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Loading home…Alcohols contain the hydroxyl group –OH attached to a saturated carbon.
General formula is often written R–OH.
They are classified as primary, secondary, or tertiary.
Classification depends on how many carbons are attached to the C bearing OH.
Methanol is wood alcohol; ethanol is grain alcohol.
This classification predicts oxidation behaviour and some substitution rates.
Methanol is CH₃OH; ethanol is CH₃CH₂OH.
Ethanol is the alcohol in fermented drinks; methanol is highly toxic.
Methanol is sometimes called wood alcohol; ethanol grain alcohol.
Ethanol gives the iodoform test; methanol does not.
Ethanol is used as a fuel additive and solvent.
Industrial ethanol may be denatured with methanol and pyridine to prevent drinking.
Ethanol reacts with iodine and alkali to give yellow crystalline iodoform (CHI₃).
The test detects the CH₃CH(OH)– group (and related CH₃CO–).
Iodoform has a characteristic antiseptic smell.
Methanol does not give this test.
Secondary alcohols of the type CH₃CH(OH)R also respond.
It is a classic qualitative test in school organic chemistry.
Fermentation converts sugars into ethanol and carbon dioxide using yeast.
Enzymes in yeast catalyse the anaerobic process.
Approximate equation: C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂.
Temperature is controlled near 25–35 °C for effective yeast action.
Distillation concentrates ethanol from the dilute brew.
This is the traditional route to grain alcohol.
With sodium: 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂.
Hydrogen gas evolution shows the acidic hydrogen of –OH is weakly acidic.
With PCl₅: CH₃CH₂OH + PCl₅ → CH₃CH₂Cl + POCl₃ + HCl.
With acetic acid (acid catalyst): ethyl ethanoate (an ester) forms.
Esterification is reversible and is driven by removing water or using excess reactant.
These reactions illustrate alcohol chemistry as acid, nucleophile precursor, and ester partner.
Ethanol blends with petrol to oxygenate fuel and reduce some emissions.
It is a renewable biofuel when made from biomass.
Ethanol is a versatile solvent for medicines, cosmetics, and labs.
It is a starting material for ethene, ethanal, ethanoic acid, and esters.
Antiseptic wipes and sanitizers use concentrated ethanol.
Industrial importance spans energy, solvents, and organic synthesis.
Fermentation uses yeast enzymes on glucose from sugar or starch sources.
Starch must first be hydrolysed to sugars before fermentation.
Anaerobic conditions favour ethanol rather than full respiration to CO₂/H₂O.
The brew contains dilute ethanol, which is distilled to higher strength.
Absolute ethanol needs special drying because ethanol–water forms an azeotrope.
Industrial ethanol is also made by hydration of ethene.
Ethene + steam with acid catalyst (e.g. H₃PO₄ on silica) gives ethanol.
Conditions involve elevated temperature and pressure.
Petrochemical ethene makes large-scale synthetic ethanol possible.
Choice between fermentation and hydration depends on feedstock economics.
Write both equations in exam answers.
State yeast/enzymes for fermentation and acid catalyst for hydration.
Mention that methanol is not produced by the same sugar fermentation path.
Denatured alcohol has additives to make it unfit to drink.
Quality control measures water content and impurities.
Bioethanol can support energy security but needs land/water management.
Lab preparation demos often use sugar, yeast, and limewater for CO₂.
Safety: ethanol is flammable; methanol contamination is toxic.
Link ethene hydration back to alkene addition chemistry.
Summarise: biological vs petrochemical routes to the same product.
Ethanol’s dual origin explains its presence in both food and fuel industries.
Alcohols burn in oxygen to CO₂ and H₂O, releasing energy.
Ethanol is therefore useful as a fuel and fuel additive.
Dehydration removes water to form alkenes or ethers depending on conditions.
Excess conc. H₂SO₄ at about 170 °C converts ethanol to ethene.
At about 140 °C with excess ethanol, diethyl ether forms.
Oxidation depends on alcohol class.
Primary alcohols oxidise to aldehydes then carboxylic acids with strong oxidants.
Controlled oxidation (distil aldehyde) can stop at the aldehyde stage.
Secondary alcohols oxidise to ketones.
Tertiary alcohols resist oxidation under mild conditions (no C–H on the carbinol carbon).
Acidified dichromate turns from orange to green on oxidation of 1°/2° alcohols.
Esterification: alcohol + carboxylic acid ⇌ ester + water (H⁺ catalyst).
Concentrated H₂SO₄ can act as catalyst and dehydrating agent.
Esters often have fruity smells and are used in flavourings.
Write equations for each reaction type with ethanol examples.
State temperatures for ethene vs ether formation clearly.
