NEET Organic Chemistry Crash Course – General Organic Chemistry (Day 3)

Reactive Intermediates

• Definition: Transient species formed between reactant + reagent and final product.
• Main types discussed
  • Carbocation $R–C^+$
  • Carbanion $R–C^-$
  • Free-radical $R–C\cdot$
  • Others (carbene, nitrene, benzyne) mentioned implicitly.
• General stability order highlighted repeatedly:
  • Carbocation : \text{BBS (bridgehead benzylic)} > \text{Aryl} > \text{Benzyl} > \text{3°} > 2° > 1° > \text{Methyl} > H
  • Carbanion : \text{BBS} > \text{Aryl} > \text{Benzyl} > \text{Methyl} > 1° > 2° > 3° (opposite inductive trend)
  • Free-radical : Similar to carbocation but less sensitive to –I/ +I.

Bond Fission

• Homolytic cleavage ➜ two radicals.
• Heterolytic cleavage ➜ ions (carbocation + carbanion).

Inductive Effect (Permanent σ-effect)

• Operates through $\sigma$-bonds; distance-dependent, practically negligible after 3-σ bonds.
• Produces partial charges (δ⁺/δ⁻).
• Reference value: $I_H = 0$ (inductive effect of H = zero).

–I series (most \rightarrow least)

\text{–NO2} > –SO3H > –CN > –CHO > –COOH > –F > –Cl > –Br > –I > –OR > –OH > –C\equiv CH > –NH2 > –C6H_5 > –H
Organometallic exception: $R$ in $RMgX, RLi$ behaves as +I (metal is δ⁺).

+I series (most \rightarrow least)

\text{(CH3)3C–} > (CH3)2CH– > CH3CH2– > CH_3– > \text{silicon, Ge atoms etc.}

Applications

1. Acidic strength ($\text{A.S} \propto -I$, opposite for +I).
   • Carboxylic acids: $Ka$ increases with –I groups.
   • Alcohols: far less acidic; –I still enhances $Ka$ but distance & number matter.
2. Basicity of aliphatic amines
   • Gas phase basicity ∝ +I only (3° > 2° > 1°).
   • Aqueous phase: solvation reverses the order (1° > 2° > 3°) because of hydrogen bonding.
3. Stability of carbocations/carbanions
   • +I stabilises cations, –I stabilises anions.

Hyperconjugation (+H/–H Effect)

• Delocalisation of $\sigma$-(C–H) electrons into adjacent empty or partially filled $p/\pi$ orbitals.
• Requirements
  1. $\alpha$-C must bear at least one H.
  2. For carbocations/free-radicals: $\alpha$-C is sp³ attached to an sp² centre.
  3. For alkenes: one $\alpha$-H on each vinylic carbon gives hyperconjugative structures.
• Number of hyperconjugative structures $=\text{No. of } \alpha\text{-H}$ (for CH bond cleavage version).
• Explains stability order of
  • Carbocation: 3°>2°>1°>CH_3^+
  • Alkene: \text{substituted}>\text{cis}<\text{trans} because trans has better hyperconjugation.
• Alternative names: no-bond resonance, Baker–Nathan effect.
• Not operative in $C=O, C=N$ due to high π-energy gap.

Mesomeric / Resonance Effect (+M, –M)

• Permanent, distance-independent π-effect via conjugation.
• +M (electron donation by lone pair/π): $–OH, –OR, –NH_2, –NHR, –X$ (halogens), $–O^-$.
• –M (electron withdrawal): $–NO<em>2, –CN, –CHO, –COR, –COOH, –SO</em>3H, –COOR$, $–C≡N$.
• Conditions for resonance:
  1. Planarity (sp² network).
  2. Conjugated system: alternating σ–π or presence of adjacent lone-pair/empty-p.
• Rules for drawing and evaluating Resonating Structures (RS)
  1. All RS must be valid Lewis structures.
  2. Neutral > charged RS.
  3. More covalent bonds → more stable.
  4. Charges on more electronegative atoms preferred; charge separation lowers stability.
  5. Like charges adjacent ⇒ highly unstable.
  6. Resonance hybrid is the weighted average; has lowest energy and equalised bond length/dipole.

Special Terms

• Delocalised vs Localised lone pair.
• Extended vs Cross conjugation (affects colour & stability).
• Equivalent RS: equal potential energy hence equal contribution.

Applications

1. Acidic strength in phenols & aromatic acids: $\text{A.S}\propto -M$, ortho/para directive.
2. Ortho-effect: o-substituted benzoic acids are stronger due to steric hindrance & intramolecular H-bond (–I dominates).
3. Hydrogen bonding impact:
   • o-nitrophenol: intramolecular H-bond ↓ acidity test detectability.
4. Overall withdrawing nature: (a) –M > –H > –I ; (b) +M from LP > +M via hyperconjugation.

