Class 11 Chemistry Notes

Organic Chemistry — Some Basic Principles and Techniques Notes

Complete, exam-ready notes on the basics of organic chemistry: why carbon is special, IUPAC nomenclature, functional groups, structural and stereoisomerism, hybridisation, electron displacement effects like inductive and resonance, reactive intermediates, and the main types of organic reactions — written for CBSE, JEE and NEET revision.

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Written byDeep Narayan· Science & Mathematics EducatorReviewed byPushpanjali

What makes carbon the backbone of organic chemistry?

Carbon forms four covalent bonds (tetravalency) and joins into long chains and rings (catenation) with remarkable strength, giving millions of stable compounds.

Why Carbon — Tetravalency and Catenation

Carbon has 4 valence electrons and forms 4 covalent bonds — tetravalency.
Catenation: carbon links to carbon forming chains, branches and rings, very stable thanks to strong C–C bonds.
Compounds can be acyclic (open chain), cyclic (ring), aromatic (benzene-like) and heterocyclic.
Tetrahedral carbon (sp³, ~109.5°) is the common building block of organic molecules.

IUPAC Nomenclature — The Rules

IUPAC naming

\text{prefix} + \text{root} + \text{primary suffix} + \text{secondary suffix}

Name in steps: find the longest parent chain (root: meth, eth, prop, but, pent...), number it to give substituents the lowest locants, add the functional group suffix (-ol, -al, -one, -oic acid) and locate double/triple bonds with -ene/-yne.

Lowest locant rule for the principal functional group decides numbering.
Substituents are named alphabetically before the root, e.g. 3-methylpentane.
Functional-group priority for the suffix: acid > ester > amide > nitrile > aldehyde > ketone > alcohol > amine.
Common prefixes: -chloro, -bromo, -nitro, and the alkyl group R– from alkanes.

Functional Groups

Functional group

\text{ROH, RC(=O)H, RC(=O)R', RCOOH, RNH}_2, \text{RX, RC}\equiv\text{N}

A functional group is the atom or bond that gives a family its characteristic reactions — alcohols (OH), aldehydes (–CHO), ketones (–C=O), carboxylic acids (–COOH), amines (–NH₂), halides (X), nitriles (–CN).The same group gives a family similar chemistry, so classification follows the functional group.

Isomerism

Structural and stereoisomerism

C_nH_{2n+2} - \text{isomers: } n\text{-butane vs isobutane}

Structural isomers share a formula but differ in connectivity — chain (n-butane/isobutane), position (1-propanol/2-propanol), functional (ethanol/dimethyl ether) and tautomerism.

Stereoisomers share connectivity but differ in 3-D arrangement — geometric (cis/trans) and optical (chiral) isomers.
A carbon with four different groups (chiral centre) gives two mirror-image (enantiomeric) forms.
cis- and trans-2-butene are geometric isomers; their physical properties differ.

Hybridisation and Bonding in Organic Molecules

sp³: four σ bonds, tetrahedral, 109.5° — alkanes.
sp²: three σ + one π, planar, 120° — alkenes and carbonyl carbon.
sp: two σ + two π, linear, 180° — alkynes and nitriles.
A σ bond is formed by head-on overlap, π by side-on overlap of p orbitals; π bonds restrict rotation and are weaker than σ bonds.

Electron Displacement Effects

Inductive effect

\delta+\!-\!-\delta-\!\!-

Sigma-bond polarisation pulls electron density along a chain. Electron-withdrawing groups (−I): F, Cl, NO₂, CN, CF₃. Electron-donating groups (+I): alkyl groups. The effect weakens sharply with distance.

Resonance and mesomeric effect

\text{two or more Lewis structures share the true molecule}

Delocalised electrons spread over several atoms, stabilising the molecule. Carboxylate and nitrate ions and benzene are resonance stabilised. The mesomeric effect (M) acts through π systems.

Hyperconjugation: σ (C–H) electrons delocalise into an empty p orbital (as in propene/toluene), stabilising carbocations.
Aromaticity: cyclic, planar, fully conjugated with (4n+2)π electrons — benzene (6π).
Electromeric effect: a temporary effect in the presence of a reagent, where a π bond polarises completely.

Reactive Intermediates and Types of Reactions

Intermediates

\text{Carbocations } R_3C^+,\;\text{ carbanions } R_3C^-,\;\text{ free radicals } R_3C\cdot

Short-lived species formed during reactions. Carbocation stability: 3° > 2° > 1° > methyl (hyperconjugation + inductive). Carbanion stability is the reverse. Free radicals are neutral with an unpaired electron.

Substitution: replacement of an atom/group (CH₄ + Cl₂ → CH₃Cl + HCl).
Addition: π bond becomes σ bonds across the double/triple bond.
Elimination: removal of atoms to form a π bond (dehydration of alcohols).
Nucleophile = electron-rich species (OH⁻, CN⁻); electrophile = electron-poor (H⁺).
Homolytic cleavage gives radicals; heterolytic cleavage gives ions.

Solved Examples

Example: Name CH₃CH(CH₃)CH₂CH₃ by IUPAC rules.

Solution: Longest chain has four carbons → butane; the methyl substituent is on carbon 2 → 2-methylbutane.

Example: Why is the tertiary carbocation most stable?

Solution: More alkyl groups donate electron density through the +I effect and hyperconjugation, spreading and stabilising the positive charge — so the stability order is 3° > 2° > 1° > CH₃⁺.

Revision

Key formulas at a glance

Memorise these before attempting numericals — most exam questions hinge on one of them.

General formula (alkanes)

C_nH_{2n+2}

Alkenes / alkynes

C_nH_{2n} / C_nH_{2n-2}

IUPAC assembly

\text{prefix} + \text{root} + \text{suffix}^{\text{primary}} + \text{suffix}^{\text{secondary}}

Carbocation stability

3^\circ > 2^\circ > 1^\circ > \text{CH}_3^+

Aromatic rule

(4n+2)\pi\;\text{electrons}

sp³/sp²/sp angles

109.5^\circ/120^\circ/180^\circ

Bond count

\sigma: \text{head-on},\; \pi: \text{side-on}

Exam tips

How this chapter is asked

Where this topic appears in CBSE, JEE Main and NEET papers.

Carbon: tetravalent, catenating — the basis of organic chemistry.
IUPAC: longest chain, lowest locants, suffix priority acid > aldehyde > ketone > alcohol.
sp³/sp²/sp → 109.5°/120°/180°.
Inductive: distance-dependent σ polarisation; resonance: delocalised π stabilisation.
Carbocation stability 3° > 2° > 1°; carbanion reverse.
Aromaticity needs cyclic, planar, (4n+2)π.
Nucleophiles and electrophiles drive substitution and addition.

FAQ

Common questions

Why is carbon the basis of organic chemistry?

Carbon is tetravalent and catenates — it forms four strong covalent bonds and chains with other carbons giving huge numbers of stable molecules in ring, chain and branched forms.

How do you name an organic compound by IUPAC rules?

Find the longest chain, number it to give lowest locants, add main functional-group suffix and secondary suffix, then list substituents alphabetically — prefix + root + suffix.

What is the difference between inductive effect and resonance?

The inductive effect is a permanent sigma-bond polarisation that falls off with distance; resonance is the delocalisation of π electrons over several atoms, giving an averaged, more stable structure than any single Lewis form.

Why is a tertiary carbocation more stable than a primary one?

Alkyl groups donate electron density by the +I effect and hyperconjugation, dispersing the positive charge. More groups around the charged carbon means more stabilisation: 3° > 2° > 1°.

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