Unit 5: Fundamentals of Organic Chemistry

Table of Contents

Electronic Displacements

These are effects that describe how electrons are distributed in a molecule, which in turn explains its reactivity, shape, and stability.

Permanent Effects (always present in the molecule)

  1. Inductive Effect (I):
    • What: The permanent partial displacement of σ (sigma) electrons along a carbon chain due to a difference in electronegativity.
    • Types:
      • -I Effect: Electron-withdrawing groups (e.g., -NO2, -CN, -Cl, -OH).
      • +I Effect: Electron-donating groups (e.g., alkyl groups like -CH3, -C2H5).
    • Key: It is a weak effect that fades quickly with distance (negligible after 3 bonds).
  2. Resonance (or Mesomeric) Effect (R or M):
    • What: The permanent delocalization of π (pi) electrons or lone pairs through a conjugated system.
    • Types:
      • -R Effect: Electron-withdrawing (e.g., -NO2, -CHO, -CN on benzene).
      • +R Effect: Electron-donating (e.g., -OH, -NH2, -Cl on benzene).
    • Key: It is a strong effect and is transmitted through the entire π-system.
  3. Hyperconjugation:
    • What: The delocalization of σ electrons from an adjacent C-H bond into an empty p-orbital or π-system. It is a "no-bond resonance."
    • Application: Used to explain the stability of carbocations and alkenes. (More alkyl groups → more hyperconjugation → more stability).

Temporary Effect (only when a reagent approaches)

  1. Electromeric Effect (E):
    • What: The complete transfer of a shared π electron pair to one of the atoms in a multiple bond, at the demand of an attacking reagent.
    • Example: When H+ attacks C=C, the π bond breaks and the pair moves to one carbon, which then bonds to the H+.

Reaction Basics: Bond Cleavage and Reagents

Cleavage of Bonds: Homolysis and Heterolysis

  1. Homolytic Cleavage (Homolysis):
    • What: The symmetrical breaking of a covalent bond, where each atom gets one electron from the shared pair.
    • Products: Forms free radicals.
    • Condition: Typically requires energy (Heat, Light) or occurs in non-polar solvents. (e.g., Cl2 \xrightarrow{UV} Cl· + Cl·).
  2. Heterolytic Cleavage (Heterolysis):
    • What: The unsymmetrical breaking of a covalent bond, where one atom gets both electrons from the shared pair.
    • Products: Forms ions (a carbocation and a carbanion).
    • Condition: Typically occurs in polar bonds and is aided by polar solvents. (e.g., (CH3)3C-Br → (CH3)3C+ + Br-).

Nucleophiles and Electrophiles

  • Nucleophile (Nu:- or Nu:):
    • "Nucleus-loving." An electron-rich species.
    • It donates an electron pair to form a new bond.
    • They are Lewis bases.
    • Examples: OH-, CN-, Br-, H2O:, :NH3.
  • Electrophile (E+):
    • "Electron-loving." An electron-deficient species.
    • It accepts an electron pair to form a new bond.
    • They are Lewis acids.
    • Examples: H+, Br+, NO2+, (CH3)3C+, AlCl3, BF3.

Reactive Intermediates

These are short-lived, high-energy species formed during a reaction.

Carbocations

  • Structure: A carbon atom with a positive charge and 6 valence electrons.
  • Shape: sp2 hybridized, trigonal planar geometry, with an empty p-orbital.
  • Stability: Stabilized by +I effect and hyperconjugation.
    Stability Order: 3° > 2° > 1° > Methyl
    (e.g., (CH3)3C+ is most stable).

Carbanions

  • Structure: A carbon atom with a negative charge and 8 valence electrons (including a lone pair).
  • Shape: sp3 hybridized, trigonal pyramidal geometry (like NH3).
  • Stability: Destabilized by +I effect (electron-donating groups) and stabilized by -I or -R effects.
    Stability Order: Methyl > 1° > 2° > 3°
    (e.g., CH3- is most stable).

Free Radicals

  • Structure: A carbon atom with an unpaired electron and 7 valence electrons.
  • Shape: sp2 hybridized, trigonal planar (or shallow pyramid).
  • Stability: Stabilized by hyperconjugation, similar to carbocations.
    Stability Order: 3° > 2° > 1° > Methyl

Strength of Organic Acids and Bases

Strength of Organic Acids

  • Definition: An acid's strength is its ability to donate a proton (H+). We measure it by Ka or pKa.
    pKa = -log(Ka)
    Stronger acid → Large Ka → Small pKa.
  • Key: Acid strength depends on the stability of the conjugate base (A-). A more stable conjugate base means a stronger acid.
  • Factors Affecting pK values:
    1. Inductive Effect: Electron-withdrawing groups (-I, e.g., -Cl) stabilize the A- and increase acidity (lower pKa). (e.g., ClCH2COOH > CH3COOH).
    2. Resonance Effect: Delocalization of the negative charge in the A- (-R) greatly stabilizes it and increases acidity. (e.g., Phenol is much more acidic than ethanol).

Strength of Organic Bases

  • Definition: A base's strength is its ability to accept a proton (H+), usually via a lone pair. We measure it by Kb or pKb.
  • Key: Base strength depends on the availability of the lone pair.
  • Factors Affecting pK values:
    1. Inductive Effect: Electron-donating groups (+I, e.g., -CH3) push electron density onto the N, making the lone pair more available and increasing basicity. (e.g., (CH3)2NH > CH3NH2 > NH3).
    2. Resonance/Hybridization: If the lone pair is delocalized (e.g., in aniline) or in an orbital with more s-character (e.g., pyridine), it is less available, making the base weaker. (e.g., NH3 > Aniline).

Aromaticity

Aromaticity is a property of certain cyclic, planar molecules with delocalized π electrons that gives them unusually high stability.

Hückel's Rule

For a molecule to be aromatic, it must meet all four criteria:

  1. Be Cyclic.
  2. Be Planar (all atoms sp2 hybridized).
  3. Be Fully Conjugated (a continuous loop of p-orbitals).
  4. Contain (4n + 2) π electrons (where n = 0, 1, 2, 3...).

Common (4n+2) numbers: 2 π e-, 6 π e-, 10 π e-, 14 π e-.

Benzenoids and Anti-Aromaticity

  • Benzenoids: Aromatic compounds containing one or more benzene rings (e.g., Benzene (6 π e-), Naphthalene (10 π e-)).
  • Non-Benzenoids: Aromatic compounds without a benzene ring (e.g., Pyridine, Pyrrole, Furan, Cyclopentadienyl anion (C5H5-)).
  • Anti-Aromatic: Molecules that are cyclic, planar, and conjugated but have (4n) π electrons (e.g., 4, 8, 12...). These are highly unstable. (e.g., Cyclobutadiene).
  • Non-Aromatic: Molecules that fail any one of the first three rules (e.g., are not cyclic, not planar, or not conjugated). (e.g., Cyclohexane).