Concept Core
Alkanes, alkenes, alkynes, and benzene — their reactions and mechanisms.
Hydrocarbon Classification & General Formulas
| Class | General Formula | Example | Bond Type |
|---|
| Alkane | CₙH₂ₙ₊₂ | CH₄, C₂H₆ | C−C (sigma only) |
| Alkene | CₙH₂ₙ | C₂H₄ (ethene) | C=C (1σ + 1π) |
| Alkyne | CₙH₂ₙ₋₂ | C₂H₂ (ethyne) | C≡C (1σ + 2π) |
| Benzene (arene) | CₙH₂ₙ₋₆ | C₆H₆ | Delocalised π |
Alkane Reactions (Free Radical Mechanism)
Halogenation proceeds via free radical substitution: (requires UV light)
Step 1 — Initiation: Cl₂ →(hν) 2Cl• (radical)
Step 2 — Propagation: Cl• + CH₄ → CH₃• + HCl
CH₃• + Cl₂ → CH₃Cl + Cl•
Step 3 — Termination: Cl• + Cl• → Cl₂; CH₃• + Cl• → CH₃Cl
Reactivity of C-H bond: 3° > 2° > 1° (easier to abstract H from more substituted carbon).
Alkene Reactions (Electrophilic Addition)
Alkenes undergo electrophilic addition across the C=C double bond.
Markovnikov's rule: H adds to C with more H atoms (forms more stable carbocation).
CH₂=CH₂ + HBr → CH₃−CH₂Br (ethyl bromide)
CH₃−CH=CH₂ + HBr → CH₃−CHBr−CH₃ (Markovnikov product)
Hydration: alkene + H₂O (H₃PO₄ catalyst) → alcohol
Hydrogenation: alkene + H₂ (Ni/Pt catalyst) → alkane
Anti-Markovnikov (Kharasch effect): In the presence of peroxides (radical mechanism), Br adds to less substituted carbon.
Alkyne Reactions
Acidic character of terminal alkynes: The terminal C−H bond is more acidic than alkene or alkane C−H (sp carbon, higher s-character). Reacts with Na metal or NaNH₂ to form acetylide:
HC≡CH + NaNH₂ → HC≡C⁻Na⁺ + NH₃ (sodium acetylide)
Addition reactions: electrophilic addition proceeds in two steps (Markovnikov at each step). Hydration of alkynes gives ketones (Markovnikov) or aldehydes (anti-Markovnikov, hydroboration).
Benzene — Electrophilic Aromatic Substitution (EAS)
Benzene's π-cloud attracts electrophiles. Key reactions:
| Reaction | Reagents | Product |
|---|
| Nitration | HNO₃/H₂SO₄ | Nitrobenzene |
| Sulfonation | Fuming H₂SO₄ | Benzene sulphonic acid |
| Halogenation | Cl₂/FeCl₃ (Lewis acid) | Chlorobenzene |
| Alkylation (F-C) | RCl/AlCl₃ | Alkylbenzene |
| Acylation (F-C) | RCOCl/AlCl₃ | Acylbenzene (ketone) |
Directing Effects in EAS
o,p-directing (activating): −OH, −NH₂, −OCH₃, alkyl groups (donate electrons by +M or +I)
m-directing (deactivating): −NO₂, −CHO, −COOH, −SO₃H (withdraw electrons by −M or −I)
Halogens: o,p-directing but deactivating (−I effect dominates over +M for rate, but directs to o,p by resonance)
Formula Vault
Hydrocarbon reaction conditions and Markovnikov's rule.
