Educational Galaxy

Chapter 9: Hydrocarbons — EG's own summary, in simple words

Book: Chemistry — Chemistry — Part II (CBSE, Class 11)  |  Chapter 9  |  Medium: English

Alkanes, alkenes, alkynes and aromatic hydrocarbons — preparation, properties and reactions.

Diagram (labels on the image): Single-bond chain, Multiple-bond chain, Ring molecule. This is an Educational Galaxy schematic drawn for this chapter; the second image below is a general illustration, not a textbook figure.

1. 1. Alkanes: the saturated hydrocarbons

Alkanes contain only carbon–carbon single bonds and have the general formula CnH2n+2. They are saturated because every carbon carries as many hydrogens as possible. Methane, ethane and propane are gases; larger alkanes become liquids and then solids as chain length grows, and boiling points rise with molecular mass.

Alkanes are made by hydrogenation of alkenes and alkynes, by decarboxylation of sodium salts of carboxylic acids with soda lime (R–COONa → R–H), and by the Wurtz reaction (two alkyl halides with sodium in dry ether give a longer alkane). Their main reaction is free-radical substitution: chlorine replaces hydrogen in sunlight, step by step, through initiation, propagation and termination. They also burn completely to carbon dioxide and water — the basis of their use as fuels.

Conformations are the shapes from rotation about a single bond. Ethane's staggered conformation (hydrogens as far apart as possible) is more stable than the eclipsed form (hydrogens aligned); the energy difference is small, so the forms interconvert rapidly at room temperature.

2. 2. Alkenes: the double bond changes everything

Alkenes contain at least one carbon–carbon double bond and follow CnH2n. They are made by dehydration of alcohols with concentrated sulphuric acid, and by dehydrohalogenation of alkyl halides with alcoholic potassium hydroxide. Both are elimination reactions: a small molecule is removed and a double bond forms.

The double bond's pi electrons make alkenes reactive towards electrophiles, so their characteristic reaction is electrophilic addition. Hydrogen adds across the double bond (with a catalyst), halogens add to give dihalides (bromine water is decolourised — the classic test for unsaturation), hydrogen halides add by Markovnikov's rule, and water adds in the presence of acid to give alcohols.

Markovnikov's rule: when HX adds to an unsymmetrical alkene, the hydrogen goes to the carbon that already has more hydrogens, and the halide to the more substituted carbon. The reason is carbocation stability: the addition goes through the more stable carbocation intermediate. Peroxides reverse the rule for HBr only (Kharasch effect), because the mechanism switches to free radicals.

Caption: An originally generated illustration for this chapter — a general scene for the topic, not a photograph of a real experiment or a copy of a textbook figure. The labelled diagram above is the one to study for the named parts.

3. 3. Alkynes: the triple bond

Alkynes contain a carbon–carbon triple bond and follow CnH2n−2. Ethyne (acetylene) is the first member, made industrially from calcium carbide and water (CaC2 + 2H2O → C2H2 + Ca(OH)2). Ethyne burns with a very hot, smoky flame and is used in oxy-acetylene welding.

Like alkenes, alkynes undergo electrophilic addition, but they can add two molecules of HX or halogen in sequence. Terminal alkynes (with –C≡CH) are weakly acidic: they react with ammoniacal silver nitrate to give a white precipitate of silver acetylide, and with ammoniacal cuprous chloride to give a red precipitate — tests that distinguish terminal alkynes from internal ones and from alkenes.

4. 4. Aromatic hydrocarbons: benzene and its special stability

Benzene, C6H6, is a planar ring of six carbons with delocalised pi electrons. It does not behave like three separate double bonds: it resists addition and prefers substitution, because substitution preserves the delocalised ring while addition would destroy it. This extra stability is aromaticity, explained by Hückel's rule (4n + 2 pi electrons; benzene has 6).

Benzene's characteristic reactions are electrophilic aromatic substitutions: nitration (with nitrating mixture, concentrated HNO3 + H2SO4) gives nitrobenzene; halogenation (with a Lewis acid catalyst like FeCl3) gives chlorobenzene; sulphonation gives benzene sulphonic acid; and Friedel–Crafts alkylation and acylation attach alkyl and acyl groups using AlCl3 as catalyst.

