Quick Summary
Organic chemistry is the chemistry of carbon compounds. This chapter lays the foundation: why carbon forms so many compounds (catenation, tetravalency), IUPAC naming rules, the shapes of carbon (hybridisation), the types of structural isomers, and how electrons shift in molecules (electronic effects). Everything in organic chemistry builds on these concepts.
Special Properties of Carbon
- Tetravalency: Carbon forms 4 bonds.
- Catenation: Carbon atoms bond to each other in long chains, branched chains, and rings.
- Bond types: Single (C−C), double (C=C), triple (C≡C).
Hybridisation of Carbon
| Hybridisation | Shape | Bond angle | Example |
|---|---|---|---|
| sp³ | Tetrahedral | 109.5° | Methane (CH₄), ethane |
| sp² | Trigonal planar | 120° | Ethene (CH₂=CH₂), benzene |
| sp | Linear | 180° | Ethyne (HC≡CH) |
Bond strength: Triple > Double > Single Bond length: Single > Double > Triple
Functional Groups
| Group | Name | Suffix |
|---|---|---|
| −OH | Hydroxyl | -ol |
| −CHO | Aldehyde | -al |
| −CO− | Ketone | -one |
| −COOH | Carboxylic acid | -oic acid |
| −NH₂ | Amine | -amine |
| −X (halogen) | Halo | halo- (prefix) |
| C=C | Alkene | -ene |
| C≡C | Alkyne | -yne |
IUPAC Nomenclature
Steps: 1. Identify the principal functional group (determines suffix). 2. Find the longest chain containing the functional group (parent chain). 3. Number the chain from the end nearest to the functional group (lowest locant rule). 4. Name substituents as prefixes with locant numbers. 5. List substituents alphabetically (ignore di/tri prefixes for alphabetical order).
Prefixes: meth- (1C), eth- (2C), prop- (3C), but- (4C), pent- (5C), hex- (6C), hept- (7C), oct- (8C), non- (9C), dec- (10C)
Isomerism
Structural isomers: Same molecular formula, different structural formula.
| Type | Description | Example |
|---|---|---|
| Chain isomers | Different carbon skeleton | n-butane vs isobutane |
| Position isomers | Same group, different position | 1-butanol vs 2-butanol |
| Functional group isomers | Different functional groups | ethanol vs dimethyl ether |
| Metamerism | Different alkyl groups on same heteroatom | — |
Stereoisomers: Same connectivity, different spatial arrangement.
- Geometric (cis-trans): around C=C double bond
- Optical: non-superimposable mirror images (chiral centre)
Electronic Effects
Inductive Effect (I)
Electron shift through σ bonds due to electronegativity.
- −I effect (electron-withdrawing): −F, −Cl, −Br, −I, −NO₂, −CN, −COOH
- +I effect (electron-donating): −CH₃, −C₂H₅ (alkyl groups)
- Effect decreases with distance from the group.
Resonance (Mesomeric Effect)
Delocalisation of π electrons or lone pairs through conjugated systems.
- +M effect: Groups that donate electrons into the ring (−NH₂, −OH, −OCH₃)
- −M effect: Groups that withdraw electrons from the ring (−NO₂, −CHO, −COOH)
Hyperconjugation
Delocalisation of σ-bonding electrons of C−H adjacent to a π bond or carbocation. Increases stability. More α-H atoms → greater hyperconjugation.
Bond Fission and Intermediates
| Type | Mechanism | Intermediate | Species |
|---|---|---|---|
| Homolytic | Equal electron split | Free radical | •CH₃ |
| Heterolytic | Unequal split | Carbocation (C⁺) or Carbanion (C⁻) | CH₃⁺, CH₃⁻ |
Stability of carbocations: 3° > 2° > 1° > methyl (due to hyperconjugation and inductive effect of alkyl groups)
Stability of free radicals: 3° > 2° > 1° > methyl (same reason)
Important Questions
One Mark
- What is the hybridisation of carbon in acetylene (ethyne)?
- Define catenation.
- Give the IUPAC name of CH₃−CH₂−OH.
- What are enantiomers?
Short Answer (3 marks)
- Explain sp², sp³ hybridisation in carbon with bond angles and examples.
- Give the IUPAC names of: (a) (CH₃)₂CHCH₂CH₃ (b) CH₃CH₂CHO (c) CH₃COCH₂CH₃.
- What is the inductive effect? Arrange −F, −Cl, −Br, −I in order of decreasing −I effect.
Long Answer (5 marks)
- Explain the different types of structural isomerism with one example each.
- Explain homolytic and heterolytic bond fission. Define free radicals, carbocations, and carbanions. Give the order of stability of carbocations with reason.
Common Mistakes
- IUPAC naming: not choosing the longest chain that contains the principal functional group.
- Forgetting to number from the end nearest to the functional group (not necessarily the nearest substituent).
- Confusing resonance (π electrons) with inductive effect (σ electrons).
- Stability of carbocations: thinking primary is more stable because it has fewer substituents — it is actually less stable.
Board Exam Tips
- IUPAC naming is a guaranteed 3-mark or 5-mark question — practise naming branched and multi-functional compounds.
- Hybridisation with bond angle and shape: learn all three types and their examples.
- "Explain the inductive effect with examples" is a standard 3-mark question.
- NEET: Electronic effects (inductive, resonance, hyperconjugation) determine reactivity — tested in almost every NEET paper.