Hydrocarbons Notes for NEET and JEE Main 2027: Key Points and PDF

Hydrocarbons is Unit 9 of NCERT Class 11 Chemistry Part II and Unit 15 of both the NEET UG and JEE Main chemistry syllabi. These notes cover alkanes and their conformations, free radical halogenation, alkenes and Markovnikov addition, the peroxide effect, alkynes and their acidity, benzene, aromaticity, electrophilic substitution and directive influence, with a free notes PDF.

Hydrocarbons Notes for NEET and JEE Main

Section-wise key points from NCERT Unit 9, reaction tables, high-yield facts and common mistakes for NEET and JEE Main 2027 revision.

Hydrocarbons at a Glance

Topics follow the NEET UG 2026 syllabus released by the National Medical Commission (NMC) in December 2025 and the JEE Main 2026 syllabus published by the National Testing Agency (NTA). Both stay the reference until the 2027 syllabi are out. Full lists: NEET syllabus and JEE Main syllabus.

How are hydrocarbons classified?

Hydrocarbons contain only carbon and hydrogen and are classified as saturated (alkanes and cycloalkanes), unsaturated (alkenes and alkynes) and aromatic.

Alkanes: Isomerism, Preparation and Properties

• Methane is tetrahedral with H–C–H angles of 109.5°; C–C and C–H bond lengths in alkanes are 154 pm and 112 pm.
• Chain isomers: C4H10 has 2, C5H12 has 3, C6H14 has 5, C7H16 has 9 and C10H22 has 75.
• Carbon types: primary (1°) is bonded to one or no carbon, secondary (2°) to two, tertiary (3°) to three, quaternary (4°) to four.

Physical Properties

• Almost non-polar, held by weak van der Waals forces; C1 to C4 are gases, C5 to C17 liquids, C18 and above solids at 298 K.
• Boiling point rises with molecular mass; branching lowers it because the molecule becomes more spherical with less surface contact.
• Isomeric pentanes: pentane 309.1 K, 2-methylbutane 300.9 K, 2,2-dimethylpropane 282.5 K.

Chemical Reactions of Alkanes

Free Radical Mechanism of Chlorination of Methane

• Initiation: light or heat homolyses Cl–Cl, the weakest bond present, into two chlorine free radicals.
• Propagation: Cl• takes H from CH4 to give •CH3 and HCl; •CH3 attacks Cl2 to give CH3Cl and a new Cl•.
• Termination: radicals combine (Cl• + Cl•, •CH3 + •CH3, •CH3 + Cl•). The •CH3 + •CH3 step explains ethane as a by-product.

Conformations of Ethane

Conformations (conformers or rotamers) are spatial arrangements that interconvert by rotation about a C–C single bond; the rotation is hindered only by a small barrier of 1-20 kJ/mol called torsional strain.

• Any conformation between the two is a skew conformation; bond angles and bond lengths stay the same in all of them.
• Sawhorse projection: the C–C bond is drawn as a longer tilted line, front carbon at the lower end, rear carbon at the upper end.
• Newman projection: viewed along the C–C bond; the front carbon is a point and the rear carbon a circle.
• The angle of rotation is the dihedral or torsional angle. The 12.5 kJ/mol barrier is crossed by collisions at ordinary temperature, so ethane conformers cannot be isolated.

Alkenes: Structure, Isomerism and Preparation

• The C=C bond is one strong σ bond (about 397 kJ/mol, sp2-sp2 overlap) plus one weak π bond (about 284 kJ/mol, sideways 2p overlap). Its loosely held π electrons attract electrophilic reagents.
• Structural isomers of C4H8: but-1-ene and but-2-ene are position isomers; each is a chain isomer of 2-methylprop-1-ene.
• Geometrical (cis-trans) isomerism arises from restricted rotation about C=C when each doubly bonded carbon carries two different groups (types XYC=CXY, XYC=CXZ, XYC=CZW).
• cis-But-2-ene has a dipole moment of 0.33 D; trans-but-2-ene is almost non-polar. In solids the trans isomer has the higher melting point.

Physical properties: the first three alkenes are gases, the next fourteen liquids and the higher ones solids. Ethene has a faint sweet smell; each added –CH2 raises the boiling point by 20-30 K, and straight chains boil higher than branched isomers.

