Organic Chemistry — Basic Principles
'Organic chemistry is the chemistry of carbon compounds. Biochemistry is the study of carbon compounds that crawl.' — Mike Adams
1. Chapter Overview
ORGANIC CHEMISTRY is the study of CARBON compounds. Carbon's ability to form FOUR covalent bonds and CHAINS of unlimited length makes it UNIQUE. This chapter covers TETRAVALENCY of carbon, CLASSIFICATION of organic compounds, IUPAC NOMENCLATURE (naming), ISOMERISM (same formula, different structure), ELECTRONIC EFFECTS (inductive, resonance, hyperconjugation), and the BASICS of reaction mechanisms.
2. Why Carbon is Special (Catenation)
Carbon's Properties
- Tetravalency (4 valence electrons → 4 covalent bonds)
- Catenation: Ability to form LONG CHAINS (C—C bonds are strong)
- Forms MULTIPLE bonds (double, triple)
- Forms bonds with MANY elements (H, O, N, S, halogens)
- Isomerism: Same formula, different structures
Hybridisation in Carbon
| Hybridisation | Bond Angles | Geometry | Examples |
|---|---|---|---|
| sp³ | 109.5° | Tetrahedral | CH₄, C₂H₆ |
| sp² | 120° | Trigonal planar | C₂H₄, C₆H₆ |
| sp | 180° | Linear | C₂H₂ |
3. Classification of Organic Compounds
Based on Carbon Skeleton
- Acyclic (Open chain): Carbon atoms in OPEN chains (alkanes)
- Cyclic (Closed chain): Carbon atoms in RINGS
- Homocyclic (Carbocyclic): Ring of only CARBON atoms
- Alicyclic: Cyclohexane, cyclopropane
- Aromatic: Benzene, naphthalene (SPECIAL stability)
- Heterocyclic: Ring contains OTHER atoms (N, O, S)
- Examples: Furan, pyridine, thiophene
- Homocyclic (Carbocyclic): Ring of only CARBON atoms
Based on Functional Groups
| Class | Functional Group | Suffix | Example |
|---|---|---|---|
| Alkane | —C—C— | -ane | CH₄ (methane) |
| Alkene | C=C | -ene | C₂H₄ (ethene) |
| Alkyne | C≡C | -yne | C₂H₂ (ethyne) |
| Alcohol | —OH | -ol | C₂H₅OH (ethanol) |
| Aldehyde | —CHO | -al | CH₃CHO (ethanal) |
| Ketone | —CO— | -one | CH₃COCH₃ (propanone) |
| Carboxylic acid | —COOH | -oic acid | CH₃COOH (ethanoic acid) |
| Amine | —NH₂ | -amine | CH₃NH₂ (methanamine) |
| Halide | —X (F, Cl, Br, I) | -halo | CH₃Cl (chloromethane) |
4. IUPAC Nomenclature
Rules for Naming Organic Compounds
- Select the LONGEST carbon chain (parent chain)
- Number the chain from the end NEAREST the functional group/substituent
- Name substituents as prefixes with their POSITION numbers
- Write name: [Position-Prefix][Parent][Suffix]
- Use COMMAS between numbers, HYPHENS between words and numbers
Priority Order of Functional Groups (for suffix selection)
Acid > Ester > Amide > Aldehyde > Ketone > Alcohol > Amine > Alkene > Alkyne
Worked Problem
Q: Name CH₃—CH(CH₃)—CH₂—CH₃ and CH₃—C(=O)—CH₃. A: First: Longest chain = 4 C (butane). CH₃ substituent at C-2. Name: 2-METHYLBUTANE. Second: 3 C chain with ketone. Name: PROPANONE (or acetone).
5. Isomerism
Structural Isomerism (Same MF, different connectivity)
| Type | Description | Example |
|---|---|---|
| Chain | Different carbon skeleton | C₅H₁₂: n-pentane, isopentane, neopentane |
| Position | Different position of functional group | C₃H₇OH: 1-propanol, 2-propanol |
| Functional | Different functional groups | C₂H₆O: ethanol (—OH), dimethyl ether (—O—) |
| Metamerism | Different alkyl groups around functional group | C₃H₈O: diethyl ether, methyl propyl ether |
Stereoisomerism (Same connectivity, different spatial arrangement)
| Type | Description | Requirement |
|---|---|---|
| Geometrical (cis-trans) | Different arrangement around DOUBLE bond | Restricted rotation (C=C or ring) |
| Optical | Mirror-image NON-SUPERIMPOSABLE | Chiral carbon (4 DIFFERENT groups) |
Worked Problem
Q: Draw all isomers of C₄H₁₀O (alcohols and ethers). A: Alcohols (4): 1-butanol, 2-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol. Ethers (3): diethyl ether, methyl propyl ether, isopropyl methyl ether.
