Learning Objectives
- Understand the classification and nomenclature (IUPAC) of organic compounds
- Learn about structural and stereoisomerism
- Understand electronic effects: inductive, resonance, hyperconjugation
- Study types of organic reactions and reaction intermediates
- Learn purification and characterisation techniques
Key Concepts
Classification of Organic Compounds
Based on carbon skeleton: Acyclic (open chain), Cyclic (alicyclic, aromatic, heterocyclic).
Based on functional groups: alkanes (-C-C-), alkenes (C=C), alkynes (C≡C), alcohols (-OH), aldehydes (-CHO), ketones (-CO-), carboxylic acids (-COOH), amines (-NH₂), etc.
Homologous series: Series of compounds with same functional group and general formula, differing by -CH₂-.
IUPAC Nomenclature
Steps:
- Find the longest carbon chain containing the functional group (parent chain).
- Number from the end closer to the principal functional group.
- Name substituents as prefixes (with position numbers).
- Functional group as suffix.
Priority order of functional groups: -COOH > -SO₃H > -COOR > -COCl > -CONH₂ > -CHO > >C=O > -OH > -NH₂ > C≡C > C=C
Name format: Prefix + Root word (chain length) + Primary suffix (saturation) + Secondary suffix (functional group)
Isomerism
Structural isomerism: Chain, position, functional group, metamerism, tautomerism.
Stereoisomerism:
- Geometrical (cis-trans): Different spatial arrangement around C=C or ring. Cis = same side, Trans = opposite side.
- Optical isomerism: Non-superimposable mirror images (enantiomers). Requires a chiral centre (asymmetric carbon with 4 different groups).
Electronic Effects
Inductive effect (I effect): Permanent shifting of sigma electrons along a chain due to electronegativity difference. -I (electron-withdrawing): -NO₂, -CN, -COOH, -F, -Cl, -Br, -OH, -OR. +I (electron-donating): alkyl groups (-CH₃, -C₂H₅, etc.).
Resonance (Mesomeric) effect: Delocalisation of pi electrons or lone pairs. +M (electron-donating): -OH, -NH₂, -OR, -X. -M (electron-withdrawing): -NO₂, -CN, -CHO, -COOH, >C=O.
Hyperconjugation (No-bond resonance): Delocalisation of C-H σ electrons with adjacent π system or empty p-orbital. More α-H → more hyperconjugation → more stability.
Electromeric effect: Temporary shift of π electrons in the presence of an attacking reagent.
Reaction Intermediates
Carbocation (R⁺): sp² hybridised, planar, electron-deficient. Stability: 3° > 2° > 1° > CH₃⁺ (hyperconjugation and inductive effect).
Carbanion (R⁻): sp³ hybridised, pyramidal. Stability: CH₃⁻ > 1° > 2° > 3° (opposite of carbocation).
Free radical (R·): sp² hybridised, planar. Stability: 3° > 2° > 1° > CH₃·.
Types of Organic Reactions
- Substitution: Nucleophilic (Sₙ1, Sₙ2) and Electrophilic.
- Addition: Electrophilic and Nucleophilic (to C=C, C=O).
- Elimination: E1 and E2 (form double bonds).
- Rearrangement: Change in carbon skeleton.
Homolytic fission: Each atom gets one electron (free radicals). Heterolytic fission: Both electrons go to one atom (ions).
Summary
Organic chemistry is based on the unique bonding ability of carbon (catenation, tetravalence). IUPAC nomenclature provides systematic naming. Isomerism creates diversity in organic compounds. Electronic effects (inductive, resonance, hyperconjugation) determine reactivity and stability. Reaction intermediates (carbocations, carbanions, free radicals) are key to understanding mechanisms.
Important Terms
- Functional Group: Atom/group determining chemical properties
- Homologous Series: Series differing by -CH₂- with similar properties
- Inductive Effect: Electron displacement through sigma bonds
- Resonance: Delocalisation of pi electrons or lone pairs
- Carbocation: Positively charged carbon intermediate
- Electrophile: Electron-pair acceptor, attacks electron-rich sites
- Nucleophile: Electron-pair donor, attacks electron-deficient sites
Quick Revision
- IUPAC: longest chain + substituents + functional group suffix
- Carbocation stability: 3° > 2° > 1° (hyperconjugation + inductive)
- +I effect: alkyl groups; -I effect: halogens, -NO₂, -CN
- +M effect: -OH, -NH₂; -M effect: -NO₂, -CHO, -COOH
- Homolytic → free radicals; Heterolytic → carbocation + carbanion
- Reactions: substitution, addition, elimination, rearrangement