Organic Chemistry - GOC & Hydrocarbon Revision

Electronic Effects and Resonance

• Electronic effects are internal factors within a molecule that influence the distribution of electron density and consequently affect reactivity and stability.
• Resonance ($R$/$M$ Effect): The flow of electrons through $\pi$ systems. It is categorized into two types: Positive Mesomeric ($+M$) effect and Negative Mesomeric ($-M$) effect.

Identification of $+M$ and $-M$ Groups

• Trick to Identify $+M$: A group exhibits a $+M$ (or $+R$) effect when the atom directly attached to the conjugated system (e.g., a benzene ring) possesses a lone pair.
  • Example Groups ($+M$):
    • $-NH_2$ (Lone pair on Nitrogen)
    • $-OH$ (Lone pair on Oxygen)
    • $-OCH_3$
    • $-NHCH_3$
    • $-O-C(=O)-CH_3$
    • $-NH-C(=O)-CH_3$
    • $-Cl$ (Halogens with lone pairs)
    • $-S-CH_3$
• Trick to Identify $-M$: A group exhibits $-M$ (or $-R$) effect when the atom directly attached to the system is connected to a more electronegative atom via a multiple bond (double or triple bond).
  • Example Groups ($-M$):
    • $-NO_2$ (Nitrogen double-bonded to Oxygen)
    • $-CN$ (Carbon triple-bonded to Nitrogen)
    • $-C(=O)-H$ (Aldehyde)
    • $-C(=O)-NH_2$ (Amide)
    • $-S(=O)_2-OH$ (Sulfonic Acid)
    • $-C(=O)-O-CH_3$ (Ester)
    • $-C(=O)-OH$ (Carboxylic Acid)
• Special Cases:
  • $-NO$ (Nitroso) and $-Ph$ (Phenyl): Can act as both $+M$ and $-M$ depending on the attached group, though $-NO$ is often primarily considered $-M$ due to the electronegative oxygen.

Ordering of $+M$ and $-M$ Strength

$+M$ Power Order

• High to Low: $-O^- > -NH_2 > -NHR > -NR_2 > -OH > -OR > -NH-C(=O)R > -O-C(=O)R > -Ph > -CH=CH_2 > -F > -Cl > -Br > -I$
• Reasoning for Amine vs. Hydroxyl ($NH_2 > OH$): Nitrogen is less electronegative than Oxygen, making it more willing to donate its lone pair into the ring.
• Reasoning for Internal Resonance: $-NH_2 > -NH-C(=O)-CH_3$ because in the latter, the lone pair on Nitrogen is involved in internal resonance with the Carbonyl oxygen, making it less available for the benzene ring.
• Atomic Size factor: $2p - 2p$ overlap (as in Nitrogen or Oxygen with Carbon) is more effective than $2p - 3p$ (Chlorine) or $2p - 4p$ (Bromine).

$-M$ Power Order

• High to Low: $-NO_2 > -CN > -SO_3H > -CHO > -C(=O)R > -C(=O)-O-C(=O)R > -C(=O)OR > -C(=O)OH > -C(=O)NH_2$

Conjugation Types and Stability

• Extended Conjugation: A continuous linear system of alternating single and multiple bonds (e.g., $A=B-C=D-E=F$).
• Cross Conjugation: A system where two different $\pi$ systems are in conjugation with the same central atom or bond, but not with each other (e.g., $A=B-C(=D)-E=F$).
• Core Principle: Extended conjugation is more stable than cross conjugation because the electrons are delocalized over a longer path.

Stability of Resonating Structures (RS)

To compare the stability of different resonating structures, follow this priority sequence:

1. Neutral vs. Charged: Neutral molecules are generally more stable than charged structures.
2. Number of $\pi$ Bonds: Stability increases as the number of $\pi$ bonds increases.
3. Octet Completion: Structures where every atom (especially $C, N, O, F$) has a complete octet are significantly more stable.
4. Electronegativity ($EN$): Negative charges are more stable on highly electronegative atoms (e.g., Oxygen); positive charges are more stable on less electronegative atoms.
5. Charge Separation:
   • Opposite charges ($+/-$) are more stable when they are closer together (attraction).
   • Like charges ($+/+$ or $-/-$) are more stable when they are farther apart (minimized repulsion).