Alkane General Formula
CₙH₂ₙ₊₂
Each new C adds CH₂ unit
Free Radical Stability
3° > 2° > 1° > methyl
More substituted = more stable radical
Markovnikov's Rule
H adds to C with more H atoms
More stable carbocation forms
Anti-Markovnikov
Peroxide (radical) mechanism
Br adds to less substituted C
Alkyne Acidity
sp > sp² > sp³ (C−H acidity)
More s-character → more acidic
Benzene Nitration
Conc. HNO₃ + H₂SO₄ → NO₂⁺ electrophile
NO₂⁺ attacks benzene ring
EAS o,p directors
−OH, −NH₂, −OR, −R (alkyl)
Activate ring; direct to ortho/para
EAS m directors
−NO₂, −CHO, −COOH, −SO₃H
Deactivate ring; direct to meta
Worked Examples
5 problems — Markovnikov, free radical, benzene EAS, acidity, and a directing effect trap.
EasyPredict Markovnikov product: propene + HBr▾
Predict the major product when propene (CH₃−CH=CH₂) reacts with HBr.
1
Markovnikov: H adds to C with more H atoms.
2
In propene: CH₂= end has 2H; CH= end has 1H.
3
H adds to CH₂ end → Br adds to CH end (middle carbon).
4
Major product: CH₃−CHBr−CH₃ (2-bromopropane)
✓ Major product: CH₃−CHBr−CH₃ (2-bromopropane)
EasyIdentify the type of reaction: CH₄ + Cl₂ → CH₃Cl + HCl (UV light)▾
Classify the reaction: CH₄ + Cl₂ →(UV) CH₃Cl + HCl
1
UV light initiates homolytic fission of Cl₂ → 2Cl• (free radicals).
2
This is free radical substitution.
3
Steps: Initiation (Cl₂ → 2Cl• by UV), Propagation, Termination.
✓ Free radical substitution (homolytic mechanism, requires UV/heat)
MediumWhich is more acidic: CH₄, C₂H₄, or HC≡CH? Explain.▾
Arrange in increasing order of acidity: CH₄, C₂H₄ (ethene), HC≡CH (ethyne).
1
Acidity of C−H depends on hybridisation:
2
CH₄: sp³ carbon, 25% s-character → least acidic
3
C₂H₄: sp² carbon, 33% s-character → intermediate
4
HC≡CH: sp carbon, 50% s-character → most s-character → electrons held closest → most acidic
5
Order: CH₄ < C₂H₄ < HC≡CH
✓ Acidity order: CH₄ < C₂H₄ < HC≡CH (terminal alkyne most acidic)
EAPCET LevelPredict EAS product when toluene reacts with HNO₃/H₂SO₄▾
Predict the major product when toluene (methylbenzene) undergoes nitration with HNO₃/H₂SO₄. Where does the −NO₂ group go?
1
−CH₃ is an alkyl group: +I effect → electron donating → activating substituent.
2
Alkyl groups are ortho,para directors (activate ring at o and p positions).
3
Nitration introduces −NO₂ group.
4
Major products: ortho-nitrotoluene and para-nitrotoluene (mixture).
5
In practice, para is slightly favoured due to less steric hindrance.
✓ Major products: ortho and para nitrotoluene (−CH₃ is o,p-director)
Trap QuestionBenzene undergoes addition reactions like alkenes — True or False?▾
A student claims: 'Benzene has three double bonds, so it undergoes addition reactions like alkenes.' Evaluate.
1
The trap: Benzene's π electrons are delocalised over all 6 carbons (aromatic system), not localised in alternating double bonds.
2
Adding across one 'double bond' would disrupt the delocalisation and destroy the aromatic stabilisation energy (~36 kcal/mol).
3
Benzene strongly prefers substitution (maintains aromaticity) over addition (destroys aromaticity).
4
Benzene does undergo addition under extreme conditions (Birch reduction, catalytic hydrogenation at high T), but EAS is overwhelmingly preferred.
✓ False — benzene prefers electrophilic aromatic SUBSTITUTION to preserve aromaticity
Mistake DNA
4 hydrocarbon errors from EAPCET distractor analysis.
🔀
Anti-Markovnikov: Peroxides Affect HCl and HI Too
Anti-Markovnikov (Kharasch) effect occurs ONLY with HBr in presence of peroxides. HCl and HI do NOT show anti-Markovnikov with peroxides.
