Class 11 Chemistry · NCERT chapter 13 · 7 topics · free to read
Alkanes — The Saturated Hydrocarbons
Alkanes (CₙH₂ₙ₊₂) are saturated hydrocarbons with only C—C and C—H single bonds. Prepared by Wurtz reaction, decarboxylation, and Kolbe's electrolysis. They undergo combustion, halogenation (free radical), and isomerization. Conformational analysis of ethane and butane is a NEET favorite.
C_nH_ 2n+2 (General formula of alkanes)
General Properties
Alkanes contain only C—C and C—H sigma bonds. Every carbon is sp³ hybridized with ~109.5° bond angles. They are the least reactive hydrocarbons — earning the name 'paraffins' (Latin: little affinity).
Alkanes are the simplest organic compounds — just carbon and hydrogen connected by single bonds. Every carbon is sp^3 hybridized (tetrahedral, 109.5°). Because there are no bonds to attack, alkanes are remarkably unreactive under normal conditions.
General formula: C_nH_ 2n+2 (open chain) or C_nH_ 2n (cycloalkanes)
All bonds are bonds — no bonds → no addition reactions
Non-polar molecules → insoluble in water, soluble in organic solvents
Boiling point increases with molecular weight (more London dispersion forces)
Branching decreases boiling point (less surface area → fewer van der Waals contacts)
Preparation of Alkanes
Alkanes can be prepared by Wurtz reaction (coupling two alkyl halides with Na), decarboxylation (heating sodium salt of carboxylic acid with NaOH/CaO), Kolbe's electrolysis (electrolysis of sodium carboxylate), and catalytic hydrogenation of unsaturated hydrocarbons.
There are four major methods to prepare alkanes. Each one gets asked in NEET — know the reagents, conditions, and the limitation of each method.
Wurtz Reaction: 2R—X + 2Na dry ether R—R + 2NaX
Wurtz Reaction: Two alkyl halides couple via sodium in dry ether → gives alkane with double the carbon count
Limitation: Only works well for symmetrical alkanes. Using two different R—X gives a mixture of 3 products (R—R, R'—R', R—R')
Only odd-carbon alkanes like CH_4 (methane) cannot be made by Wurtz reaction — you'd need C_ 0.5 !
Alkanes undergo combustion (complete and incomplete), free radical halogenation (Cl₂/Br₂ with UV or heat), and catalytic isomerization. Free radical halogenation follows the chain mechanism: initiation → propagation → termination.
Alkanes are unreactive — they need harsh conditions (UV light, high temperature, or catalysts) to react. The two most important reactions for NEET are combustion and free radical halogenation.
Termination: Any two radicals combine → R—R , R—X , or X—X
Ease of H-abstraction: 3° H > 2° H > 1° H (more substituted radical = more stable)
Br₂ is highly selective — preferentially attacks 3° C—H
Cl₂ is less selective — attacks all positions, giving a mixture of products
Conformational Analysis
Different spatial arrangements of atoms arising from rotation about a C—C single bond are called conformations (or conformers/rotamers). Ethane has staggered (most stable) and eclipsed (least stable) conformations. Butane has additional gauche and anti conformations. The energy barrier to rotation is ~12.5 kJ/mol for ethane.
Unlike double bonds, single bonds allow free rotation. As one carbon rotates relative to the other, the hydrogens (and substituents) adopt different spatial arrangements. These are called conformations — not isomers, because they interconvert rapidly at room temperature.
Conformations: Different arrangements due to rotation about C—C bond
They are NOT isomers — they interconvert too rapidly to be isolated
Best visualized using Newman projections (view along the C—C bond axis)
Front carbon = dot, Back carbon = circle
Ethane ( C_2H_6 ) has two extreme conformations:
Dihedral angle = 60°
Dihedral angle = 0°
H atoms are as far apart as possible
H atoms are as close as possible
Minimum energy
Maximum energy (+12.5 kJ/mol)
Torsional strain = zero
Maximum torsional strain
Butane ( CH_3—CH_2—CH_2—CH_3 ) — rotation about the C2—C3 bond gives four notable conformations:
Anti: Two CH_3 groups 180° apart — most stable (minimum steric strain)
Gauche: Two CH_3 groups 60° apart — slightly less stable than anti (+3.8 kJ/mol)
Eclipsed (partial): CH_3 eclipses H — less stable
Fully eclipsed: Two CH_3 groups 0° apart — least stable (maximum steric + torsional strain)
Quick recall
In ethane, what is the energy difference between staggered and eclipsed conformations?
12.5 kJ/mol (or ~3 kcal/mol). This is the torsional barrier to rotation. It's small enough that ethane rotates essentially freely at room temperature.
Alkenes — Addition Reactions & Markovnikov's Rule
Alkenes (CₙH₂ₙ) contain at least one C=C double bond. Prepared by dehydration of alcohols and dehydrohalogenation of alkyl halides (Saytzeff's rule). They undergo electrophilic addition (H₂, HX, X₂, H₂O), with HX following Markovnikov's rule. Anti-Markovnikov addition occurs with HBr/peroxide. Ozonolysis is used for locating double bonds.
C_nH_ 2n (General formula of alkenes)
Preparation of Alkenes
Alkenes are prepared by elimination reactions: dehydration of alcohols (conc. H₂SO₄ or Al₂O₃ at 443 K), dehydrohalogenation of alkyl halides (alcoholic KOH), and dehalogenation of vicinal dihalides (Zn dust). Saytzeff's rule predicts the major product — the more substituted alkene is preferred.
Alkenes are prepared by elimination — removing two atoms/groups from adjacent carbons to form a C=C double bond. The key question is always: which alkene forms as the major product? Saytzeff's rule answers that.
The C=C π bond is the reactive site. Alkenes undergo electrophilic addition: catalytic hydrogenation (H₂/Ni,Pt,Pd), halogenation (X₂ in CCl₄), hydrohalogenation (HX → Markovnikov), and acid-catalyzed hydration (H₂O/H⁺). These are the most frequently tested reactions in NEET.
The bond in alkenes is the electron-rich site that attracts electrophiles. All addition reactions follow the pattern: the bond breaks, and two new bonds form — one to each carbon of the former double bond.
Halogenation ( Br_2 /CCl₄): anti-addition via cyclic bromonium ion intermediate
HX addition: Follows Markovnikov's rule — H goes to the C with more H's
H₂O/H⁺ (hydration): Also Markovnikov — OH goes to the more substituted carbon
Markovnikov's Rule
In the addition of HX to an unsymmetrical alkene, the hydrogen adds to the carbon with more hydrogens, and X adds to the carbon with fewer hydrogens (more substituted carbon). The real reason: the reaction proceeds through the more stable carbocation intermediate.
Markovnikov's rule (1870): When HX adds to an unsymmetrical alkene, H attaches to the carbon that already has more hydrogens, and X attaches to the carbon with fewer hydrogens. But the real explanation is carbocation stability.
Mechanism: The electrophile H^+ attacks the bond first → forms a carbocation on the adjacent carbon
The H adds to give the more stable carbocation intermediate (3° > 2° > 1°)
Then X^- attacks the carbocation → product
This is an electrophilic addition mechanism — the electrophile ( H^+ ) attacks first
In the presence of organic peroxide (ROOR), HBr adds to unsymmetrical alkenes in anti-Markovnikov fashion: Br goes to the less substituted carbon. This is because the mechanism changes from ionic to free radical. This peroxide effect works ONLY with HBr — not HCl or HI.
When HBr is added to an unsymmetrical alkene in the presence of benzoyl peroxide ( (C_6H_5COO)_2 ), the addition is reversed — Br goes to the less substituted carbon. This is the Kharasch effect or peroxide effect.
Br^ adds to the less substituted carbon → gives the more stable radical intermediate on the other carbon
Then H from HBr adds to the radical center → anti-Markovnikov product
The selectivity is governed by radical stability (3° > 2° > 1°) — same order as carbocations!
Quick recall
Propene + HBr → ? (a) without peroxide (b) with peroxide
(a) 2-bromopropane (Markovnikov — Br on 2° C) (b) 1-bromopropane (Anti-Markovnikov — Br on 1° C, radical mechanism)
Ozonolysis
Ozonolysis cleaves the C=C bond completely: ozone (O₃) adds across the double bond to form an ozonide, which is then reduced (Zn/H₂O) to give aldehydes and/or ketones. This reaction is used to locate the position of the double bond in an unknown alkene.
Ozonolysis is the analytical superpower for alkenes — it breaks the double bond cleanly into two carbonyl fragments. From the products, you can work backward to figure out where the double bond was in the original molecule.
Step 1: O_3 adds across C=C → forms an unstable ozonide (1,2,3-trioxolane)
Step 2: Ozonide is cleaved by Zn/H_2O (reductive workup) → gives aldehydes and/or ketones
If C of the double bond has H → gives aldehyde ( RCHO )
If C of the double bond has two R groups → gives ketone ( R_2CO )
Terminal =CH_2 → gives formaldehyde ( HCHO )
Quick recall
Ozonolysis of but-2-ene gives what product(s)?
2 moles of acetaldehyde ( CH_3CHO ). Both carbons of the double bond each carry one H and one CH_3 → both give aldehyde.
Alkynes — Triple Bonds & Acidic Hydrogen
Alkynes (CₙH₂ₙ₋₂) contain at least one C≡C triple bond. Prepared from calcium carbide (CaC₂) and by double dehydrohalogenation. Terminal alkynes have acidic hydrogen (pKa ~25) and react with NaNH₂, Na metal, and ammoniacal AgNO₃/Cu₂Cl₂. Addition reactions occur in two stages.
C_nH_ 2n-2 (General formula of alkynes)
Preparation of Alkynes
Acetylene (the simplest alkyne) is prepared industrially from calcium carbide (CaC₂ + H₂O). Higher alkynes are prepared by double dehydrohalogenation of vicinal or geminal dihalides using alcoholic KOH or NaNH₂.
Acetylene ( C_2H_2 ) is the parent compound of all alkynes. Its industrial preparation from calcium carbide is one of the oldest and most famous reactions in organic chemistry.
From CaC _2 : CaC_2 + 2H_2O arrow HC CH + Ca(OH)_2
Geminal dihalides (both X on same carbon): 2 mol alc. KOH → alkyne
NaNH₂ (stronger base) gives better yields — forces both eliminations to completion
These are double elimination reactions — two molecules of HX are removed
Acidic Hydrogen
Terminal alkynes (R—C≡C—H) have acidic hydrogen (pKa ~25) due to the high s-character of the sp carbon (50% s). They react with strong bases (NaNH₂) and metal ions (Ag⁺, Cu⁺) to form metal acetylides — a key identification test.
The H attached to a terminal C C is weakly acidic — it can be removed by very strong bases. This is the most distinctive property of terminal alkynes and separates them from internal alkynes, alkenes, and alkanes.
Alkynes undergo the same addition reactions as alkenes but in two stages (triple bond → double bond → single bond). Partial hydrogenation with Lindlar's catalyst gives cis-alkene. Polymerization of acetylene gives polyacetylene (conducting polymer) or cyclotrimerization gives benzene.
Alkynes have two bonds, so they can undergo addition reactions twice — first giving an alkene intermediate, then an alkane. The challenge in NEET is knowing when to stop at the alkene stage.
