Introduction to Organic Chemistry
Organic chemistry is the study of carbon compounds. Carbon is unique in its ability to form chains (catenation) and complex structures.
Reasons for Large Number of Organic Compounds
- Catenation: C-C bonds are strong and stable.
- Carbon can form single, double, and triple bonds.
- Isomerism is very common.
- Carbon bonds with many elements (H, O, N, S, halogens).
Tetravalence of Carbon
Carbon has 4 valence electrons (2s^2 2p^2). It forms 4 covalent bonds through sp^3, sp^2, or sp hybridisation.
Hybridisation in Carbon
| Hybridisation | Bond Type | Geometry | Bond Angle | Example |
|---|---|---|---|---|
| sp^3 | Single | Tetrahedral | 109.5 | CH4 |
| sp^2 | Double | Trigonal planar | 120 | C2H4 |
| sp | Triple | Linear | 180 | C2H2 |
Sigma (sigma) and Pi (pi) Bonds
- Sigma bond: End-on overlap, stronger, free rotation possible.
- Pi bond: Side-on overlap, weaker, restricts rotation.
- Single bond: 1 sigma. Double bond: 1 sigma + 1 pi. Triple bond: 1 sigma + 2 pi.
Functional Groups
A functional group is an atom or group of atoms that determines the characteristic properties of an organic compound.
| Class | Functional Group | Suffix/Prefix |
|---|---|---|
| Alkane | C-C | -ane |
| Alkene | C=C | -ene |
| Alkyne | C(triple)C | -yne |
| Alcohol | -OH | -ol |
| Aldehyde | -CHO | -al |
| Ketone | -CO- | -one |
| Carboxylic acid | -COOH | -oic acid |
| Ester | -COOR | -oate |
| Amine | -NH2 | -amine |
| Haloalkane | -X (Cl, Br, I) | halo- |
IUPAC Nomenclature
Rules for Naming
- Identify the longest continuous carbon chain (parent chain).
- Number the chain from the end nearest the principal functional group.
- Identify substituents and their positions.
- Write name: substituents (alphabetical order) + parent chain + suffix.
- Use prefixes: di-, tri-, tetra- for multiple identical substituents.
Examples
CH3-CH2-CH2-CH3: Butane.CH3-CH(CH3)-CH3: 2-Methylpropane.CH3-CH2-OH: Ethanol.CH3-CH2-CHO: Propanal.CH3-CO-CH3: Propanone.CH3-CH2-COOH: Propanoic acid.CH3-CH=CH2: Propene.HC(triple)CH: Ethyne.
Common vs IUPAC Names
- CH3OH: Methyl alcohol (common), Methanol (IUPAC).
- CH3COOH: Acetic acid (common), Ethanoic acid (IUPAC).
Isomerism
Compounds with same molecular formula but different structures or arrangements.
Structural Isomerism
Chain isomerism: Different carbon skeleton. C4H10: Butane and 2-Methylpropane.
Position isomerism: Different position of functional group. C3H7OH: Propan-1-ol and Propan-2-ol.
Functional group isomerism: Different functional groups. C2H6O: Ethanol and Methoxymethane (ether).
Metamerism: Different alkyl groups attached to same functional group. C4H10O: Diethyl ether and methyl propyl ether.
Tautomerism: Dynamic equilibrium between two forms. Keto-enol tautomerism.
Stereoisomerism
Same structure but different spatial arrangement (will be covered in detail in Class 12).
Electronic Effects in Organic Compounds
Inductive Effect (I)
Permanent displacement of electrons along a sigma bond due to electronegativity difference.
-I groups: Electron withdrawing (down the bond).
Examples: -NO2 > -CN > -F > -Cl > -Br > -I > -OH > -OR
+I groups: Electron releasing (up the bond).
Examples: -C(CH3)3 > -CH(CH3)2 > -CH2CH3 > -CH3 > -H
Resonance Effect (Mesomeric Effect, M)
Delocalisation of pi electrons through a conjugated system.
+M groups: Electron donating through resonance.
Examples: -NH2, -OH, -OR, -X (halogens have -I but +M).
-M groups: Electron withdrawing through resonance.
Examples: -NO2, -CN, -CHO, -COOH, -COOR.
Hyperconjugation (Baker-Nathan Effect)
Delocalisation of sigma electrons (C-H bonds) with adjacent pi system or empty p-orbital.
- Also called 'no-bond resonance'.
- Stability increases with more alpha-hydrogens.
- Explains stability of alkenes: more substituted alkene is more stable.
Methods of Purification
Common Techniques
- Filtration: Separating solids from liquids.
- Crystallisation: Purifying solids based on solubility.
- Distillation: Separating liquids based on boiling point differences.
- Sublimation: For solids that vapourise directly.
- Chromatography: Adsorption-based separation (paper, column, TLC).
Qualitative Analysis (Detection of Elements)
- Lassaigne's test: Na fusion extract tests for N, S, halogens.
- N: Prussian blue colour with FeSO4 + FeCl3.
- S: Blood red colour with sodium nitroprusside.
- Halogens: AgNO3 gives AgX precipitate.
Worked Examples
Example 1: Write IUPAC name of CH3-CH(CH3)-CH2-COOH.
Solution: Parent chain: 4 carbons (butanoic acid). Substituent: methyl at C-3. Name: 3-Methylbutanoic acid.
Example 2: Explain why acetic acid (CH3COOH) is more acidic than ethanol (CH3CH2OH). Solution: In acetic acid, the -COOH group has resonance stabilisation of the conjugate base (carboxylate ion). In ethanol, the conjugate base (ethoxide ion) has no such stabilisation.
Common Mistakes
- Functional group priority: When naming, identify the principal functional group with highest priority.
- Resonance vs tautomerism: Resonance is not a dynamic equilibrium; tautomerism is.
- Hyperconjugation requires alpha H: If there is no alpha hydrogen, hyperconjugation is not possible.
- Inductive effect is distance-dependent: Effect diminishes rapidly with distance.
ISC Exam Focus
- Theory (70%): IUPAC nomenclature, hybridisation, isomerism types, electronic effects.
- Application (30%): Naming compounds, identifying isomer types, applying inductive/resonance effects.
- ISC frequently asks: "Write IUPAC name of ..." and "Explain the acidic nature of ... using resonance/inductive effect."
- Nomenclature questions are essential for exams.
Self-Test Questions
Q1: Write the IUPAC name of CH3-CH(CH3)-CH=CH2.
Answer: Parent: 4 carbons with double bond (but-1-ene). Substituent: methyl at C-3. Name: 3-Methylbut-1-ene.
Q2: What are structural isomers? Give an example. Answer: Same molecular formula, different structure. Example: C4H10 (butane and 2-methylpropane).
Q3: Explain the inductive effect of -NO2 group. Answer: -NO2 is a strong -I group (electron withdrawing). It withdraws electron density through sigma bonds.
Q4: Differentiate between sp, sp^2, and sp^3 hybridisation. Answer: sp: 50% s-character, linear, 180. sp^2: 33% s, trigonal planar, 120. sp^3: 25% s, tetrahedral, 109.5.
Q5: Define hyperconjugation with an example. Answer: Delocalisation of sigma electrons of C-H with pi system. Example: In propene, the CH3 group shows hyperconjugation.
Q6: Write the IUPAC name of CH3-CH2-CO-CH2-CH3.
Answer: Pentan-3-one.
