Hydrocarbons is the first organic chemistry chapter that actually applies the naming and mechanism principles from the previous chapter to real reactions — and the alkane, alkene, alkyne and aromatic families covered here form the structural backbone that nearly every later organic chapter builds functional groups onto.
Alkanes, containing only single bonds, are relatively unreactive and their chemistry is dominated by free radical substitution. Alkenes and alkynes, containing double and triple bonds respectively, are considerably more reactive because that extra pi bond is a ready source of electron density for an attacking electrophile — which is exactly why addition reactions, and Markovnikov's rule specifically, are this chapter's most heavily tested territory. Aromatic hydrocarbons, built around benzene's uniquely stable delocalised ring, follow yet a third distinct reactivity pattern, favouring substitution over addition specifically to preserve that stabilising aromaticity.
Key hydrocarbon reactions
| Reaction | What it does |
|---|---|
| Free radical halogenation of alkanes | substitution via a chain mechanism (initiation, propagation, termination) |
| Markovnikov's rule | H adds to the carbon with more hydrogens already attached in HX addition to an unsymmetrical alkene |
| Anti-Markovnikov (peroxide effect) | reverses addition orientation specifically for HBr with peroxides present |
| Ozonolysis | cleaves a double bond to give two carbonyl compounds, revealing the original alkene structure |
| Hydration of alkynes | gives an enol that tautomerises to a ketone (or acetaldehyde for ethyne) |
| Friedel-Crafts alkylation/acylation | electrophilic aromatic substitution introducing an alkyl or acyl group onto benzene |
| Directing effects in benzene | activating groups direct ortho/para; deactivating groups direct meta |
Topics covered
- Classification of hydrocarbons — saturated, unsaturated and aromatic, and their general reactivity patterns
- Nomenclature and isomerism — IUPAC naming applied specifically to hydrocarbon chains and rings
- Preparation of alkanes — Wurtz reaction, decarboxylation, and catalytic hydrogenation
- Free radical substitution in alkanes — the full chain mechanism, and why multiple products typically form
- Preparation and properties of alkenes — dehydration and dehydrohalogenation, and Saytzeff's rule for the major product
- Addition reactions of alkenes — hydrogenation, halogenation, hydrohalogenation and Markovnikov's rule, plus the peroxide-effect exception
- Preparation and properties of alkynes — and their characteristic addition and acidic terminal hydrogen reactions
- Aromaticity and benzene's structure — Huckel's rule, and why benzene resists addition reactions that alkenes readily undergo
- Electrophilic aromatic substitution — nitration, sulfonation, halogenation, and Friedel-Crafts reactions, with directing effects of existing substituents
Why this chapter matters
In NEET and JEE, Markovnikov's rule (including the peroxide-effect exception) and electrophilic aromatic substitution directing-effect questions are among the single most reliably repeated organic chemistry question types across every year. Reaction mechanism identification, particularly distinguishing free radical, addition and substitution pathways, is equally consistent.
For Class 11 boards, expect a mechanism-based question on free radical halogenation or electrophilic addition, and a question on benzene's electrophilic substitution with directing effects.
Mistakes students make in this chapter
- Forgetting the peroxide effect reverses Markovnikov's rule specifically for HBr, not HCl or HI. In the presence of peroxides, HBr addition follows anti-Markovnikov orientation via a free radical mechanism, but this specific exception does not apply to HCl or HI addition.
- Assuming benzene undergoes addition reactions the way alkenes do. Benzene's delocalised aromatic ring is unusually stable, and reacting via addition would destroy that stabilisation — benzene therefore strongly favours substitution reactions instead, which preserve the aromatic ring.
- Mixing up which substituents are ortho/para directing versus meta directing on benzene. Electron-donating groups (like -OH, -NH₂, alkyl groups) are ortho/para directing and activating; electron-withdrawing groups (like -NO₂, -COOH) are meta directing and deactivating — reversing this leads to predicting the wrong substitution product.
- Applying Saytzeff's rule incorrectly in elimination reactions. It states the more substituted (generally more stable) alkene is the major elimination product, not the less substituted one — a commonly reversed rule.
- Forgetting terminal alkynes have an acidic hydrogen. The hydrogen on a terminal triple-bonded carbon is measurably acidic due to the high s-character of that carbon, allowing terminal alkynes to react with strong bases or form metal acetylides, a reactivity alkenes and alkanes don't share.
Worked example: the peroxide effect, and why only HBr shows it
- Case (a): no peroxide — ionic mechanism. HBr adds as H⁺ and Br⁻. The proton attaches first, and it attaches wherever it leaves behind the more stable carbocation:CH₃–CH=CH₂ + H⁺ → CH₃–CH⁺–CH₃ (2°, stable) not → CH₃–CH₂–CH₂⁺ (1°, unstable)
- Complete the ionic addition. Bromide attacks the secondary cation, so Br lands on the middle carbon — exactly what Markovnikov's rule predicts:major product = CH₃–CHBr–CH₃ (2-bromopropane)
- Case (b): with peroxide — free radical mechanism. The peroxide homolyses and generates a bromine radical, so now Br• adds to the double bond first, not H⁺:Br• adds first → radical forms on the other carbon
- Apply the same stability logic to radicals. Br• adds to the terminal CH₂ because that leaves the unpaired electron on the secondary carbon, and secondary radicals are more stable than primary:BrCH₂–ĊH–CH₃ (2° radical) → BrCH₂–CH₂–CH₃major product = 1-bromopropane (anti-Markovnikov)
- Now the restriction to HBr. The radical route needs the H–X bond to break easily and both propagation steps to be exothermic. H–Cl is too strong to homolyse under these conditions, while H–I breaks easily but the iodine radical adds to the alkene endothermically and reverses:HCl : bond too strong · HI : addition step reverses
What the notes contain
The PDF works through the free radical substitution chain mechanism for alkanes step by step, covers Markovnikov's rule and its peroxide-effect exception with several worked addition reactions, and explains electrophilic aromatic substitution with a full directing-effects reference table for common substituents.
How to use this: build the concept from these notes first, then do one quick NCERT pass before the exam for exact wording and diagram labels, and spend whatever time is left on previous year questions.
Questions
What exactly is Markovnikov's rule and why does it work that way?
When HX adds across an unsymmetrical alkene, hydrogen adds to the carbon that already has more hydrogen atoms attached, while X adds to the more substituted carbon. This happens because the reaction proceeds through the more stable carbocation intermediate, which forms preferentially at the more substituted carbon — the full mechanism is worked through in the PDF.
Why does the peroxide effect only apply to HBr and not HCl or HI?
The peroxide effect works through a free radical mechanism that's only kinetically favourable for HBr specifically — the bond energies and reaction steps involved for HCl and HI don't favour this same free radical pathway, so they still follow ordinary Markovnikov addition even when peroxides are present.
Why does benzene undergo substitution rather than addition?
Benzene's ring of six delocalised pi electrons gives it substantial extra stability (aromaticity). An addition reaction would break this delocalisation and destroy that stability, so benzene strongly favours substitution reactions, which keep the aromatic ring fully intact throughout the reaction.
Does this include a full table of ortho/para versus meta directing groups?
Yes, with a reference table covering the most commonly tested substituents, along with the electronic reasoning (resonance and inductive effects) explaining why each group directs the way it does.
All other chapters
Every chapter of Class 11 and Class 12 Physics, Chemistry and Biology — each with its own notes and PDF.