H_2O addition (Hg²⁺/H⁺): Terminal alkyne → acetaldehyde (tautomerism of enol)
HC CH + H_2O Hg^ 2+ /H^+ [CH_2=CHOH] tautomerism CH_3CHO
Benzene & Aromaticity — The magical stability of the ring
Benzene (C₆H₆) has a planar hexagonal structure with delocalized π electrons above and below the ring. Kekulé's oscillating structure was refined by resonance theory. Hückel's rule (4n+2 π electrons) predicts aromaticity. Benzene's extraordinary stability makes it prefer substitution over addition.
Hückel's Rule: (4n+2) electrons (n = 0, 1, 2, )
Structure of Benzene
Kekulé (1865) proposed alternating single and double bonds in a hexagonal ring. Modern understanding: benzene is a resonance hybrid with all six C—C bonds identical (1.39 Å, between single 1.54 Å and double 1.34 Å). All 6 π electrons are delocalized over the ring.
Benzene has the molecular formula C_6H_6 — this gives a degree of unsaturation = 4 (3 double bonds + 1 ring). Yet benzene doesn't behave like a typical unsaturated compound. It resists addition and prefers substitution. Why?
Kekulé structure (1865): Alternating C=C and C—C in a hexagon, rapidly oscillating between two forms
Problem with Kekulé: Should have 3 short (C=C) and 3 long (C—C) bonds — but X-ray shows ALL bonds are equal (1.39 Å)
Resonance model: Benzene is a hybrid of two Kekulé structures — neither structure alone is correct
MO model: 6 carbon p -orbitals combine to form 6 molecular orbitals — 3 bonding (filled) + 3 antibonding (empty) → delocalized cloud
All 6 C atoms are sp^2 hybridized → planar hexagonal structure
Bond angle = 120° everywhere
C—C bond length = 1.39 Å (between 1.34 Å for C=C and 1.54 Å for C—C)
Each C has one unhybridized p -orbital perpendicular to the ring plane → these 6 p -orbitals overlap laterally to form the delocalized system
Hückel's Rule
A compound is aromatic if it is cyclic, planar, fully conjugated, and has (4n+2) π electrons where n = 0, 1, 2... This gives the magic numbers: 2, 6, 10, 14 π electrons. Anti-aromatic compounds have 4n π electrons and are extremely unstable.
Not every ring with double bonds is aromatic. Hückel (1931) discovered that a molecule must satisfy four criteria simultaneously to be aromatic:
Cyclic — must be a ring (open-chain compounds are never aromatic)
Planar — all atoms in the ring must lie in the same plane (allows p -orbital overlap)
Completely conjugated — every atom in the ring has a p -orbital (no sp^3 atom breaking the conjugation)
(4n+2) electrons — the ring must contain 2, 6, 10, 14... electrons ( n = 0, 1, 2, 3... )
Benzene (6 π e⁻, n=1)
Cyclobutadiene (4 π e⁻, n=1)
Cyclopentadienyl anion (6 π e⁻)
Cyclopentadienyl cation (4 π e⁻)
Tropylium cation C₇H₇⁺ (6 π e⁻)
Cyclooctatetraene-planar (8 π e⁻)
Extra stable
Extra unstable
Non-aromatic: cyclic but NOT fully conjugated (e.g., cyclohexane — no electrons)
Anti-aromatic: cyclic, planar, conjugated, but 4n electrons → destabilized
Cyclooctatetraene ( C_8H_8 ) avoids anti-aromaticity by becoming tub-shaped (non-planar) → it's non-aromatic, not anti-aromatic
Electrophilic Aromatic Substitution — The signature reaction of benzene
Benzene undergoes electrophilic substitution (not addition) to preserve aromaticity. The five major reactions: halogenation (X₂/Lewis acid), nitration (HNO₃/H₂SO₄), sulphonation (oleum), Friedel-Crafts alkylation (RX/AlCl₃), and Friedel-Crafts acylation (RCOCl/AlCl₃). All follow the same general mechanism: generation of electrophile → attack on ring → loss of H⁺.