Localized vs. Delocalized Electrons

• Localized Lone Pair: A lone pair that is not in conjugation and cannot participate in resonance. These stay on the specific atom.
• Delocalized Lone Pair: A lone pair that is in conjugation (next to a $\pi$ bond or empty orbital) and participates in resonance. These are spread over the molecule.

Acidic Strength in Aromatic Carboxylic Acids

• General Rule: Acidic strength $\propto \frac{\text{Stability of Conjugate Base}}{\text{Strength of O-H bond}}$.
• Effect of Substituents:
  • Electron Withdrawing Groups (EWG): Standardly increase acidic strength via $-M$ and $-I$ effects.
  • Electron Donating Groups (EDG): Standardly decrease acidic strength via $+M$ and $+I$ effects.

Critical Rules for Substituents:

1. Meta Position Limitation: Mesomeric effects ($M$ or $R$) do not operate at the meta position. Only Inductive effects ($I$) apply there.
2. Relative Dominance (Except Halogens): Generally, $|M| > |I|$.
   • For $-OH, -OCH_3, -NH_2$, the group is $+M$ and $-I$; the $+M$ dominates.
3. Halogen Special Case: For $-Cl, -Br, -I$, the Inductive effect $-I$ is stronger than the Mesomeric effect $+M$ ($-I > +M$).
4. Alkyl Groups: $+M$ (via hyperconjugation) of a lone pair is stronger than the $+M$ of an alkyl group.

The Ortho Effect (Steric Inhibition of Resonance - SIR)

• Definition: When a bulky group is present at the ortho position of benzoic acid, steric hindrance forces the $-COOH$ group to rotate out of the plane of the benzene ring.
• Consequence: Resonance between the benzene ring and the $-COOH$ group is inhibited ($SIR$). This prevents the ring from donating electron density to the carboxyl carbon, effectively making the $-COOH$ group more independent and increasing the acid strength significantly.
• Rule: Ortho-substituted benzoic acids are generally more acidic than their meta or para isomers, regardless of whether the substituent is an EWG or EDG (with some exceptions like small groups).
• Bulky Groups that show SIR: $-CH_3, -C_2H_5, -iPr, -tBu, -NO_2, -COOH, -SO_3H, -NR_2, -I, -Br, -Cl$.
• Non-bulky Groups (No SIR): $-OH, -NH_2, -CN, -F$.

Practice Problems and Applications

Comparing Acidic Strength

• Example 1: Nitro-Benzoic Acids

  • (a) Benzoic Acid: No effect.
  • (b) o-nitro: $SIR$ effect + $-I$ + $-M$. (Most acidic)
  • (c) m-nitro: Only $-I$.
  • (d) p-nitro: $-M$ + $-I$.
  • Order: Ortho > Para > Meta > Benzoic Acid.

• Example 2: Methoxy-Benzoic Acids

  • Ortho > Benzoic Acid > Meta > Para.
  • (Ortho due to Ortho effect; Para is least due to strong $+M$ dominating over $-I$; Meta is acidic due to only $-I$ acting).

• Example 3: Phenol vs. Ortho-Nitrophenol

  • Intramolecular H-bonding in ortho-nitrophenol can sometimes affect acidity, though typically $-M$ and $-I$ of $NO_2$ increase acidity overall ($P > O > M$ relative to phenol).

Bond Length Comparison

• Comparison of $C-N$ bond lengths in various substituted rings:
• Structures with more resonance character in the $C-N$ bond (more double bond character) will have shorter bond lengths.
• If $SIR$ is present, resonance is inhibited, preventing the single bond from gaining double bond character, resulting in a longer bond length.
• Example: In a highly substituted ortho system with $SIR$, the $C-N$ bond length is closer to a pure single bond ($1.47\text{\AA}$) compared to a system with resonance ($\approx 1.35\text{\AA}$).