Organic chemistry contributes a large, predictable slice of the NEET chemistry paper, and almost every question from later chapters (hydrocarbons, haloalkanes, aldehydes, amines, biomolecules) silently assumes you already know the material here. Master naming, isomerism and the electronic effects once, and the rest of organic chemistry becomes pattern recognition instead of memorisation.
Carbon: Tetravalency, Hybridisation and Ways of Drawing Molecules
Carbon has four valence electrons and forms four covalent bonds, which it does using hybrid orbitals. The three hybridisation states you must be able to spot instantly are:
- sp³ — four sigma bonds, tetrahedral, bond angle ≈ 109.5°, as in methane. All four bonds are equivalent.
- sp² — three sigma bonds plus one unhybridised p orbital, trigonal planar, ≈ 120°, as in ethene or a carbonyl carbon. The leftover p orbitals overlap sideways to give a π bond.
- sp — two sigma bonds plus two p orbitals, linear, 180°, as in ethyne or the central carbon of an allene or of CO₂.
A quick shortcut: count the number of σ bonds plus lone pairs on the carbon. Four → sp³, three → sp², two → sp. Since s orbitals hold electrons closer to the nucleus, greater s character means shorter, stronger bonds and greater electronegativity of that carbon. Hence C–H bond length falls in the order ethane > ethene > ethyne, and the acidity of the terminal hydrogen rises in the reverse order (ethyne is acidic enough to react with sodium metal or ammoniacal silver nitrate; ethene and ethane are not).
Structures can be written in three ways, and NEET expects fluency in converting between them:
- Complete (Lewis/dash) formula — every atom and bond shown.
- Condensed formula — CH₃CH₂CH₂OH, or with brackets, (CH₃)₂CHCl.
- Bond-line (zig-zag) notation — lines represent bonds, carbon atoms sit at the ends and corners, hydrogens on carbon are not drawn. Heteroatoms (O, N, Cl, S) are always written explicitly.
Three-dimensional shape is shown by wedge–dash formulae: a solid wedge points towards the viewer, a hashed wedge points behind the plane, and ordinary lines lie in the plane of the paper.
Classification, Functional Groups and Homologous Series
Organic compounds are first split by skeleton:
- Acyclic (open-chain / aliphatic) compounds — straight or branched chains.
- Cyclic compounds, which subdivide into:
- Alicyclic (carbocyclic rings that are not aromatic, e.g. cyclohexane),
- Aromatic — benzenoid (benzene, naphthalene, phenol) and non-benzenoid (tropone), plus heterocyclic aromatic rings such as furan, pyrrole, thiophene and pyridine,
- Heterocyclic alicyclic rings such as tetrahydrofuran.
A functional group is the atom or group of atoms that gives a molecule its characteristic chemistry — –OH, –CHO, –COOH, –CO–, –NH₂, –X, –CN, –NO₂, –SO₃H, and the C=C and C≡C linkages. Compounds carrying the same functional group behave similarly, which is why organic chemistry can be learned family by family.
A homologous series is a family of compounds with the same functional group and the same general formula, in which successive members differ by a –CH₂– unit (molar mass difference 14 u). Members of a homologous series show a gradual gradation in physical properties (boiling point, density) and closely similar chemical properties; the individual members are homologues. Note that CH₄ and C₂H₆ are homologues, but CH₃OH and CH₃CH₂CHO are not — the functional group must be the same.
IUPAC Nomenclature
An IUPAC name has the pattern: secondary prefix + primary prefix + word root + primary suffix + secondary suffix. The working procedure:
- Pick the parent chain: the longest continuous carbon chain. If two chains are equally long, choose the one with more branches. If a functional group is present, the parent chain must contain the principal functional group.
- Number the chain so that the principal functional group (or, in a hydrocarbon, the multiple bond) gets the lowest possible locant. If there is a choice, apply the lowest set of locants rule: compare the two sets term by term and choose the one that is lower at the first point of difference. If still tied, the substituent cited first alphabetically gets the lower number.
- Name the substituents alphabetically, using di-, tri-, tetra- to indicate repetition. Multiplying prefixes are ignored when alphabetising, but prefixes that are part of a complex substituent's name (iso, cyclo, and the first letter of a bracketed group) are counted. Thus "ethyl" precedes "dimethyl", but isopropyl is alphabetised under "i".
- Attach the suffix: -ane / -ene / -yne for the skeleton, then the secondary suffix for the functional group (-ol, -al, -one, -oic acid, -amine, -nitrile, -carboxylic acid, etc.). The terminal "e" of the primary suffix is dropped before a suffix beginning with a vowel (propane + ol → propan-1-ol), but retained before a consonant (propanenitrile).
When more than one functional group is present, only one is the principal group (expressed as suffix) and the rest become prefixes. The decreasing order of seniority you should know is:
–COOH > –SO₃H > ester > acid halide > amide > –CN > –CHO > –CO– > –OH > –NH₂ > alkene/alkyne
Groups such as –X (halo), –NO₂, –OR (alkoxy) and –R are always prefixes and are never expressed as a suffix.
Cyclic compounds take the prefix "cyclo"; if the ring has fewer carbons than the attached chain, the ring is treated as the substituent. For benzene derivatives with two substituents, ortho (1,2), meta (1,3) and para (1,4) are still accepted. Many retained trivial names are examinable: toluene, phenol, aniline, styrene, acetophenone, benzaldehyde, benzoic acid, acetic acid, acetone.
Isomerism
Compounds with the same molecular formula but different properties are isomers. Two broad classes:
A. Structural (constitutional) isomerism — different connectivity.
- Chain (skeletal) isomerism: n-butane vs isobutane.
- Position isomerism: propan-1-ol vs propan-2-ol; but-1-ene vs but-2-ene.
- Functional group isomerism: same formula, different functional group — propanal (aldehyde) and propanone (ketone), C₃H₆O; ethanol and dimethyl ether, C₂H₆O.
- Metamerism: unequal distribution of carbon atoms on either side of a divalent heteroatom — CH₃OC₃H₇ vs C₂H₅OC₂H₅.
- Tautomerism: a dynamic equilibrium between two structures differing in the position of a proton and a double bond, classically keto ⇌ enol.
B. Stereoisomerism — same connectivity, different spatial arrangement. This includes geometrical (cis–trans) isomerism, which needs restricted rotation (a C=C or ring) and two different groups on each doubly bonded carbon, and optical isomerism, which needs chirality (commonly an asymmetric carbon bearing four different groups). Optical isomers rotate plane-polarised light in opposite directions.
Electron Displacement Effects
Reactivity in organic chemistry is largely explained by how electron density is redistributed in a molecule.
1. Inductive effect (I). Permanent polarisation transmitted through a σ-bond chain because of an electronegativity difference. It is a distance-dependent effect and dies out beyond roughly three bonds. Electron-withdrawing groups (–NO₂, –CN, –COOH, –X) show –I; alkyl groups show +I. The –I effect of halogens falls F > Cl > Br > I, which is why fluoroacetic acid is the strongest of the haloacetic acids.
2. Electromeric effect (E). A temporary, complete shift of a π-electron pair to one of the bonded atoms, occurring only in the presence of an attacking reagent, and reversing when the reagent is removed. +E is when the shift is towards the attacking reagent (as in acid addition to an alkene); –E is away from it (as in cyanide attack on a carbonyl).
3. Resonance (mesomeric) effect. When a molecule cannot be represented by a single Lewis structure, it is described as a resonance hybrid of several canonical forms. The rules for canonical structures: the positions of nuclei stay fixed, only electron pairs move; all forms must have the same number of unpaired electrons; the real molecule is more stable (has lower energy) than any contributing structure, the difference being the resonance energy. Structures with more covalent bonds, less charge separation, and negative charge on the more electronegative atom contribute more.
- +R (+M): groups that release electrons to the conjugated system — –OH, –OR, –NH₂, –NHR, –X, –O⁻.
- –R (–M): groups that withdraw electrons — –NO₂, –CN, –CHO, –COOH, –COR, –SO₃H.
Halogens are a classic exam trap: they exert –I but +R, which is why chlorobenzene is deactivated yet ortho/para directing.
4. Hyperconjugation. Delocalisation of the σ electrons of a C–H bond adjacent to an sp² carbon (a double bond, a carbocation or a radical) into the empty or partly filled p orbital — sometimes called "no-bond resonance". Its magnitude increases with the number of α-hydrogens. Hyperconjugation explains why 2-methylbut-2-ene is more stable than but-1-ene, and why the stability order of carbocations is:
tertiary > secondary > primary > CH₃⁺ (9, 6, 3, 0 hyperconjugative structures respectively).
Reaction Intermediates and Types of Organic Reactions
A covalent bond can break in two ways:
- Homolytic fission — the bonding pair splits symmetrically, one electron going to each fragment. This needs an energy input (heat, light, or a peroxide initiator) and typically occurs in the gas phase or in non-polar solvents, generating free radicals: neutral species with an unpaired electron, denoted with a dot (Cl•). A radical is reactive not because it is charged but because the unpaired electron seeks a partner.
- Heterolytic fission — the bonding pair moves entirely onto one atom, usually the more electronegative one, generating an ion pair. This is favoured in polar solvents, which can stabilise the resulting ions through solvation.
Heterolysis produces two important intermediates. A carbocation is an sp² hybridised, planar carbon with an empty p orbital; it is stabilised by anything that feeds electron density into that empty orbital — hyperconjugation and the +I effect of alkyl groups (stability order tertiary > secondary > primary > methyl), or resonance donation from an adjacent lone pair or π bond (allylic and benzylic cations are unusually stable). A carbanion is a pyramidal, sp³ carbon holding a lone pair; its stability runs the opposite way, favoured by electron-withdrawing groups and disfavoured by electron-donating ones, so the order is methyl > primary > secondary > tertiary. A free radical's stability tracks the carbocation's, tertiary > secondary > primary, again through hyperconjugative delocalisation of the odd electron.
Organic reactions themselves fall into four broad mechanistic families: substitution (one atom or group replaces another, the degree of saturation unchanged, e.g. CH₄ + Cl₂ → CH₃Cl + HCl); addition (reagent fragments add across a multiple bond, converting it to a single bond, as with HBr or H₂ across an alkene); elimination (atoms are lost from adjacent carbons to create a multiple bond, as in dehydrohalogenation with alcoholic KOH); and rearrangement (the carbon skeleton itself reorganises, usually through a carbocation migrating a hydride or alkyl group to a more stable cation).
Reagents are further classified by what they seek. Electrophiles ('electron lovers') are electron-deficient species — cations, or neutral molecules with an incomplete octet or an accessible empty orbital (H⁺, NO₂⁺, BF₃, carbocations) — that attack electron-rich sites. Nucleophiles ('nucleus lovers') are electron-rich species — anions, or neutral molecules with a lone pair (OH⁻, CN⁻, NH₃, H₂O) — that attack electron-deficient sites. The same species can play either role depending on context, so classification depends on what it is doing in that particular step, not on some fixed label.
Common Mistakes and Exam Traps
Mixing up the –I and +R effects of halogens. Halogens withdraw electron density inductively (–I, raising the acidity of nearby protons and destabilising adjacent carbocations) but donate a lone pair by resonance into an attached π system (+R, which is why a halogen substituent on benzene is ortho/para directing despite being deactivating). A question about acidity usually wants the inductive picture; a question about directing effects on an aromatic ring wants the resonance picture.
Ranking carbocations and carbanions the same way. Alkyl groups stabilise carbocations, which are electron-deficient and want more electron density, but destabilise carbanions, which already have surplus electron density. Defaulting to 'tertiary is always most stable' without checking which intermediate is in question is a common slip.
Applying the lowest-locant rule substituent by substituent instead of as a set. The rule compares the entire set of locants at the first point of difference; alphabetical priority is only the tie-breaker when two numbering schemes give identical locant sets, not the first criterion.
Picturing resonance structures as separate molecules in rapid equilibrium. They are not interconverting species; the real molecule is a single hybrid at all times, closer in energy and structure to whichever canonical form contributes most, never oscillating between the drawn forms.