General Organic Chemistry (GOC) – Comprehensive Notes

General Organic Chemistry (GOC) forms the backbone of organic chemistry. A strong grasp of these fundamental concepts is crucial for understanding reaction mechanisms and predicting products, which are vital for NEET and JEE Main.

1. Introduction to Organic Chemistry

• Definition: The study of hydrocarbons and their derivatives. Carbon’s unique ability to catenate (form chains/rings) and form multiple bonds (C−C,C=C,C≡C) leads to millions of organic compounds.
• Bonding in Organic Compounds: Primarily covalent bonds.
• Bond Fission:
  • Homolytic Fission: Symmetrical cleavage of a covalent bond, where each atom gets one electron. Forms free radicals (species with unpaired electrons).
    • Example: A−B→A⋅+B⋅
    • Favored by: High temperature, UV light, non-polar solvents, peroxides.
  • Heterolytic Fission: Unsymmetrical cleavage of a covalent bond, where one atom gets both bonding electrons.
    • Forms carbocations (carbon with positive charge, electron deficient) or carbanions (carbon with negative charge, electron rich).
    • Example: A−B→A++:B− (or A−+B+)
    • Favored by: Polar solvents.
• Reagents:
  • Electrophiles (Electron Loving): Electron-deficient species that seek electrons. Lewis acids.
    • Examples: H+,NO2+​,R+,AlCl3​,BF3​,SO3​.
  • Nucleophiles (Nucleus Loving): Electron-rich species that seek positive centers. Lewis bases.
    • Examples: OH−,CN−,R3​N:,H2​O:,R−O−R:,R−Mg−X.

2. Electronic Displacement Effects

These effects describe how electrons are distributed or moved within a molecule, influencing its reactivity and stability.

2.1. Inductive Effect (I-Effect)

• Definition: Permanent displacement of σ-electrons along a carbon chain towards a more electronegative atom or group (or away from an electron-donating group). It is a permanent effect.
• Nature: Operates through sigma bonds only.
• Distance Dependent: Decreases rapidly with distance, usually negligible after 3-4 carbon atoms.
• Types:
  • -I Effect (Electron Withdrawing Inductive Effect): Groups that pull electron density towards themselves.
    • Order: −NR3+​>−NO2​>−CN>−SO3​H>−CHO>−COOH>−F>−Cl>−Br>−I>−OH>−OR>−NH2​>−C6​H5​>−H
  • +I Effect (Electron Donating Inductive Effect): Groups that push electron density away from themselves.
    • Order: −(CH3​)3​C>−(CH3​)2​CH>−CH3​CH2​>−CH3​>−D>−T>−H (Alkyl groups show +I effect).
• Applications:
  • Stability of Carbocations: +I groups stabilize (e.g., 3∘>2∘>1∘>CH3+​).
  • Stability of Carbanions: -I groups stabilize (e.g., CH3−​>1∘>2∘>3∘).
  • Stability of Free Radicals: Similar to carbocations (3∘>2∘>1∘>CH3⋅​).
  • Acidity of Carboxylic Acids: -I groups increase acidity, +I groups decrease acidity. (e.g., F3​CCOOH>Cl3​CCOOH>CH3​COOH).
  • Basicity of Amines: +I groups increase basicity, -I groups decrease basicity.