Link oxidation products to carbonyl chapter chemistry.
Industrial esters are made under controlled equilibrium conditions.
Exam answers should separate physical burning from chemical functional-group reactions.
Mention that complete combustion needs sufficient air.
These four reaction types cover most school alcohol mechanisms/tests.
Sodium metal reacts with alcohols to give alkoxides and hydrogen gas.
This shows alcohols are weakly acidic compared with water in some contexts, but less acidic than carboxylic acids.
PCl₅ replaces –OH by Cl, giving alkyl chloride, POCl₃, and HCl.
HBr (or KBr/H₂SO₄) converts alcohols to alkyl bromides.
Reactivity with HX often follows 3° > 2° > 1°.
Lucas reagent (ZnCl₂/HCl) distinguishes classes by speed of cloudiness.
Acetic acid forms ethyl acetate with ethanol under acid catalysis.
The esterification mechanism involves protonation and nucleophilic attack by alcohol.
These reactions convert alcohols into ionic alkoxides, halogenoalkanes, or esters.
Each product class opens further synthetic routes.
Write balanced equations for Na, PCl₅, HBr, and CH₃COOH reactions.
Note observations: H₂ evolution with Na; white fumes with PCl₅.
Dry conditions improve many halogenation procedures.
Compare PCl₅ with SOCl₂ as chlorinating options.
School practicals emphasise observations and product identification.
Safety: PCl₅ and HBr are corrosive; handle in fume hoods.
Link halogenoalkane products to the previous chapter’s reactions.
Ester names follow alkyl alkanoate pattern (ethyl ethanoate).
Summarise reagents → products in a quick-revision table mentally.
These transformations are frequently examined as short equations.
Always specify heat/reflux when esterification is slow at room temperature.
Classification rests on the number of carbons attached to the C–OH carbon.
Primary: RCH₂OH; secondary: R₂CHOH; tertiary: R₃COH.
Oxidation: 1° → aldehyde/acid; 2° → ketone; 3° resistant to mild oxidation.
Acidified K₂Cr₂O₇ colour change supports 1°/2° oxidation; 3° usually negative.
Lucas test: 3° reacts fastest (immediate cloudiness), 2° slower, 1° slowest.
Iodoform test is positive for ethanol and for CH₃CH(OH)– secondary alcohols.
Spectroscopy (IR OH stretch; NMR) distinguishes structures in advanced labs.
Boiling points are high relative to hydrocarbons due to hydrogen bonding.
Solubility in water falls as the alkyl chain lengthens.
Tertiary alcohols often dehydrate especially readily to alkenes.
Write structural examples: ethanol, propan-2-ol, 2-methylpropan-2-ol.
State reagents and observations for each distinguishing test.
Do not rely on smell alone for identification.
Mechanism differences (SN1 vs SN2 tendencies of derived cations) also relate to class.
Exam answers should give definition + two chemical tests minimum.
Link oxidation products to Tollens/Fehling tests for aldehydes formed.
Emphasise that methanol is primary but fails iodoform.
Safety: dichromate is toxic/oxidising — school demos need care.
Tabulate class / oxidation product / Lucas speed for revision.
Correct classification prevents wrong predictions in synthesis.
Practice with unknown isomeric alcohols of formula C₄H₁₀O.
Methanol is a feedstock for formaldehyde, acetic acid (some processes), and fuels.
It is used in biodiesel transesterification and as a laboratory solvent.
Methanol is toxic; ingestion can cause blindness or death.
Ethanol is a beverage alcohol, solvent, antiseptic, and biofuel component.
Gasohol blends reduce reliance on pure petrol in some countries.
Ethanol is a starting material for ethene, acetaldehyde, acetic acid, and esters.
Pharmaceutical tinctures historically used ethanol as solvent.
Cosmetics and personal-care products use ethanol widely.
Higher alcohols are used in plasticisers, detergents, and fragrances.
Polyols (ethylene glycol, glycerol) are important related hydroxy compounds.
Industrial production scales from fermentation plants to petrochemical units.
Denaturing prevents diversion of industrial ethanol to beverages.
Quality specs control water, methanol impurity, and other contaminants.
Green chemistry seeks bio-based routes with lower carbon footprints.
Energy balance of bioethanol depends on farming and processing efficiency.
Exam answers should name uses in fuel, solvent, synthesis, and health.
Contrast toxicity of methanol with relative safety of ethanol (still intoxicating).
Link industrial importance to everyday student experience (sanitizer, fuel).
Mention regulatory control of alcohol for tax and safety reasons.
Summarise: alcohols bridge fuels, materials, and fine chemicals.
Their –OH chemistry underpins a large fraction of applied organic industry.