Substituents already on the ring direct the next group: activating groups such as –OH and –NH2 direct ortho and para; deactivating groups such as –NO2 and –COOH direct meta. This directive influence decides the major product and is a favourite exam question. Benzene is also a health hazard — it is carcinogenic — so its use is controlled, and fuels and solvents are formulated to limit exposure.

5. 5. Putting it together: choosing the right hydrocarbon reaction

When a question gives an unknown hydrocarbon, work backwards: decolourisation of bromine water means unsaturation; a white precipitate with ammoniacal silver nitrate means a terminal alkyne; combustion data gives the formula. When a question asks for a preparation, match the starting material to the method: alcohol → alkene by dehydration, alkyl halide → alkene by alcoholic KOH, carboxylate salt → alkane by soda lime.

A final exam habit: in addition reactions, always show the intermediate or the rule you used. Markovnikov's rule is not a magic phrase — the examiner wants the more stable carbocation, named, as the reason the product is what it is.

Hydrocarbon families at a glance

Family	General formula	Characteristic reaction	Key test
Alkanes	CnH2n+2	Free-radical substitution	Burn with a clean flame
Alkenes	CnH2n	Electrophilic addition	Decolourise bromine water
Alkynes	CnH2n−2	Electrophilic addition (twice)	Terminal: white ppt with ammoniacal AgNO3
Arenes (benzene)	CnH2n−6	Electrophilic substitution	Resists addition; substitution instead

Going further 1 — More preparations and reactions: alkanes, alkenes, alkynes (NCERT §9.2–§9.4)

Three more preparations complete the set. Kolbe's electrolytic method makes alkanes at the anode: two acetate ions lose electrons and carbon dioxide, and the two methyl radicals join to give ethane — from sodium propanoate the product is butane. Aromatisation turns a straight alkane into benzene: n-hexane over chromium oxide on alumina at 773 K and 10–20 atm gives benzene. Pyrolysis, or cracking, breaks higher alkanes into smaller ones, for example dodecane into heptane and pentene, which is how refineries raise the yield of petrol-range molecules.

For alkenes and alkynes, three reactions identify and transform. Ozonolysis cuts a double bond and reports the pieces: but-2-ene gives two molecules of acetaldehyde, so the carbonyl products read back the double bond's position. Cold dilute potassium permanganate oxidises an alkene to a glycol and loses its purple colour — the Baeyer test for unsaturation. Alkynes hydrate by Markovnikov's rule with mercuric sulphate and sulphuric acid: ethyne gives acetaldehyde and propyne gives propanone, through an unstable enol that rearranges. And ethyne polymerises in a red-hot tube to benzene itself, closing the loop back to aromatics.

Exam note: ozonolysis products are carbonyls — aldehydes or ketones — never alcohols; if your products contain –OH, the mechanism is wrong.

Going further 2 — Benzene from scratch, and when it does add (NCERT §9.5–§9.6)

Benzene is made, not just found. Cyclic polymerisation passes ethyne through a red-hot iron or copper tube to give benzene directly. Decarboxylation heats sodium benzoate with soda lime, driving off carbon dioxide and leaving benzene. Both routes matter because they connect the families: an alkyne becomes an arene, and a carboxylate salt becomes a hydrocarbon.

Benzene resists addition because addition would destroy its delocalised ring — but under drastic conditions it does add. With hydrogen over nickel at high temperature and pressure it gives cyclohexane; with chlorine in ultraviolet light it gives benzene hexachloride. Oxidation attacks the side chain instead of the ring: toluene with hot alkaline permanganate gives benzoic acid, the ring untouched. And the hazard note closes the chapter: polynuclear hydrocarbons with more than two fused benzene rings, such as benzpyrene from tobacco smoke and coal tar, are toxic and carcinogenic — which is why benzene exposure is controlled.

Exam note: if a benzene reaction keeps the ring, it is substitution; if the ring disappears, it is addition under drastic conditions — the conditions tell you which one the examiner wants.

Practice set — try it yourself

1. Ethene decolourises bromine water but ethane does not. Explain the difference in terms of bonding.
2. Propene reacts with HBr (a) without peroxides and (b) with peroxides. Give the major product in each case and explain the difference.
3. Why does benzene undergo substitution rather than addition, even though it contains double bonds in its Kekulé structures?
4. How would you distinguish a terminal alkyne from an internal alkyne in the laboratory?
5. A substituent –NO2 is already on a benzene ring. Where will the next electrophile attack, and why?