2.2. Resonance Effect (Mesomeric Effect, M-Effect)

• Definition: Delocalization of π-electrons or lone pair electrons within a conjugated system. It is a permanent effect.
• Conditions: Presence of conjugation (alternating single and multiple bonds, or a lone pair/empty orbital adjacent to a multiple bond).
• Resonance Structures (Canonical Forms): Hypothetical structures that contribute to the overall hybrid structure. The true structure is a resonance hybrid.
• Resonance Hybrid: The actual structure, which is more stable than any single canonical form.
• Resonance Energy: The difference in energy between the resonance hybrid and the most stable canonical form. Higher resonance energy means greater stability.
• Rules for Drawing Resonance Structures:
  • All structures must be valid Lewis structures.
  • Nuclei positions remain fixed.
  • Only electrons move.
  • Same number of paired and unpaired electrons.
  • Minimize charge separation.
  • More covalent bonds = more stable.
  • Negative charge on more electronegative atom = more stable. Positive charge on less electronegative atom = more stable.
• Types:
  • +M Effect (Electron Donating Resonance Effect): Groups that donate electrons to the conjugated system.
    • Groups: −OH,−OR,−NH2​,−NHR,−NR2​,−O−,−Cl,−Br,−I (atoms with lone pairs).
  • -M Effect (Electron Withdrawing Resonance Effect): Groups that withdraw electrons from the conjugated system.
    • Groups: −NO2​,−CN,−CHO,−COOH,−COOR,−SO3​H,−COR.
• Applications:
  • Stability of Carbocations/Carbanions: Resonance dramatically stabilizes both.
  • Acidity/Basicity: Major impact. Phenols are acidic due to resonance stabilization of phenoxide ion. Anilines are less basic than aliphatic amines due to resonance delocalization of nitrogen’s lone pair.
  • Electrophilic/Nucleophilic Substitution in Benzene: +M groups are ortho-para directing and activating; -M groups are meta-directing and deactivating. Halogens are ortho-para directing but deactivating (due to strong -I overriding +M).

2.3. Hyperconjugation (“No-Bond Resonance” / σ-p Overlap)

• Definition: Delocalization of σ-electrons of a C-H bond (alpha to a pi system or a carbocation/free radical center) into an adjacent empty p-orbital or a p-orbital containing electrons. It is a permanent effect.
• Condition: Presence of α-hydrogens (hydrogens on carbon adjacent to the unsaturated system or charged carbon).
• Effect: More α-hydrogens = more hyperconjugative structures = greater stability.
• Applications:
  • Stability of Alkenes: More substituted alkenes are more stable (Saytzeff’s Rule).
  • Stability of Carbocations: 3∘>2∘>1∘>CH3+​ (stabilized by hyperconjugation).
  • Stability of Free Radicals: 3∘>2∘>1∘>CH3⋅​ (stabilized by hyperconjugation).
  • Directing Effect of Alkyl Groups: Alkyl groups are ortho-para directing in electrophilic aromatic substitution due to hyperconjugation.

2.4. Electromeric Effect (E-Effect)

• Definition: Temporary and complete transfer of shared π-electron pair to one of the bonded atoms in the presence of an attacking reagent. It is a temporary effect.
• Nature: Occurs in unsaturated compounds (double or triple bonds).
• Types:
  • +E Effect: The electron pair shifts towards the attacking reagent. (e.g., in alkenes during electrophilic addition, electrons shift towards the carbocation forming carbon)
  • -E Effect: The electron pair shifts away from the attacking reagent. (e.g., in carbonyl compounds during nucleophilic addition, electrons shift towards oxygen).

3. Isomerism

Compounds having the same molecular formula but different structural or spatial arrangements of atoms.

3.1. Structural Isomerism (Constitutional Isomerism)

Different connectivity of atoms.

• Chain Isomerism: Difference in the arrangement of the carbon skeleton (straight vs. branched chain).
  • Example: n-Butane and Isobutane.
• Position Isomerism: Difference in the position of functional group, substituent, or multiple bond on the same carbon skeleton.
  • Example: Butan-1-ol and Butan-2-ol.
• Functional Group Isomerism: Different functional groups.
  • Example: Ethanol (CH3​CH2​OH) and Dimethyl ether (CH3​OCH3​).
• Metamerism: Different alkyl groups attached to the same polyvalent functional group.
  • Example: Diethyl ether (CH3​CH2​OCH2​CH3​) and Methyl propyl ether (CH3​OCH2​CH2​CH3​).
• Tautomerism: A special type of functional group isomerism where isomers exist in dynamic equilibrium, interconverting by the migration of a proton and a double bond. (Most common is keto-enol tautomerism).
  • Conditions: Presence of an α-hydrogen and a highly electronegative atom (like O or N) connected by a multiple bond.
  • Example: Acetaldehyde ⇌ Vinyl alcohol.