❌ Wrong
Propene + HCl (peroxide):
Cl adds to more substituted C
(anti-Markovnikov) ✗
✓ Correct
Anti-Markovnikov: only HBr ✓
HCl: too strong (radical not
stable); HI: too slow ✓
Only HBr radical stable ✓
Only HBr shows anti-Markovnikov with peroxides because Br• radicals are stable enough to add to alkenes. HCl reaction is too fast (HCl too reactive) and HI• radicals are too stable to add.
⬆️
Halogens are m-Directing in EAS
Halogens (−F, −Cl, −Br, −I) are ortho,para directing (though deactivating). This surprises students who expect deactivators to be m-directors.
❌ Wrong
Cl substituent in benzene:
m-directing (deactivating) ✗
✓ Correct
Cl: o,p-directing but
deactivating ✓
(lone pair donation
overrides −I for direction)
Halogens have both −I (deactivating) and +M (lone pair to ring, o,p-directing). The +M effect wins for direction (positions electrons at o,p), but the −I effect dominates the rate (makes ring less reactive overall).
🧪
Free Radical Mechanism Requires Dark (Not UV Light)
Free radical halogenation of alkanes REQUIRES UV light (or heat) for initiation. In dark, reaction does not proceed.
❌ Wrong
CH₄ + Cl₂ (dark) →
CH₃Cl + HCl ✗
(dark: no reaction)
✓ Correct
CH₄ + Cl₂ →(UV/heat)
CH₃Cl + HCl ✓
UV or heat splits Cl₂
into radicals
The initiation step (Cl₂ → 2Cl•) requires energy from UV light or heat to break the Cl−Cl bond homolytically. In the dark at room temperature, chlorine does not react with methane.
🔘
Benzene + Cl₂ Without Lewis Acid: EAS or Not?
Benzene reacts with Cl₂ ONLY in the presence of a Lewis acid catalyst (FeCl₃ or AlCl₃). Without catalyst, no reaction occurs at room temperature.
❌ Wrong
Benzene + Cl₂ (no catalyst):
chlorobenzene formed ✗
(no Lewis acid → no Cl⁺)
✓ Correct
Need FeCl₃ or AlCl₃ ✓
Generates Cl⁺ electrophile ✓
Without catalyst: no EAS ✓
Lewis acid catalyst activates Cl₂: Cl₂ + FeCl₃ → Cl⁺ + FeCl₄⁻. This Cl⁺ (electrophile) then attacks the benzene ring. Without Lewis acid, Cl₂ is not electrophilic enough.
Chapter Intelligence
Hydrocarbons is the entry point to all of organic chemistry — master Markovnikov and EAS directing effects.
EAPCET Weightage (2019–2024)
Markovnikov's rule (alkene addition)~8 EAS directing effects (benzene)~7 Free radical mechanism (alkane)~6 Alkyne acidity and reactions~5 Benzene reactions (specific)~4 High-Yield PYQ Patterns
Predict Markovnikov/anti-MarkovnikovIdentify free radical vs ionic mechanismEAS o/p or m directing predictionTerminal alkyne + NaNH₂ reactionBenzene nitration product positionWhy benzene prefers substitutionHydration of alkene → alcohol
Exam Strategy
- Markovnikov: H adds to C with more H → more substituted carbocation intermediate. Anti-Markovnikov: only with HBr + peroxides (radical mechanism).
- EAS directing groups: +M or +I groups → o,p activating directors. −M or −I groups → m deactivating directors. Halogens = exception: o,p directing but deactivating.
- Alkyne acidity: sp hybridised C has 50% s-character → electrons closer to nucleus → bonds more acidic. HC≡CH is acidic enough to react with Na metal.
- Benzene always prefers substitution over addition — it preserves aromaticity. The aromatic stabilisation energy of ~36 kcal/mol is the driving force.