Organic Chemistry Notes

5.1 INTRODUCTION TO ORGANIC CHEMISTRY

  • Organic chemistry focuses on compounds containing carbon.
  • Students should be able to classify organic molecules by functional groups, draw structural formulas (expanded, condensed, skeletal), explain covalent bond cleavage (homolytic fission forming free radicals, heterolytic fission forming ions), and describe electrophiles/nucleophiles.

Carbon's Unique Properties

  • Carbon can form single, double, and triple bonds, leading to chains of varying lengths called catenation.
  • It forms cyclic or ring structures.
  • Carbon can bind with O, S, N, P, halogens, and metals.

5.1.1 Homologous Series and Functional Groups

  • Organic molecules are classified based on functional groups and homologous series.
  • A functional group is an atom or atom group in a molecule responsible for its chemical behavior.
  • The chemical behavior of a functional group remains the same, regardless of the molecule's size or location of the functional group.

Characteristics of Homologous Series

  • Compounds with the same functional group belong to the same homologous series.
    • They can be represented by the same general formula.
    • They have the same functional groups and chemical properties.
    • They can be prepared using similar methods.
    • Successive members differ by a -CH2 group.
    • Physical properties (boiling point, density) progressively change with increasing molecular mass.

Examples of Functional Groups, Homologous Series, and Examples

  • None: Alkane (e.g., Propane)
  • Carbon-carbon double bond: Alkene (e.g., Propene)
  • Halogen: Alkyl halide (e.g., Bromopropane)
  • Hydroxyl: Alcohol (e.g., Propanol)
  • Carbonyl: Aldehyde (e.g., Propanal)
  • Carbonyl: Ketone (e.g., Propanone)
  • Carboxyl: Carboxylic acid (e.g., Propanoic acid)
  • Carboalkoxy: Ester (e.g., Methyl ethanoate)

5.1.2 Representation of Organic Molecules

  • Four ways to write structural formulas:
    1. Expanded structural formula: Shows each atom and covalent bond (single, double, triple).
    2. Condensed structural formula: Represents atom arrangement linearly, omitting bonds.-
    • For repeating -CH₂- units, enclose in parentheses, e.g., (CH₂)n
    • Lone pairs on O, N, or halogens are omitted.
    1. Skeletal structural formula: Shorthand, with carbon and hydrogen bonds as lines.
    2. 3D structural formula: Depicts 3D spatial arrangement using wedge-dash diagrams.
      • Wedged line: Bond lies on the planar of the paper.
      • Dashed line: Bond projects in front of the plane of the paper towards the viewer.
      • Solid line: Bond behind the plane of the paper, away from the viewer.

5.1.3 Organic Chemistry Reactions: Fission and Mechanistic Arrows

  • Homolytic fission: Occurs in nonpolar covalent bonds. Even breaking where each atom gets one electron, forming free radicals.
    • Represented by half-headed arrow (fishhook arrow) showing single electron movement.
    • Example: Cl−Cl→2ClCl-Cl \rightarrow 2Cl (under UV light)
  • Heterolytic fission: Occurs in polar covalent bonds. Uneven breaking where one atom takes both electrons, forming an anion, and the other becomes a cation.
    • Represented by full-headed arrow showing pair of electrons movement.

Nucleophiles, Electrophiles, and Free Radicals

  • Nucleophile (Nu-): Electron-rich species (neutral or negatively charged) with available electron pairs (Lewis bases, electron pair donors).
    • Examples: Hydroxide ion, Bromide ion, Iodide ion, Water, Ammonia, Amine, C=C double or triple bond (Ethylene, Ethyne), Carbanion.
  • Electrophile (E+): Electron-poor species (neutral or positively charged) with vacant orbitals (Lewis acids, electron pair acceptors).
    • Examples: Hydrogen cation, Hydronium ion, Boron trifluoride, Aluminum trichloride, Carbocation (Propyl carbocation), Double-bonded electronegative atoms, Single-bonded electronegative atoms.
  • Free radicals: Species containing one or more unpaired electrons in their outermost shell, represented by a single dot.

