopropanol), 3° (2-methyl-2-propanol)

│ ├── Dihydric: Ethylene glycol; Trihydric: Glycerol; Polyhydric: Sorbitol, Glucose

│ └── Industrial Manufacture of Ethanol: Fermentation of Molasses (Sucrose) or Hydration of Ethene

├── 2. Chemical Oxidation of Alcohols (using acidified K2Cr2O7 or KMnO4)

│ ├── Primary (1°): Oxidizes in 2 steps: Alcohol → Aldehyde → Carboxylic acid

│ ├── Secondary (2°): Oxidizes in 1 step: Alcohol → Ketone

│ └── Tertiary (3°): Does NOT oxidize under ordinary conditions (no H on carbinol C)

└── 3. Phenols (Carbolic Acid C6H5OH)

├── Acidity of Phenol: Strongly more acidic than alcohols due to benzene ring drawing electrons, weakening O-H bond

├── Reactions: Reacts with NaOH to form sodium phenoxide (alcohols DO NOT react with NaOH)

└── Diagnostic Tests for Phenol:

├── Addition of FeCl3 solution produces a deep violet coloration

└── Addition of bromine water produces a white precipitate of 2,4,6-tribromophenol

📋 Master Teacher's Blackboard Rules & Exam Secrets

RULE 1: Tertiary alcohols resist oxidation with acidified dichromate because their carbinol carbon is attached to zero hydrogen atoms.

RULE 2: Phenols are significantly more acidic than alcohols (hence called Carbolic acid). Both react with active sodium metal, but ONLY phenols react with aqueous NaOH; alcohols DO NOT react with NaOH.

RULE 3: Diagnostic test for suspected drunk drivers: Breathing into a tube containing acidified potassium dichromate impregnated on silica gel; breath containing alcohol vapors changes the color from orange to green.

📊 Detailed Comparison Matrix: Comparative Analysis: Aliphatic Alcohols vs. Aromatic Phenols