ArH + E^+ arrow ArE + H^+
General EAS Mechanism
All electrophilic aromatic substitution reactions follow the same two-step mechanism: (1) the electrophile attacks the π cloud to form a resonance-stabilized carbocation intermediate (arenium ion/sigma complex), and (2) a proton is lost to restore aromaticity.
Benzene refuses to undergo addition because that would destroy its aromatic stabilization (150 kJ/mol). Instead, it undergoes substitution — one H is replaced by the electrophile, and the ring stays intact.
Step 1 (slow, RDS): Electrophile E^+ attacks the cloud → forms the arenium ion (also called -complex or Wheland intermediate)
The arenium ion is a resonance-stabilized carbocation — positive charge is delocalized over 3 positions in the ring
Step 2 (fast): A proton ( H^+ ) is lost from the carbon bearing E → aromaticity is restored
The driving force is restoration of aromaticity — the ring would rather lose H^+ than keep it and lose aromaticity
Five Key EAS Reactions
The five electrophilic aromatic substitution reactions differ only in how the electrophile is generated. The ring mechanism is identical in all five. Know the reagents, catalysts, electrophile, and products for each.
Every EAS reaction has the same ring mechanism — what changes is the electrophile and how it's made. Here are the five reactions NEET expects you to know cold:
FC Alkylation electrophile: R^+ (carbocation from RCl + AlCl_3 )
FC Acylation electrophile: RCO^+ (acylium ion from RCOCl + AlCl_3 )
Limitation of FC alkylation: Polyalkylation occurs (product is more reactive than benzene → reacts again)
FC acylation advantage: No polyacylation (acyl group is deactivating → product is less reactive)
Directing Effects — Ortho/para vs meta orientation
Substituents already on the benzene ring direct the incoming electrophile to specific positions. Electron-donating groups (EDGs: —OH, —NH₂, —OR, —R, —X) are ortho/para directors and ring activators (except halogens which are deactivating o/p directors). Electron-withdrawing groups (EWGs: —NO₂, —COR, —COOH, —CN) are meta directors and ring deactivators.
EDG arrow ortho/para EWG arrow meta
Ortho/Para Directors
Groups that donate electron density to the ring (by +M or +I effect) activate it toward EAS and direct the incoming electrophile to the ortho and para positions. These include —OH, —NH₂, —OR, —NHCOR, —R (alkyl), and halogens (special case: deactivating but o/p directing).
When a substituent is already on the benzene ring, it controls where the next group goes. Ortho/para directors funnel the electrophile to the 2,4,6-positions (ortho and para to themselves) and generally make the ring more reactive than unsubstituted benzene.
Moderately activating (o/p): —NHCOR (lone pair partially delocalized into C=O)
Weakly activating (o/p): —R (alkyl groups — hyperconjugation/+I effect)
Deactivating but o/p directing: —F , —Cl , —Br , —I (the lone pair donates via +M, but strong -I effect reduces overall electron density)
Meta Directors
Groups that withdraw electron density from the ring (by −M or −I effect) deactivate it toward EAS and direct the incoming electrophile to the meta position. These include —NO₂, —CN, —COOH, —COOR, —COR, —CHO, —SO₃H, and —CF₃.
Meta directors pull electrons away from the ring, making it less reactive toward electrophiles. They direct the incoming E⁺ to the meta position (3 and 5 positions) because ortho/para attack would place the positive charge directly on the carbon bearing the electron-withdrawing group — maximally destabilizing.
All have a partial or full positive charge on the atom directly attached to the ring, or strong -M effect
They make the ring so electron-poor that EAS becomes difficult — needs harsher conditions than unsubstituted benzene
—OH, —NH₂, —OR, —R, —X
—NO₂, —CN, —COR, —COOH, —SO₃H
Donate electrons to ring
Withdraw electrons from ring
Activate (except halogens)
Always deactivate
Have lone pair or +I/+M effect
Have −M or −I effect
E⁺ goes to o/p positions
E⁺ goes to meta position
Quick recall
Predict the major product of nitration of toluene (methylbenzene).
ortho-nitrotoluene and para-nitrotoluene (roughly 60:40 o:p). The —CH_3 group is a weakly activating ortho/para director (+I and hyperconjugation). The reaction is also faster than nitration of benzene.
