Exam Master Review Sheet - Electronic Effects

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EXAM MASTER REVIEW SHEET

Electronic Effects in Organic Chemistry (GOC)

Strategic Foundation for Reaction Mechanisms

1. Inductive Effect (\(I\))

Permanent polarization of a \(\sigma\)-bond due to electronegativity difference. Decreases rapidly with distance (negligible after 3 carbons).

$-I$ Effect (EWG)

Order of Strength:

$-NR_3^+ > -NO_2 > -CN > -SO_3H > -CHO > -COOH > -F > -Cl > -Br > -I > -OH > -OR > -NH_2 > -C_6H_5 > -H$

$+I$ Effect (EDG)

Order of Strength:

$-O^- > -COO^- > (CH_3)_3C- > (CH_3)_2CH- > CH_3CH_2- > -CH_3 > -D > -H$

2. Resonance / Mesomeric Effect

Permanent effect involving delocalization of \(\pi\)-electrons or lone pairs. More powerful than \(I\)-effect (except for halogens).

$+M$ / $+R$ Effect

Releases \(e^-\) into the system:

$-O^- > -NH_2 > -NHR > -OH > -OR > -NHCOR > -Ph > -F > -Cl > -Br > -I$

$-M$ / $-R$ Effect

Withdraws \(e^-\) from the system:

$-NO_2 > -CN > -CHO > >C=O > -COOR > -COOH > -CONH_2$

Stability of Resonating Structures (\(RS\)):

• Octet Rule: Structures with complete octets are most stable.
• Neutrality: Non-polar structures > Polar structures.
• Charge Placement: Negative on more EN atom, Positive on less EN atom.
• Charge Separation: Avoid like-charge proximity; minimize unlike-charge distance.

3. Hyperconjugation (Baker-Nathan)

Delocalization of \(\sigma\)-electrons of \(C-H\) bond to an adjacent unsaturated system or empty \(p\)-orbital.

Fundamental Law:

Stability \(\propto\) Number of \(\alpha\)-Hydrogens

Total structures = (No. of \(\alpha\)-H) + 1

Main Applications

• • Alkenes: More \(\alpha\)-H \(\implies\) More Stable.
• • Carbocations: \(3^\circ > 2^\circ > 1^\circ > \text{methyl}\).
• • Heat of Hydrogenation: Inversely proportional to stability.

Special Cases

Reverse Hyperconjugation:

Observed in $-CF_3$ or $-CCl_3$ groups attached to \(\pi\)-systems.

4. Intermediate Stability Reference

Intermediate	Stabilized by (EDG)	Destabilized by (EWG)
Carbocation (\(C^+\))	\(+M, +H, +I\)	\(-M, -I\)
Free Radical (\(C^\cdot\))	\(+M, +H, +I\)	\(-M, -I\)
Carbanion (\(C^-\))	\(-M, -I\)	\(+M, +H, +I\)

Strength Priority: Resonance (M) > Hyperconjugation (H) > Inductive (I)

5. Acids and Bases Logic

Acidic Strength: \[ \text{Acidity} \propto K_a \propto \frac{1}{pK_a} \propto \text{Stability of C.B.} \]

\(\uparrow\) Increase:
\(-M, -I\)

\(\downarrow\) Decrease:
\(+M, +I\)

Basic Strength: \[ \text{Basicity} \propto K_b \propto \frac{1}{pK_b} \propto \text{L.P. Availability} \]

\(\uparrow\) Increase:
\(+M, +I\)

\(\downarrow\) Decrease:
\(-M, -I\)

THE ORTHO EFFECT (High Yield)

Ortho-substituted benzoic acids are generally more acidic than their meta/para isomers and benzoic acid itself, regardless of whether the group is $+I$ or $-I$. This is primarily due to Steric Inhibition of Resonance (SIR).

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