Notes › Organic Chemistry

General Organic Chemistry

1. Electronic Effects

Electronic effects determine the reactivity and stability of organic molecules.

  • Inductive Effect: Polarisation of $\sigma$-bonds due to electronegativity. It decreases rapidly with distance.
  • Resonance Effect: Delocalisation of $\pi$-electrons. It is a permanent effect and much stronger than the inductive effect.
  • Hyperconjugation: 'No-bond resonance' involving $\sigma$-electrons of C-H bonds adjacent to unsaturated systems.

2. Reaction Intermediates

Stability orders are critical for predicting major products:

  • Carbocations ($C^+$): $3^\circ > 2^\circ > 1^\circ > methyl$. Stabilized by $+I$ and $+M$ effects.
  • Free Radicals: Follow the same order as carbocations.
  • Carbanions ($C^-$): $methyl > 1^\circ > 2^\circ > 3^\circ$. Stabilized by $-I$ and $-M$ effects.

🔬 Mechanism Insight

Always look for Carbocation Rearrangement (Hydride or Methyl shift) if a more stable carbocation can be formed. This is a favourite JEE topic!

Interactive diagram: open this page in a browser to use it.

3. Electromeric Effect

A temporary, complete transfer of a shared π-pair to one atom at the demand of an attacking reagent. It disappears once the reagent is removed. +E: electrons move towards the attacking electrophile; −E: electrons move away from the attacking nucleophile.

4. Order of Effects

When they compete, the usual priority is: Mesomeric (resonance) > Hyperconjugation > Inductive. Halogens are the classic exception in aromatic substitution: their −I effect deactivates the ring, but their +M effect still directs ortho/para.

5. Nucleophiles and Electrophiles

  • Nucleophiles (electron-rich): $OH^-$, $CN^-$, $RO^-$, $NH_3$, $H_2O$, $RNH_2$.
  • Electrophiles (electron-poor): $H^+$, $NO_2^+$, $R^+$, $BF_3$, $AlCl_3$, carbonyl carbon.

6. Acidity and Basicity Trends

  • Acidity rises when the conjugate base is stabilised: $-I$ and $-M$ groups help, $+I$ groups hurt.
  • Phenol ($pK_a \approx 10$) is more acidic than ethanol ($\approx 16$) because the phenoxide ion is resonance-stabilised. p-Nitrophenol is more acidic than phenol; p-cresol is less.
  • Ethyne is more acidic than ethene and ethane: an $sp$ carbon has 50% s-character and holds the negative charge closer to the nucleus.

7. Fission of Covalent Bonds

Homolytic fission gives free radicals (favoured by light, heat, peroxides, non-polar solvents). Heterolytic fission gives ions (carbocations and carbanions), favoured by polar solvents.

Practice questions

  1. The most stable carbocation is:

    1. CH₃⁺
    2. CH₃CH₂⁺
    3. (CH₃)₂CH⁺
    4. (CH₃)₃C⁺
    Answer

    D. Tertiary carbocations are best stabilised by the +I effect and hyperconjugation from nine C–H bonds.

  2. Which effect is temporary and operates only in the presence of a reagent?

    1. Inductive
    2. Mesomeric
    3. Electromeric
    4. Hyperconjugation
    Answer

    C. The electromeric effect disappears when the attacking reagent is removed.

  3. Which of the following is the strongest acid?

    1. Phenol
    2. p-Nitrophenol
    3. p-Cresol
    4. Ethanol
    Answer

    B. The −M and −I effect of the nitro group stabilises the phenoxide ion further.

  4. Which of these is an electrophile?

    1. NH₃
    2. H₂O
    3. BF₃
    4. CN⁻
    Answer

    C. BF₃ has an empty orbital on boron and accepts an electron pair.

  5. Hyperconjugation is also known as:

    1. Electromeric effect
    2. No-bond resonance
    3. Field effect
    4. Steric effect
    Answer

    B. It involves delocalisation of σ C–H electrons and is often called no-bond resonance or Baker–Nathan effect.

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