6. Electronic Effects
Inductive Effect (I-effect)
- Definition: POLARISATION of σ-bonds due to electronegativity difference
- —I groups: Electron WITHDRAWING (—F, —Cl, —Br, —NO₂, —CN, —COOH)
- +I groups: Electron RELEASING (—CH₃, —C₂H₅, —O⁻)
- Key: Inductive effect DECREASES with distance (attenuates after 3 carbons)
Resonance Effect (Mesomeric Effect, M-effect)
- Definition: DELOCALISATION of π-electrons through conjugated system
- +R groups: Electron RELEASING through resonance (—OH, —NH₂, —OR, —Cl)
- —R groups: Electron WITHDRAWING through resonance (—NO₂, —CN, —COOH, —CHO)
- Conditions: Planarity, conjugation, p-orbitals available
Hyperconjugation (Baker-Nathan Effect)
- Definition: DELOCALISATION of σ-electrons (C—H) into adjacent π-system
- No. of α-H atoms = stability order: Tertiary > Secondary > Primary
- Explains stability of ALKENES: More alkyl substitution → more stable
Electromeric Effect (E-effect)
- Temporary effect: Complete TRANSFER of π-electrons in presence of attacking reagent
- Occurs at the MOMENT of reaction
7. Reaction Mechanisms — Basic Concepts
Types of Bond Cleavage
| Type | Description | Result |
|---|---|---|
| Heterolytic | Unequal sharing → both e⁻ go to ONE atom | Forms CARBOCATION (R⁺) or CARBANION (R⁻) |
| Homolytic | Equal sharing → each atom gets ONE e⁻ | Forms FREE RADICALS (R•) |
Types of Reagents
| Reagent | Definition | Examples |
|---|---|---|
| Electrophile | Electron-LOVING (attacks electron-rich sites) | H⁺, NO₂⁺, Cl⁺, BF₃, AlCl₃ |
| Nucleophile | Nucleus-LOVING (attacks electron-deficient sites) | OH⁻, CN⁻, NH₃, H₂O, ROH |
Types of Organic Reactions
| Type | Description | Example |
|---|---|---|
| Substitution | Atom/group REPLACED by another | CH₄ + Cl₂ → CH₃Cl + HCl |
| Addition | Atoms ADDED across double/triple bond | C₂H₄ + HCl → C₂H₅Cl |
| Elimination | Atoms REMOVED, forming multiple bond | C₂H₅OH → C₂H₄ + H₂O |
| Rearrangement | Atoms REARRANGE within molecule | Isomerisation reactions |
8. Common Mistakes
- Functional group priority: —COOH > —CHO > —OH. Many students pick the wrong suffix
- Numbering the chain: Number from the END NEAREST the FUNCTIONAL GROUP, not from any end
- Resonance ≠ tautomerism: Resonance is delocalisation within a molecule; tautomerism is equilibrium between isomers
- Hyperconjugation is NOT a permanent effect: It's a stabilising interaction, not a bond
- Optical isomerism requires a CHIRAL carbon: All four groups around carbon must be DIFFERENT
9. CBSE Exam Focus
- IUPAC naming of organic compounds (3/5-mark)
- Structural and stereoisomerism identification (3-mark)
- Inductive and resonance effects — comparing acidity/basicity (5-mark)
- Stability of carbocations, free radicals (3-mark)
- Hybridisation of carbon in organic compounds
- Electrophiles and nucleophiles identification
10. Key Formulas (Conceptual)
- NO specific formulas, but CONCEPTUAL understanding of:
- Stability: Tertiary carbocation > Secondary > Primary > Methyl
- Acidity order: —COOH > —OH > —NH₂ > —CH₃
- C—H bond energy determines reactivity in free radical substitution
11. Self-Test (5+ Q&A)
Q1: Name CH₃—CH(CH₃)—CH=CH₂ according to IUPAC. A: Longest chain with double bond = 4 C. Number from end near C=C. Name: 3-METHYLBUT-1-ENE.
Q2: Arrange in order of acidity: ethanol, acetic acid, phenol, water. A: Acetic acid (pK_a ≈ 4.8) > Phenol (pK_a ≈ 10) > Water (pK_a ≈ 15.7) > Ethanol (pK_a ≈ 16).
Q3: What is a chiral carbon? Give an example. A: Carbon bonded to FOUR DIFFERENT groups. Example: Lactic acid CH₃—C*H(OH)—COOH (the starred C is chiral).
Q4: Why is a tertiary carbocation more stable than a primary carbocation? A: TERTIARY carbocation has THREE alkyl groups with +I effect (+ hyperconjugation from 9 α-H) → better charge stabilisation. Primary has only ONE alkyl group (3 α-H).
Q5: Define resonance. What are the conditions for resonance? A: Resonance is the DELOCALISATION of π-electrons across a conjugated system. Conditions: PLANAR molecule, CONJUGATED system (alternating double/single bonds), p-orbitals available for overlap.
12. Conclusion
Organic chemistry's basic principles are the FOUNDATION for understanding the chemistry of life and synthetic compounds. IUPAC nomenclature gives EVERY organic compound a UNIQUE name. Isomerism explains WHY the same molecular formula can give VASTLY different compounds. Electronic effects (inductive, resonance) PREDICT reactivity and stability. Mastering these fundamentals is ESSENTIAL — they will be used in EVERY subsequent organic chemistry topic, from hydrocarbons to biomolecules.
