Chemca Formula Sheet - Electronic Effects (GOC)

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EXAM MASTER FORMULA 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. It is distance-dependent and becomes negligible after 3 carbons.

$-I$ Effect (EWG)

Order of Strength (Electron Withdrawing):

$-NF_3^+ > -NR_3^+ > -NH_3^+ > -NO_2 > -CN > -SO_3H > -CHO > -COOH > -F > -Cl > -Br > -I > -OH > -OR > -NH_2 > -C\equiv CH > -C_6H_5 > -CH=CH_2 > -H$

$+I$ Effect (EDG)

Order of Strength (Electron Donating):

$-CH_2^- > -NH^- > -O^- > -COO^- > 3^\circ\text{ alkyl } [-(C(CH_3)_3)] > 2^\circ\text{ alkyl } [-CH(CH_3)_2] > 1^\circ\text{ alkyl } [-CH_2CH_3] > -CH_3 > -D > -H$

The D.N.P. Rule (For Acid/Base Strength):

When comparing the Inductive effect of multiple substituents, evaluate them in this strict priority order:

Distance: A closer group has a much stronger effect than a further one.
Number: More groups exert a stronger effect.
Power: Only if Distance and Number are same, compare the strength order from the series above.

2. Resonance / Mesomeric Effect ($M$ or $R$)

Permanent effect involving delocalization of $\pi$-electrons or lone pairs. More powerful than $I$-effect (except for halogens, where $-I > +M$).

Conditions for Conjugation (Resonance):

$\pi - \sigma - \pi$

$\pi - \sigma - (+)$

$\pi - \sigma - (-)$ / LP

$\pi - \sigma - (\cdot)$

LP $- \sigma - (+)$

$+M$ / $+R$ Effect

Releases $e^-$ into the conjugated system (increases $e^-$ density at ortho/para):

$-O^- > -NH_2 > -NHR > -NR_2 > -OH > -OR > -NHCOR > -O-CO-R > -Ph > -F > -Cl > -Br > -I$

$-M$ / $-R$ Effect

Withdraws $e^-$ from the conjugated system (decreases $e^-$ density at ortho/para):

$-NO_2 > -CN > -SO_3H > -CHO > >C=O > -COCl > -COOR > -COOH > -CONH_2$

Stability of Resonating Structures:

Octet Rule: Structures with complete octets are most stable (even if charged).
Neutrality: Non-polar structures > Polar (charged) structures.
Charge Placement: Negative charge is more stable on highly electronegative atom (e.g., O); Positive charge on less EN atom (e.g., C).
Charge Separation: Avoid proximity of like charges; minimize distance between opposite charges.

Cross vs Extended Conjugation:

Extended (Linear) Conjugation: Delocalization occurs in one continuous path. Generally more stable.

Cross Conjugation: Three groups are present, two are not conjugated with each other but both are conjugated with the third. Less stable.

3. Hyperconjugation (Baker-Nathan Effect)

Delocalization of $\sigma$-electrons of $C-H$ bond into an adjacent empty $p$-orbital or $\pi^*$ orbital. Also called No-bond resonance.

Fundamental Law:

Stability $\propto$ Number of $\alpha$-Hydrogens

Total Hyperconjugative Structures = (No. of $\alpha$-H) + 1

Main Applications

Stability of Alkenes: More $\alpha$-H $\implies$ More Stable (Saytzeff's Rule). E.g., $(CH_3)_2C=C(CH_3)_2$ is most stable.
Stability of Carbocations/Radicals: $3^\circ > 2^\circ > 1^\circ > \text{methyl}$.
Heat of Hydrogenation (HOH): $\text{HOH} \propto \frac{1}{\text{Stability of Alkene}}$.
$C-C$ Bond Length: Shortens single bonds and lengthens double bonds adjacent to the $\alpha$-C.

Special Cases

Reverse Hyperconjugation ($-H$):

Observed when $-CF_3$ or $-CCl_3$ groups are attached to $\pi$-systems. Electrons flow from $\pi$-system to the $C-X$ anti-bonding orbital, withdrawing electron density from the ring.

Isotope Effect:

The $C-H$ bond is weaker than $C-D$ bond. Therefore, Hyperconjugative effect order: $-CH_3 > -CD_3 > -CT_3$.

4. Electromeric Effect ($E$)

A temporary effect observed only in the presence of an attacking reagent. Involves complete transfer of $\pi$-electrons to one of the atoms.

$+E$ Effect (Electrophilic Addition):

$\pi$-electrons transfer to the atom to which the attacking reagent gets attached.
Occurs in Alkenes/Alkynes reacting with $H^+$.

$-E$ Effect (Nucleophilic Addition):

$\pi$-electrons transfer to the atom other than the one to which the attacking reagent gets attached.
Occurs in Carbonyls ($>C=O$) reacting with $CN^-$.

5. 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 ($C^-$)	$-M, -I$	$+M, +H, +I$

Master Stability Order of Carbocations:

Tropylium ion > Triphenylmethyl cation > Diphenylmethyl cation > Benzyl cation $\approx$ Allyl cation $\approx$ $3^\circ$ Alkyl > $2^\circ$ Alkyl > $1^\circ$ Alkyl > Methyl > Vinyl > Phenyl

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

6. Acids and Bases Logic

Acidic Strength:

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

$\uparrow$ Increase:
$-M, -I$ (Withdraw $e^-$)

$\downarrow$ Decrease:
$+M, +H, +I$ (Donate $e^-$)

Basic Strength:

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

$\uparrow$ Increase:
$+M, +I$ (Donate $e^-$)

$\downarrow$ Decrease:
$-M, -I$ (Withdraw $e^-$)

Amine Basicity Trends (Highly Tested)

Gas Phase / Non-Polar Solvents:

$3^\circ > 2^\circ > 1^\circ > NH_3$ (Only $+I$ operates)

Aqueous Phase (Methyl substituted):

$2^\circ > 1^\circ > 3^\circ > NH_3$ (Solvation + $+I$ + Steric)

Aqueous Phase (Ethyl substituted):

$2^\circ > 3^\circ > 1^\circ > NH_3$ (Steric hindrance dominates over solvation)

The Ortho Effect (Benzoic Acids)

Ortho-substituted benzoic acids are generally much more acidic than their meta/para isomers and benzoic acid itself, regardless of whether the group is $+I$ or $-I$.

Reason: Steric Inhibition of Resonance (SIR). The bulky ortho group twists the $-COOH$ group out of the ring plane, breaking conjugation with the ring, which stabilizes the resulting carboxylate anion.

Steric Inhibition of Protonation (SIP)

Ortho-substituted anilines are generally less basic than aniline itself, irrespective of the nature of the substituent.

Reason: Protonation of the $-NH_2$ group changes its hybridization from $sp^3$ (pyramidal) to a bulkier $-NH_3^+$ group, severely increasing steric clash with the ortho substituent, making protonation thermodynamically unfavorable.

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Exam Master Review Sheet - Electronic Effects