Questions with Answers — full answers

1. Why are alkanes called saturated hydrocarbons? Answer: Alkanes contain only carbon–carbon single bonds, so each carbon atom is bonded to as many hydrogen atoms as possible. No more hydrogen can be added without breaking a bond, so the hydrocarbon is saturated.
2. State Markovnikov's rule and explain it using carbocation stability. Answer: When HX adds to an unsymmetrical alkene, the hydrogen attaches to the carbon already bearing more hydrogens, and the halide to the more substituted carbon. This happens because addition proceeds through the more stable carbocation: adding H+ to the less substituted carbon leaves the positive charge on the more substituted carbon, which is stabilised by neighbouring alkyl groups.
3. How is ethyne prepared from calcium carbide? Answer: Calcium carbide reacts with water: CaC2 + 2H2O → C2H2 + Ca(OH)2. Ethyne gas is evolved and calcium hydroxide remains. This is a standard laboratory and industrial preparation of acetylene.
4. What is aromaticity, and why does benzene show it? Answer: Aromaticity is the extra stability of a planar, cyclic, fully conjugated system with (4n + 2) pi electrons (Hückel's rule). Benzene is planar and cyclic with 6 delocalised pi electrons (n = 1), so it is aromatic and resists reactions that would destroy the delocalised system.
5. What is the directive influence of –OH and –NO2 groups on a benzene ring? Answer: The hydroxyl group –OH is activating and ortho/para directing: it donates electrons into the ring and stabilises attack at the ortho and para positions. The nitro group –NO2 is deactivating and meta directing: it withdraws electrons, so attack occurs at the meta position where the destabilisation is least.

Extension Questions with Answers — going further

1. What does ozonolysis of but-2-ene give, and what does it prove? Answer: Two molecules of acetaldehyde, CH3CHO. The two identical carbonyls show the double bond was in the middle of a four-carbon chain, symmetrical about the bond.
2. Propyne reacts with water in the presence of HgSO4 and H2SO4. What is the product, and why? Answer: Propanone (acetone). Water adds by Markovnikov's rule, with –OH going to the more substituted carbon, giving an unstable enol that rearranges to the ketone.
3. Sodium propanoate solution is electrolysed (Kolbe's method). What alkane forms at the anode? Answer: Butane. Two propanoate ions each lose CO2 at the anode, and the two ethyl radicals join: CH3CH2CH2CH3.
4. Why does benzene not decolourise bromine water, although ethene does? Answer: Ethene's double bond undergoes electrophilic addition with bromine at once. Benzene's delocalised ring resists addition, because addition would destroy aromatic stability; it reacts by substitution instead, and only with a Lewis acid catalyst.
5. Toluene is heated with hot alkaline KMnO4. What forms, and what happens to the ring? Answer: Benzoic acid. The methyl side chain is oxidised to –COOH while the benzene ring stays intact — oxidation attacks the side chain, not the ring.

Board-pattern Questions with Answers

1. How would you distinguish ethane, ethene and ethyne in the laboratory? Answer: Bromine water: ethene and ethyne decolourise it, ethane does not. Ammoniacal silver nitrate: only ethyne, the terminal alkyne, gives a white precipitate of silver acetylide. Together the two tests separate all three.
2. Propene reacts with HBr (a) without peroxides and (b) with peroxides. Give the major product in each case. Answer: (a) 2-bromopropane, by Markovnikov's rule through the more stable secondary carbocation. (b) 1-bromopropane, by the peroxide effect: the free-radical mechanism puts bromine on the less substituted carbon. The peroxide effect applies to HBr only.
3. Benzene reacts with methyl chloride in the presence of anhydrous AlCl3. Name the product and the reaction type. Answer: Toluene (methylbenzene), by Friedel–Crafts alkylation — an electrophilic aromatic substitution in which the methyl electrophile replaces a ring hydrogen.
4. Why is the Wurtz reaction not used to make unsymmetrical alkanes? Answer: Two different alkyl halides give three products: the two symmetrical alkanes and the mixed one, in one mixture that is hard to separate. The method is practical only for symmetrical alkanes from a single halide.
5. Give the conditions for converting n-hexane into benzene. Answer: Chromium oxide on alumina (Cr2O3/Al2O3) as catalyst, 773 K, and 10–20 atm pressure. This is aromatisation: cyclisation plus dehydrogenation in one step.