3.2. Stereoisomerism

Same connectivity of atoms, but different spatial arrangement.

• Conformational Isomerism (Conformers/Rotamers): Isomers that can be interconverted by rotation around single bonds.
  • Representation: Newman projections (e.g., ethane, butane), Sawhorse projections.
  • Stability: Staggered > Gauche > Eclipsed.
• Configurational Isomerism: Isomers that cannot be interconverted without breaking and reforming bonds.
  • Geometric Isomerism (Cis-Trans / E-Z): Arises due to restricted rotation around a double bond or in cyclic compounds.
    • Conditions: Each carbon of the double bond (or ring) must be attached to two different groups.
    • Cis/Trans: For simpler cases with two identical groups.
    • E/Z (Entgegen/Zusammen): For more complex cases using Cahn-Ingold-Prelog (CIP) priority rules.
  • Optical Isomerism (Enantiomerism, Diastereomerism): Arises due to the presence of chiral centers (asymmetric carbon atoms).
    • Chiral Carbon: A carbon atom bonded to four different groups.
    • Chiral Molecule: A molecule that is non-superimposable on its mirror image (lacks plane of symmetry, center of symmetry).
    • Enantiomers: Stereoisomers that are non-superimposable mirror images of each other.
      • Properties: Identical physical and chemical properties (except rotation of plane-polarized light and reaction with other chiral molecules).
      • Optical Activity: Rotate plane-polarized light in equal but opposite directions (dextro-rotatory (+) or levo-rotatory (-)).
    • Diastereomers: Stereoisomers that are not mirror images of each other.
      • Properties: Different physical and chemical properties.
    • Meso Compounds: Molecules with chiral centers but are optically inactive due to the presence of a plane of symmetry (internal compensation).
    • Racemic Mixture: An equimolar mixture of a pair of enantiomers. Optically inactive due to external compensation.
    • Resolution: Process of separating enantiomers from a racemic mixture.
    • R/S Configuration (Absolute Configuration – JEE specific): Rules for assigning configuration to chiral centers using CIP priority rules.

4. Nomenclature (IUPAC System)

Systematic naming of organic compounds.

• Steps for Naming:
  1. Longest Carbon Chain (Parent Chain): Select the longest continuous carbon chain containing the principal functional group and multiple bonds (if any).
  2. Numbering: Number the chain from the end that gives the lowest locant to the principal functional group. If no functional group, then to multiple bonds. If both, apply lowest set of locants. If still tie, then to substituents.
  3. Prioritization (Functional Groups): Carboxylic acid > Sulfonic acid > Ester > Acid halide > Amide > Nitrile > Aldehyde > Ketone > Alcohol > Amine > Alkene > Alkyne > Alkane > Ether > Halogen > Nitro > Alkyl.
  4. Substituents: Name alkyl groups and other substituents (e.g., halo, nitro) alphabetically. Use prefixes (di-, tri-, tetra-) for multiple identical substituents.
  5. Alphabetical Order: When naming different substituents, arrange them alphabetically.
• Common Prefixes/Suffixes:
  • Prefixes: For substituents (e.g., chloro-, bromo-, methyl-, ethyl-).
  • Suffixes: For functional groups (e.g., -ol for alcohol, -al for aldehyde, -one for ketone, -oic acid for carboxylic acid, -ene for double bond, -yne for triple bond).

5. Reaction Intermediates

Transient, highly reactive species formed during organic reactions. Their stability dictates reaction pathways.

• Carbocations (Carbonium Ions): Carbon with a positive charge.
  • Hybridization: sp2 hybridized, trigonal planar geometry.
  • Stability Order: Allyl/Benzyl > 3∘>2∘>1∘>CH3+​ (Stabilized by +I, +M, hyperconjugation).
  • Rearrangements: Can undergo 1,2-hydride or 1,2-alkyl shifts to form more stable carbocations (especially in JEE).
• Carbanions: Carbon with a negative charge.
  • Hybridization: sp3 hybridized, pyramidal geometry (or sp2 if stabilized by resonance).
  • Stability Order: CH3−​>1∘>2∘>3∘ (Destabilized by +I, stabilized by -I, -M).
  • Example: CH2​=CH−CH2​ (allyl carbanion) is stabilized by resonance.
• Free Radicals: Carbon with an unpaired electron.
  • Hybridization: sp2 hybridized, planar (or pyramidal).
  • Stability Order: Allyl/Benzyl > 3∘>2∘>1∘>CH3⋅​ (Stabilized by +I, hyperconjugation, resonance).
• Carbenes: Neutral species with a divalent carbon containing two non-bonding electrons.
  • Singlet Carbene (sp2, paired electrons, diamagnetic) and Triplet Carbene (sp, unpaired electrons, paramagnetic).
• Nitrenes: Neutral species with a monovalent nitrogen containing two non-bonding electrons and one bonding electron.

6. Acidity and Basicity in Organic Compounds

Understanding the factors that influence acid and base strength is crucial.

• Acids (Proton Donors):
  • Key Principle: The stability of the conjugate base formed after proton donation. More stable conjugate base = stronger acid.
  • Factors Affecting Acidity:
    • Electronegativity: Acidity increases across a period (e.g., CH4​<NH3​<H2​O<HF).
    • Size of Atom (down a group): Acidity increases down a group (e.g., HF<HCl<HBr<HI – due to increased bond length and weaker bond).
    • Inductive Effect (-I): Electron-withdrawing groups (-I) stabilize the conjugate base, increasing acidity.
    • Resonance Effect (-M): Electron-withdrawing resonance groups (-M) delocalize negative charge on conjugate base, significantly increasing acidity (e.g., carboxylic acids, phenols).
    • Hybridization: Acidity order: sp>sp2>sp3 (e.g., terminal alkynes are weakly acidic). More s-character = more electronegative = better at stabilizing negative charge.
• Bases (Proton Acceptors / Electron Pair Donors):
  • Key Principle: Availability of the lone pair of electrons for donation (for Lewis bases) or the stability of the conjugate acid formed after proton acceptance.
  • Factors Affecting Basicity:
    • Inductive Effect (+I): Electron-donating groups (+I) increase electron density on the basic atom, increasing basicity (e.g., 3∘>2∘>1∘ for aliphatic amines in gas phase).
    • Resonance Effect (-M): If the lone pair is involved in resonance, its availability for donation decreases, reducing basicity (e.g., Aniline is less basic than aliphatic amines).
    • Steric Hindrance: Bulky groups can hinder the approach of a proton, decreasing basicity (e.g., in solution, solvation effects play a major role for amines, leading to order like 2∘>1∘>3∘).
    • Hybridization: Basicity order: sp3>sp2>sp (e.g., amines are more basic than imines, which are more basic than nitriles).

7. Important Concepts & Miscellaneous

• Aromaticity (Hückel’s Rule): Cyclic, planar, completely conjugated system with (4n+2)π electrons. Aromatic compounds are highly stable.
• Anti-aromaticity: Cyclic, planar, completely conjugated system with 4nπ electrons. Highly unstable.
• Non-aromaticity: Not cyclic, not planar, or not completely conjugated.
• Steric Hindrance: Repulsion between bulky groups, affecting reactivity and stability.
• Hydrogen Bonding: Can influence physical properties (boiling point, solubility) and sometimes reactivity (e.g., intramolecular H-bonding).
• Basicity of Amines in Solution: Solvation effects play a crucial role. Primary and secondary amines are better solvated than tertiary amines. The overall order can vary:
  • In aqueous solution: 2∘>1∘>3∘>NH3​ (for methyl amines).
  • In gas phase: 3∘>2∘>1∘>NH3​.