Tests for Acidic Strength

1. $\text{Na}, \text{NaH}, \text{NaNH}<em>2$: deprotonate species more acidic than terminal alkyne $pK</em>a\approx25$.
2. $\text{NaOH}$: reacts if pK_a<15.7 (roughly phenol & stronger).
3. $\text{NaHCO}<em>3$: reacts only with carboxylic acids (pKa<6.3).
4. Feasibility rule (F.W.R): reaction proceeds towards formation of weaker acid (higher $pK_a$).

Keto–Enol Tautomerism

• Base-catalysed mechanism proceeds via carbanion; major product depends on anion stability.
• Enol content enhanced by
  1. Chelation (intramolecular H-bond) e.g. β-diketones.
  2. Conjugation with aromatic ring.
  3. Non-polar solvent.
• Relative order of enol %: $\beta$-diketone > keto-aldehyde > dialkone > keto-ester > di-ester > simple ketone > aldehyde.

S.I.R & S.I.P Effects (Steric Inhibition)

• Steric Inhibition of Resonance (SIR): bulky ortho substituents twist ring, decreasing –M; hence basicity of 2°,3° arylamines increases.
• Steric Inhibition of Protonation (SIP): ortho bulk blocks approach of H⁺; lowers basicity of primary arylamines.

Aromaticity

• Hückel rule: cyclic, planar, fully conjugated, $4n+2\,\pi$ electrons ($n=0,1,2…$) ⇒ aromatic.
• $4n\,\pi$ electrons with all other conditions ⇒ anti-aromatic (unstable).
• If any prerequisite fails ⇒ non-aromatic.
• Stability: Aromatic > Non-aromatic > Anti-aromatic.
• Examples analysed: [5]-, [6]-, [7]-, [10]-annulenes; tropylium radical with 7 π electrons aromatic ( NEET-2013 question – answer: 6 π orbitals & 6 unpaired e⁻ ).

Thermodynamic Criteria

• Heat of Hydrogenation (HoH): less negative ⇒ more stable alkene.
• Heat of Combustion (HoC): more branching ⇒ lower HoC ⇒ more stable alkane.

Physical Properties & Reactivity via Resonance

• Greater resonance ⇒
  • Shorter bond length (C–O in carboxylate).
  • Lower dipole moment if charge separation reduced.
  • Higher rotational barrier (restricted C–N in amides).
  • Lower rate of protonation/nucleophilic addition (amide vs amine).

Basic Strength Trends

1. Aliphatic amines (aqueous): 2°>1°>3°>NH_3 (due to balance of +I & solvation).
2. Gas phase: +I dominates ⇒ 3°>2°>1°>NH_3.
3. Heteroatom hybridisation: \text{sp} < \text{sp}^2 < \text{sp}^3 in basicity (lone-pair availability).
   • Order given: \text{Guanidine} > R2N–C=NH > R2C=NH > R–C≡N.
4. Aromatic vs Aliphatic: aniline < ammonia < aliphatic amine (–I, –M withdraw in ring).
5. SIR/SIP modifies ortho-substituted anilines.

Carbocation / Carbanion / Free-Radical Properties

• Geometry: trigonal planar (cation & radical sp²), pyramidal for carbanion (sp³) except if conjugated.
• Magnetic behaviour: cation & anion diamagnetic, radical paramagnetic.
• Role in mechanisms: electrophile (cation), nucleophile (anion), ambiphile (radical).

Examination Shortcuts & Mnemonics

• "Father Collector Beta Inspector Mummy" for halogen –I order –F > –Cl > –Br > –I.
• "Bredt's rule": no double bond at bridgehead of small bicyclic systems.
• Priority rule for stabilisation: +M > +H > +I > –I > –H > –M (for carbocation).

Selected Competitive-Exam Facts

• Most stable carbocation among isomers often benzylic/allylic with +M / +H assist.
• Pyridine less basic than triethylamine because N is sp² and LP is not delocalised (answer b).
• p-Nitrophenol more acidic than o- due to intramolecular H-bond lowering ability to release H⁺.
• Reactivity towards AgNO₃/SbCl₅ tests for carbocation generation; benzylic > 3° > 2° > 1°.

Practice Problem Nuggets (with answers)

• Strongest base list (AIIMS-2004): $NH_2^-$ in sp³ environment.
• Highest $K_a$ (AIIMS-2004): $p$-nitrophenol.
• Aromatic compound identification (AIIMS-2004): tropylium cation.
• Acid order FCH2COOH > ClCH2COOH > BrCH2COOH > CH3COOH (inductive magnitude correlates with electronegativity).
• Basicity order for methyl amines in water: CH3NH2 > (CH3)2NH > (CH3)3N (steric hindrance on solvation).

Numerical / Statistical References

• $pK<em>a = -\log K</em>a$ ; $pH = -\log[H^+]$.
• Cutting point for NaOH test pK_a < 15.7 (acid stronger than water).
• Hyperconjugative structures count example:
  • \text{(CH3)3C^+} has 9 α-H hence 9 H.C.S.

Ethical & Practical Implications

• Understanding substituent effects crucial for drug-design (acidity/basicity tuning).
• Knowledge of aromaticity informs stability of pollutants and pharmaceuticals.
• Inductive & mesomeric insights applied in polymer and material science for electronic properties.