Reactions of Alkenes: Markovnikov Rule and Peroxide Effect

Alkenes mainly undergo electrophilic addition across the double bond; unsymmetrical alkenes follow Markovnikov’s rule, under which the negative part of the adding molecule goes to the carbon with fewer hydrogen atoms.

• Peroxide effect was reported by M.S. Kharash and F.R. Mayo in 1933. It fails with HCl because the H–Cl bond (430.5 kJ/mol) is too strong to be cleaved by the free radical, and with HI because iodine radicals combine to form I2 (H–I is 296.8 kJ/mol, H–Br 363.7 kJ/mol).
• In the peroxide effect the secondary free radical is more stable than the primary one, which is why 1-bromopropane is the major product.

Alkynes: Structure, Acidity and Reactions

• General formula CnH2n–2. Ethyne (acetylene) is the first stable member and is used in the oxyacetylene flame for welding.
• But-1-yne and but-2-yne are position isomers; C5H8 has three alkyne isomers (pent-1-yne, pent-2-yne, 3-methylbut-1-yne).
• Each carbon of ethyne is sp hybridised; the molecule has one C–C σ bond, two C–H σ bonds and two C–C π bonds. The electron cloud is cylindrically symmetrical, so ethyne is linear.
• Preparation: CaC2 + 2H2O gives Ca(OH)2 + C2H2 (CaC2 is made by heating CaO with coke); vicinal dihalides with alcoholic KOH give an alkenyl halide, which with sodamide gives the alkyne.
• Physical properties: the first three are gases, the next eight liquids and the higher ones solids; ethyne has a characteristic odour, the others are odourless.

Why are terminal alkynes acidic?

The sp orbital has 50% s character, so the sp carbon is the most electronegative and holds the C–H electron pair more tightly; the H can leave as a proton.

• Ethyne reacts with Na or NaNH2 to give sodium ethynide (acetylide) and H2; ethene and ethane do not, which helps distinguish them.
• Acidity order: HC≡CH > H2C=CH2 > CH3–CH3, and HC≡CH > CH3–C≡CH >> CH3–C≡C–CH3. Only hydrogens on a triply bonded carbon are acidic.

Benzene: Structure, Resonance and Aromaticity

• Michael Faraday isolated benzene in 1825. It forms a triozonide, which suggests three double bonds, yet gives only one monosubstituted and one ortho-disubstituted product.
• August Kekulé proposed the cyclic structure with alternate single and double bonds in 1865 and explained the single ortho product by oscillating double bonds.
• Benzene is a resonance hybrid of the two Kekulé structures, shown as a hexagon with a circle for six delocalised π electrons.
• All six carbons are sp2 hybridised; the unhybridised p orbitals overlap to give π electron clouds above and below the planar ring.
• X-ray data show all six C–C bonds equal (139 pm), between a C–C single bond (154 pm) and a C=C double bond; this explains why benzene resists addition.
• Aromaticity (Hückel rule): the ring must be planar, have complete delocalisation of π electrons and contain (4n + 2) π electrons, where n = 0, 1, 2 and so on.
• Preparation: cyclic polymerisation of ethyne; heating sodium benzoate with soda lime; passing phenol vapour over heated zinc dust. Commercially it is isolated from coal tar.
• Arenes are non-polar, immiscible with water and burn with a sooty flame.

Electrophilic Substitution Reactions of Benzene

Arenes mainly undergo electrophilic substitution, which keeps the aromatic ring intact; addition and oxidation need special conditions.

Mechanism (SE)

• Generation of the electrophile: the Lewis acid AlCl3 produces Cl+, R+ or RCO+; protonation of HNO3 gives NO2+.
• Formation of the σ-complex (arenium ion): one ring carbon becomes sp3, so the ion loses aromaticity but is resonance stabilised.
• Removal of a proton by [AlCl4]– or [HSO4]– restores aromaticity.

Addition and Combustion

• With H2 and Ni at high temperature and pressure, benzene gives cyclohexane.
• With Cl2 under UV light, benzene adds three Cl2 molecules to give benzene hexachloride (BHC, gammaxane), C6H6Cl6.
• General combustion: CxHy + (x + y/4) O2 gives x CO2 + (y/2) H2O.

Directive Influence of Groups in Monosubstituted Benzene

Where a second group enters depends on the substituent already on the ring, not on the incoming group.

Carcinogenicity and Toxicity

Benzene and polynuclear hydrocarbons with more than two fused benzene rings are toxic and carcinogenic. They form on incomplete combustion of tobacco, coal and petroleum and damage DNA in the body.

• Examples in NCERT: 1,2-benzanthracene, 3-methylcholanthrene, 1,2-benzpyrene, 1,2,5,6-dibenzanthracene and 9,10-dimethyl-1,2-benzanthracene.

Most-asked Facts for NEET and JEE Main: Hydrocarbons

Revise these one-line facts from the NCERT chapter just before the exam.

• Wurtz reaction suits alkanes with an even number of carbons; Kolbe electrolysis cannot give methane.
• Decarboxylation with soda lime removes one carbon atom.
• Rate of halogenation F2 > Cl2 > Br2 > I2; ease of H replacement 3° > 2° > 1°.
• Ethane forms as a by-product in chlorination of methane through radical termination.
• Branching lowers boiling point: neopentane boils at 282.5 K, n-pentane at 309.1 K.
• Staggered ethane is more stable than eclipsed by about 12.5 kJ/mol.
• Lindlar’s catalyst gives cis-alkenes; Na in liquid NH3 gives trans-alkenes.
• cis-But-2-ene has μ = 0.33 D; trans-but-2-ene is about zero.
• Peroxide effect works only with HBr (Kharash and Mayo, 1933).
• Baeyer’s reagent is cold, dilute, aqueous KMnO4 and gives vicinal glycols.
• Bond lengths: C–C 154, C=C 134, C≡C 120, benzene C–C 139 pm.
• sp carbon has 50% s character, so ethyne is the most acidic of ethane, ethene and ethyne.
• Hydration of ethyne with HgSO4/H2SO4 at 333 K gives ethanal.
• Ethyne through a red hot iron tube at 873 K gives benzene.
• Hückel rule: planar, fully delocalised, (4n + 2) π electrons.
• Nitronium ion NO2+ is the electrophile in nitration.
• Benzene + Cl2 in UV light gives benzene hexachloride (gammaxane).
• –NO2, –CN, –CHO, –COOH, –SO3H are meta directors; halogens are deactivating but ortho-para directing.

Common Mistakes to Avoid

Watch for these mix-ups when you answer questions on this chapter.

• Applying the peroxide effect to HCl or HI. It works only with HBr.
• Expecting cis-alkenes from Na in liquid NH3. That gives trans-alkenes; Lindlar’s catalyst gives cis.
• Calling halogens activating because they are ortho-para directing. They deactivate the ring.
• Predicting n-propylbenzene from benzene and 1-chloropropane. Isopropylbenzene forms.
• Assuming all hydrogens of an alkyne are acidic. Only hydrogens on the triply bonded carbon are.
• Thinking branched alkanes boil higher. Branching reduces surface area and lowers the boiling point.
• Calling eclipsed ethane the most stable form. The staggered form has the least torsional strain.
• Writing that benzene has alternating 154 pm and 134 pm bonds. All six C–C bonds are 139 pm.
• Treating benzene hexachloride as a substitution product. It is an addition product formed under UV light; hexachlorobenzene (C6Cl6) is the substitution product.
• Saying iodine adds readily to alkenes. It does not add under normal conditions.

Hydrocarbons Notes PDF and Related Pages

Download the AglaSem notes for Hydrocarbons (key notes and important questions with answers) free: View PDF

• All chapters: NEET and JEE Main Chemistry notes
• Full topic list: NEET syllabus and JEE Main syllabus
• Practice: NEET question papers and JEE Main question papers
• Source text: NCERT Class 11 Chemistry Unit 9 (NCERT, Class 11 Chemistry Part II)

Chemistry Notes for NEET and JEE Main: All Chapters

• Some Basic Concepts of Chemistry
• Structure of Atom
• Chemical Kinetics
• Equilibrium
• Organic Chemistry: Some Basic Principles and Techniques
• Electrochemistry
• Amines
• Solutions
• Thermodynamics
• Coordination Compounds
• Haloalkanes and Haloarenes
• p-Block Elements
• Alcohols, Phenols and Ethers
• Hydrocarbons
• Aldehydes, Ketones and Carboxylic Acids

All chapters with key points and notes PDFs are on the Chemistry notes for NEET and JEE Main page.

Hydrocarbons Notes for NEET and JEE Main FAQs