Types of Organic Reactions

  • Addition Reactions: Two reactants add to form one product (A+B→ABA + B \rightarrow AB). Includes electrophilic and nucleophilic addition.
  • Elimination Reactions: One reactant splits into two products (AB→A+BAB \rightarrow A + B), often with a small molecule formed (e.g., H₂O, HBr).
  • Substitution Reactions: Two molecules exchange an atom or group of atoms (AB+C→AC+BAB + C \rightarrow AC + B). Can be nucleophilic, electrophilic, or free radical.
  • Rearrangement Reactions: A molecule changes how its atoms are connected (A−B−A→B−A−AA-B-A \rightarrow B-A-A).

5.2 ISOMERISM

  • Isomerism occurs when compounds have the same molecular formula but different structural formulas.

Types of Isomerism

  • Constitutional Isomerism: Same molecular formula, different structural formulas (different connectivity of atoms).
    • Chain Isomerism
    • Positional Isomerism
    • Functional Group Isomerism
  • Stereoisomerism: Same molecular formula and connectivity, different arrangements of atoms in space.
    • Geometrical Isomerism (Cis-Trans Isomerism)
    • Optical Isomerism (Enantiomerism)

5.2.1 Constitutional Isomerism

  • Compounds with identical molecular formulas but distinct structural formulas due to different bonds or connectivity.
Types of Constitutional Isomers
  • Chain/Branch Isomers: Variations in carbon chain arrangement, resulting in different chain lengths. Different physical properties but similar chemical properties.
  • Positional Isomers: Same carbon chain length, but differ in functional group location. Different physical properties but similar chemical properties.
  • Functional Group Isomers: Different functional groups due to variations in functional group arrangement. Belong to different homologous series. Different physical and chemical properties. Compounds from different homologous series with the same general formula can exhibit functional group isomerism.
Functional Group/Homologous Series Isomers
  • Isomers belonging to different homologous series but sharing the same functional group.
    • Example: C3H6O (Propanal and Propanone), both have a carbonyl group, but one is an aldehyde and the other is a ketone.

5.2.2 Stereoisomerism

  • Molecules with the same molecular and structural formulas but different orientations of atoms in space.
    • Geometrical Isomerism (cis-trans isomers)
      • Different chemical, physical, biological, and pharmacological effects.
    • Enantiomerism (optical isomerism)
      • Nearly identical chemical and physical properties but can have significantly different biological and pharmacological effects.
Geometrical Isomers
  • Molecule contains a carbon-carbon double bond (C=C) or a cyclic structure that prevents carbon atom rotation.
  • Rotation is not possible around a carbon-carbon double bond or within a cyclic structure.
Disubstituted Double Bond
  • Two substituent groups (atoms other than hydrogen) attached to carbon atoms in the double bond.
    • Cis: Similar groups on the same side of the double bond.
    • Trans: Similar groups on opposite sides of the double bond.
Trisubstituted or Tetrasubstituted Double Bond
  • Three/four specific groups or atoms (other than hydrogen) bonded to carbon atoms in the double bond.
    • Cis: Higher molecular weight groups on the same side.
    • Trans: Higher molecular weight groups on the opposite side.
Cyclic Structure
  • Use wedge-dash projections to represent geometrical isomerism.
    • Cis: Substituents on the same side/plane.
    • Trans: Substituents on opposite sides/plane.
  • Cis-trans stereoisomerism is not possible if:
    • One of the doubly bonded carbons bears two identical substituents.
    • Two identical substituents bonded to one carbon in the ring structure.
Enantiomerism
  • Molecules share the same molecular formula and structural connectivity but differ in 3D spatial arrangement around chiral centers.
  • Chiral centers are sp3 carbon atoms with four different substituent groups, making them asymmetric.
  • Chiral carbon atoms are referred to as