Quick recall
Predict the major product of nitration of nitrobenzene.
meta-dinitrobenzene. The —NO_2 group is a strong meta director and deactivator. The reaction requires harsh conditions (fuming HNO_3 , conc. H_2SO_4 , prolonged heating).
Disubstituted Benzene
When two substituents are already on the ring, they may agree or conflict on where the next group goes. If both direct to the same position, that's the major product. If they conflict, the stronger activating group wins. Steric effects also play a role — bulky groups disfavor the ortho position between two substituents.
In disubstituted benzene, the incoming electrophile must satisfy the directing preferences of both existing groups. Three scenarios arise:
Both groups agree: E.g., 4-methylphenol — both —OH (o/p) and —CH_3 (o/p) direct to the same positions → clean major product
Groups conflict: The more powerful activating group wins the directing contest
Polycyclic aromatic hydrocarbons (PAHs) like benzo[a]pyrene are potent carcinogens found in coal tar, cigarette smoke, and charred food. Hydrocarbons contribute to the greenhouse effect (CH₄ is 80× more potent than CO₂ over 20 years). Petroleum fractionation separates crude oil into useful hydrocarbon fractions based on boiling points.
CNG = CH_4 LPG = C_3H_8 + C_4H_ 10
Polycyclic Aromatic Hydrocarbons
PAHs are hydrocarbons with two or more fused benzene rings. Some PAHs are powerful carcinogens — they metabolize in the body to epoxides that bind to DNA, causing mutations. Benzo[a]pyrene (from coal tar, tobacco smoke, charred meat) is the most studied carcinogenic PAH.
When two or more benzene rings share edges (fused rings), they form polycyclic aromatic hydrocarbons (PAHs). While benzene itself is a mild carcinogen, some PAHs are among the most potent cancer-causing agents known.
Naphthalene ( C_ 10 H_8 ): 2 fused rings — used in mothballs, mildly toxic
Anthracene ( C_ 14 H_ 10 ): 3 fused rings in a line — used in dyes
Phenanthrene ( C_ 14 H_ 10 ): 3 fused rings in an angular arrangement — found in coal tar
Benzo[a]pyrene ( C_ 20 H_ 12 ): 5 fused rings — potent carcinogen found in cigarette smoke, coal tar, and charred/smoked food
Sources of PAHs: incomplete combustion of coal, oil, gas, tobacco, wood, charred food
Benzene itself is a confirmed carcinogen (causes leukemia with chronic exposure)
NCERT specifically mentions coal tar and tobacco smoke as sources
Greenhouse Effect & Petroleum
Methane is a potent greenhouse gas. Incomplete combustion of hydrocarbons produces CO (toxic) and unburnt hydrocarbons (smog). Petroleum (crude oil) is a complex mixture of hydrocarbons separated by fractional distillation into fractions (petrol, kerosene, diesel, etc.) based on boiling point ranges.
Hydrocarbons are the backbone of modern energy — but burning them has massive environmental consequences. From greenhouse gases to urban smog, the chemistry of hydrocarbon combustion affects the entire planet.
Greenhouse effect: CO_2 and CH_4 absorb infrared radiation emitted by Earth → trap heat → global warming
Methane ( CH_4 ) is ~80× more potent than CO_2 as a greenhouse gas (over 20 years)
Sources of CH_4 : cattle (enteric fermentation), rice paddies, natural gas leaks, landfills
Incomplete combustion of hydrocarbons → CO (carbon monoxide — toxic, binds to hemoglobin) + soot (particulate matter)
Petroleum fractionation separates crude oil into fractions based on boiling point ranges: