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Revision: Some Basic Principles of Organic Chemistry JEE Main Some Basic Principles of Organic Chemistry

Definitions [7]

Definition: Organic Compounds

Organic compounds are the compounds of carbon.

Define isomerism.

Compounds having the same molecular formula but different structural formulas are called isomers and the phenomenon is called isomerism.

Define of chain isomerism.

When two or more compounds have a similar molecular formula but are different in the arrangement of carbon atoms in straight or branched chains, the compounds are referred to as chain isomers.

Define a functional group.

An atom or a group of atoms attached to an alkyl group, which takes part in most of the chemical reactions, is called a functional group or functional radical.

Definition: Nomenclature

Nomenclature is the system of assignment of names to organic compounds.

Definition: Isomers

Compounds having the same molecular formula but different structural formula are known as Isomers and the phenomenon is known as Isomerism. (iso = same, meros = parts).

Definition: Hyperconjugation

The stabilising interaction that involves delocalisation of σ-electrons of C–H bond of an alkyl group linked directly to an atom of unsaturated system or to an atom having unshared p-orbital, is called hyperconjugation.

e.g.

Formulae [1]

Give the general formula of carboxylic acids.

C_nH_2n+1COOH (or RCOOH)

Key Points

Key Points: Tetravalence of Carbon & Shapes of Organic Compounds

Carbon has 4 electrons in its outermost shell → forms 4 covalent bonds → tetravalent.

Determination of Hybridisation (First Method):

Count the number of pi (π) bonds on that carbon atom.
Single-bonded C → sp³ hybridisation
C with one π bond → sp²
C with two π bonds → sp

Bonds on C	Hybridisation	Geometry	Example
4 sigma, 0 pi	sp³	Tetrahedral	Methane (CH₄)
3 sigma, 1 pi	sp²	Trigonal planar	Ethene (C₂H₄)
2 sigma, 2 pi	sp	Linear	Ethyne (C₂H₂)

Shapes of molecules:

Methane (CH₄) → sp³ hybrid orbitals
Ethene (C₂H₄) → sp² hybrid orbitals
Ethyne (C₂H₂) → sp hybrid orbitals

Key Points: Organic Compounds

Specification: Organic chemistry studies carbon compounds (excluding CO₂, carbonates, etc.).
Sources: Found in plants, animals, coal, petroleum, wood, and by fermentation.
Lab Synthesis: Many organic compounds are made in labs today.
Daily Use: Found in soaps, clothes, fuels, medicines, food, etc.
Importance: Organic compounds are used in almost every part of life.

Key Points: Classification of Organic Compounds

(i) Based on Functional Group:

(ii) Based on Carbon Skeleton:

Key points: Nomenclature of Carbon Compounds

The IUPAC system provides a unique, systematic way to name carbon compounds based on structure, replacing confusing common names.
An IUPAC name has three parts: prefix, parent, and suffix, reflecting the carbon chain and functional group.
The parent name is based on the longest carbon chain, and its ending changes to –ane, –ene, or –yne depending on the number of bonds.
Functional groups are shown as prefixes or suffixes, and the chain is numbered to give them the lowest possible number.
If the suffix begins with a vowel, the final ‘e’ in the parent alkane name is dropped (e.g., propane → propanone).

Key Points: Types of Isomerism

1. Structural Isomerism

Same molecular formula but different connectivity (bonding) of atoms.

Types:

Chain isomerism: Different carbon skeleton (straight/branched)
Position isomerism: Functional group at different positions
Functional isomerism: Different functional groups (e.g., alcohol vs ether)
Metamerism: Different alkyl groups on either side of same functional group
Tautomerism: Dynamic equilibrium between two forms (keto ↔ enol)

2. Stereoisomerism

Same molecular formula and bonding but different spatial arrangement.

Types:

I. Geometrical isomerism:

Due to restricted rotation (double bond)
Forms cis (same side) and trans (opposite side)

II. Optical isomerism:

Mirror image isomers (enantiomers)
Show optical activity (rotate plane polarized light)

Key Points: Inductive Effect

Permanent effect due to shift of σ-electrons
Electrons move towards more electronegative atom
Effect decreases with distance (negligible after 3 carbons)

e.g-

Type	Meaning	Example
–I effect	Electron withdrawing group	–NO₂, –Cl
+I effect	Electron donating group	–CH₃

Key Points: Resonance Effect

Resonance:

When a single Lewis structure cannot explain all properties of a compound, two or more structures (canonical forms) are written
The real molecule does not exist as separate forms, it exists as a resonance hybrid
Resonance hybrid is a weighted average of all canonical forms
Due to resonance, electrons are delocalised over the molecule
Resonance hybrid is more stable (lower energy) than any single structure

Resonance Effect (Mesomeric Effect):

Type	Explanation	Effect on Electron Density	Examples
+R effect	Group donates electrons to conjugated system	Increases electron density at certain positions	–OH, –OR, –NH₂, –NHR
–R effect	Group withdraws electrons from conjugated system	Decreases electron density at o- and p-positions	–NO₂, –CN, –CHO, –COOH

Key Points: Electromeric Effect

A temporary effect involving the complete transfer of shared π-electrons to one of the atoms in a multiple bond, in the presence of an attacking reagent.

Type	Direction of π-electron transfer	Example
+E Effect	π electrons transferred to the atom to which the reagent gets attached
–E Effect	π electrons transferred to the atom to which the reagent does not attach

Key Points: Hyperconjugation

Also called the Baker-Nathan effect or no-bond resonance.

Conditions for hyperconjugation:

The compound must have at least one sp²-hybridised carbon (alkene, alkyl carbocation, or alkyl free radical)
The α-carbon must have at least one H–C bond attached to the unsaturated system
More the number of H–C bonds at α-carbon → greater the stabilisation → more stable the alkene/carbocation

Resonance vs. Hyperconjugation: