Introduction
Reactive intermediates are the fleeting species that form during reactions - they exist for nanoseconds but determine the entire outcome! Understanding them is the KEY to mastering organic reaction mechanisms.
Classification of Reactive Intermediates
graph TD
A[Reactive Intermediates] --> B[Carbocation R₃C⁺]
A --> C[Carbanion R₃C⁻]
A --> D[Free Radical R₃C·]
A --> E[Carbene R₂C:]
B --> B1[sp², planar, empty p]
C --> C1[sp³, pyramidal, lone pair]
D --> D1[sp², planar, 1 unpaired e⁻]
E --> E1[sp², singlet or triplet]Carbocations (Carbonium Ions)
Structure
Definition: Carbon with positive charge and only 6 electrons in valence shell.
$$\boxed{R_3C^+ \text{ (electron-deficient)}}$$Electronic Configuration:
- Hybridization: sp²
- Geometry: Trigonal planar
- Bond angle: 120°
- Empty p-orbital: Perpendicular to plane
R
|
R - C⁺ ← Planar
| Empty p-orbital ⊥
R
Characteristics:
- Electrophilic (electron-loving)
- Lewis acid (accepts electron pair)
- Extremely reactive (short-lived)
- Forms in polar solvents
Stability Order
$$\boxed{3° > 2° > 1° > CH_3^+ > \text{vinyl} > \text{phenyl}}$$Detailed:
$$\boxed{(CH_3)_3C^+ > (CH_3)_2CH^+ > CH_3CH_2^+ > CH_3^+ > CH_2=CH^+ > C_6H_5^+}$$Factors Affecting Stability:
1. Inductive Effect (+I)
Alkyl groups donate electrons, stabilizing positive charge.
(CH₃)₃C⁺: 3 methyl groups donating
(CH₃)₂CH⁺: 2 methyl groups
CH₃CH₂⁺: 1 methyl group
CH₃⁺: 0 methyl groups (least stable)
2. Hyperconjugation
α-Hydrogens delocalize into empty p-orbital.
Number of α-H:
- tert-Butyl: 9 α-H → 9 hyperconjugative structures
- Isopropyl: 6 α-H → 6 structures
- Ethyl: 3 α-H → 3 structures
- Methyl: 0 α-H → 0 structures
More α-H → More stable
3. Resonance
Delocalization through π-system enormously stabilizes.
Example: Allyl cation (CH₂=CH-CH₂⁺)
CH₂=CH-CH₂⁺ ↔ ⁺CH₂-CH=CH₂
Positive charge delocalized over 2 carbons
Much more stable than propyl cation!
Example: Benzyl cation (C₆H₅-CH₂⁺)
Resonance with benzene ring
Positive charge delocalized into ring
Very stable!
Stability with resonance:
$$\boxed{\text{Allyl/Benzyl} > 3° > 2° > 1°}$$Special Cases
Vinyl and Phenyl Cations
Extremely unstable!
Vinyl cation (CH₂=CH⁺):
- Positive charge on sp² carbon
- High s-character (50%) holds electrons tightly
- No hyperconjugation possible
- Very unstable
Phenyl cation (C₆H₅⁺):
- Disrupts aromaticity
- Extremely unstable
Bridgehead Cations
Also very unstable - violate Bredt’s rule.
Cannot form planar sp² geometry at bridgehead.
Carbocation Rearrangements
Carbocations rearrange to form more stable species!
1. Hydride Shift (1,2-H shift)
H with electron pair migrates to adjacent carbon.
Example:
CH₃ CH₃
| |
CH₃-CH-CH₂⁺ → CH₃-C⁺-CH₃
2° carbocation → 3° carbocation
(less stable) (more stable)
Driving force: Forms more stable carbocation
2. Methyl Shift (1,2-CH₃ shift)
Alkyl group migrates with electron pair.
Example:
CH₃ CH₃ CH₃ CH₃
| | | |
CH₃-C---CH⁺ → CH₃-C⁺--CH₂
| |
CH₃ CH₃
2° carbocation → 3° carbocation
General rule: Always shifts to form more stable carbocation
Mistake: Forgetting about carbocation rearrangements in mechanisms.
Reality: Carbocations ALWAYS rearrange if it gives more stability!
Example: Hydration of 3-methyl-1-butene
Expected: 3-methylbutan-2-ol Actual: 2-methylbutan-2-ol (after rearrangement!)
CH₃-CH(CH₃)-CH=CH₂ + H₂O
↓ H⁺
CH₃-CH(CH₃)-CH₂-CH₂⁺ (2°)
↓ rearrangement
CH₃-CH(CH₃)-CH(OH)-CH₃ WRONG!
Correct: H⁺ adds to form 3° cation after rearrangement!
This is THE most common mechanism trap!
Carbanions
Structure
Definition: Carbon with negative charge and 8 electrons (lone pair).
$$\boxed{R_3C^- \text{ (electron-rich)}}$$Electronic Configuration:
- Hybridization: sp³
- Geometry: Pyramidal (like NH₃)
- Bond angle: ~109°
- Lone pair in sp³ orbital
R
|
R - C⁻ ← Pyramidal
| Lone pair in sp³
R
Characteristics:
- Nucleophilic (nucleus-loving)
- Lewis base (donates electron pair)
- Very reactive
- Forms in aprotic solvents (no acidic H)
Stability Order
OPPOSITE of carbocations!
$$\boxed{CH_3^- > 1° > 2° > 3°}$$Detailed:
$$\boxed{CH_3^- > CH_3CH_2^- > (CH_3)_2CH^- > (CH_3)_3C^-}$$Also:
$$\boxed{\text{Phenyl} > \text{Vinyl} > \text{sp}^3 \text{ carbanions}}$$Factors Affecting Stability:
1. Inductive Effect (-I)
Electron-withdrawing groups stabilize negative charge.
CF₃CH₂⁻ > CH₃CH₂⁻
-CF₃ withdraws electrons, stabilizes negative charge
Order with -I groups:
$$\boxed{Cl_3C-CH_2^- > Cl_2CH-CH_2^- > ClCH_2-CH_2^- > CH_3-CH_2^-}$$2. Hyperconjugation
Destabilizes carbanions! (opposite of carbocations)
More α-H → Less stable carbanion
Why? H atoms are electron-donating, increase electron density on already negative center.
3. Resonance
Enormously stabilizes by delocalizing negative charge.
Example: Cyanide ion (CN⁻)
:C≡N:⁻ ↔ ⁻:C=N:
Negative charge delocalized
Very stable!
Example: Acetate ion (CH₃COO⁻)
O⁻ O
‖ ↔ ‖
CH₃-C CH₃-C
| |
O O⁻
Resonance makes acetic acid acidic!
4. Hybridization Effect
More s-character → More stable (holds electrons closer)
$$\boxed{sp (50\%) > sp^2 (33\%) > sp^3 (25\%)}$$Example:
HC≡C⁻ > CH₂=CH⁻ > CH₃CH₂⁻
Acetylide (sp) most stable
Ethyl (sp³) least stable
Acidity order matches:
$$\boxed{HC \equiv CH \text{ (pKa 25)} > CH_2=CH_2 \text{ (44)} > CH_3CH_3 \text{ (50)}}$$Carbanion Reactions
Strong bases and nucleophiles
Common reactions:
- Nucleophilic substitution
- Addition to carbonyls
- Elimination reactions
- Deprotonation
Free Radicals
Structure
Definition: Species with unpaired electron.
$$\boxed{R_3C \cdot \text{ (one unpaired electron)}}$$Electronic Configuration:
- Hybridization: sp²
- Geometry: Planar (or nearly planar)
- Bond angle: ~120°
- Unpaired electron in p-orbital
R
|
R - C· ← Planar
| Unpaired e⁻ in p
R
Characteristics:
- Highly reactive
- Neutral (no charge)
- Paramagnetic (unpaired electron)
- React with almost anything!
Stability Order
Same as carbocations!
$$\boxed{3° > 2° > 1° > CH_3 \cdot}$$Detailed:
$$\boxed{(CH_3)_3C \cdot > (CH_3)_2CH \cdot > CH_3CH_2 \cdot > CH_3 \cdot}$$Special cases:
$$\boxed{\text{Allyl/Benzyl} \cdot > 3° > 2° > 1°}$$Factors:
1. Hyperconjugation
More α-H → More stable
Same as carbocations!
2. Resonance
Example: Allyl radical
CH₂=CH-CH₂· ↔ ·CH₂-CH=CH₂
Unpaired electron delocalized
Very stable!
Example: Benzyl radical
Resonance with benzene ring
Very stable - used in polymerization!
Free Radical Reactions
1. Halogenation
Example: CH₄ + Cl₂ → CH₃Cl + HCl
Mechanism:
Initiation:
Cl₂ --hν--> 2Cl·
Propagation:
CH₄ + Cl· → CH₃· + HCl
CH₃· + Cl₂ → CH₃Cl + Cl·
Termination:
CH₃· + Cl· → CH₃Cl
CH₃· + CH₃· → C₂H₆
Cl· + Cl· → Cl₂
2. Anti-Markovnikov Addition (Peroxide Effect)
Example: CH₃CH=CH₂ + HBr –peroxide–> CH₃CH₂CH₂Br
Normal (without peroxide): Markovnikov (CH₃CHBrCH₃) With peroxide: Anti-Markovnikov (CH₃CH₂CH₂Br)
Mechanism:
Initiation:
ROOR → 2RO·
RO· + HBr → ROH + Br·
Propagation:
Br· + CH₃CH=CH₂ → CH₃CHBrCH₂· (2° radical, more stable)
CH₃CHBrCH₂· + HBr → CH₃CHBrCH₃ + Br·
Wait - this gives Markovnikov!
Actually:
Br· + CH₃CH=CH₂ → CH₃CH·-CH₂Br (less stable, but forms)
OR
→ ·CH₂-CHBr-CH₃ (2°, preferred!)
Correct mechanism: Br· adds to give more stable radical intermediate!
Peroxide effect (Anti-Markovnikov) works ONLY with HBr!
NOT with HCl or HI
Why?
- HCl: C-Cl bond too strong, propagation too slow
- HI: H-I bond too weak, reverses too easily
- HBr: Just right! (Goldilocks)
This specificity is heavily tested!
Interactive Demo: Visualize Carbocation Rearrangements
Watch how carbocations rearrange to achieve greater stability.
Carbenes
Structure
Definition: Neutral carbon with only 6 valence electrons and no charge.
$$\boxed{R_2C: \text{ (divalent carbon)}}$$Two types:
Singlet Carbene
- Paired electrons in same orbital
- sp² hybridized
- Angular geometry
- Diamagnetic
R
|
C: ← Bent, sp²
| Lone pair in sp²
R Empty p
Triplet Carbene
- Two unpaired electrons
- sp hybridized
- Linear geometry
- Paramagnetic
R-C-R ← Linear
: : Two unpaired in p orbitals
Generation
1. From Haloforms:
CHCl₃ + KOH → :CCl₂ + KCl + H₂O
Dichlorocarbene
2. From Diazomethane:
CH₂N₂ --hν--> :CH₂ + N₂
Methylene (carbene)
Reactions
Highly reactive - inserts into bonds!
1. Addition to alkenes (cyclopropane formation):
:CH₂ + CH₂=CH₂ → Cyclopropane
2. Insertion into C-H bonds
Nitrenes
Similar to carbenes but with nitrogen
$$\boxed{R-N: \text{ (monovalent nitrogen)}}$$Generation from azides:
R-N₃ --Δ--> R-N: + N₂
Comparison Summary
| Intermediate | Structure | Hybridization | Geometry | Stability Order |
|---|---|---|---|---|
| Carbocation | R₃C⁺ | sp² | Planar | 3° > 2° > 1° |
| Carbanion | R₃C⁻ | sp³ | Pyramidal | 1° > 2° > 3° |
| Free Radical | R₃C· | sp² | Planar | 3° > 2° > 1° |
| Carbene | R₂C: | sp² (singlet) | Angular | - |
Key Differences:
| Property | Carbocation | Carbanion | Free Radical |
|---|---|---|---|
| Charge | +1 | -1 | 0 |
| Electrons | 6 | 8 | 7 |
| Nature | Electrophile | Nucleophile | Both |
| +I effect | Stabilizes | Destabilizes | Stabilizes |
| Stability | 3° > 1° | 1° > 3° | 3° > 1° |
Practice Problems
Level 1: Basic Concepts
Identify the intermediate formed:
- a) CH₃CH₂Cl + AlCl₃
- b) CH₃CH₂Br + Mg
- c) (CH₃)₃C-Cl –hν–>
Arrange in order of stability:
- a) CH₃⁺, CH₃CH₂⁺, (CH₃)₂CH⁺, (CH₃)₃C⁺
- b) CH₃⁻, CH₃CH₂⁻, (CH₃)₂CH⁻, (CH₃)₃C⁻
Which is most stable?
- a) CH₂=CH-CH₂⁺
- b) CH₃-CH₂-CH₂⁺
- c) (CH₃)₂CH⁺
- d) CH₃⁺
Level 2: Application
Explain why:
- Allyl cation is more stable than propyl cation
- Acetylene is more acidic than ethane
- tert-Butyl radical is more stable than methyl radical
Predict the major product:
CH₃-CH(CH₃)-CH=CH₂ + HCl → (Consider carbocation rearrangement!)Why does peroxide effect work only with HBr?
Level 3: JEE Advanced
The most stable carbocation is:
- (a) (CH₃)₃C⁺
- (b) CH₂=CH-CH₂⁺
- (c) C₆H₅-CH₂⁺
- (d) CH₃-CH₂⁺
Which rearrangement is NOT possible?
- (a) Hydride shift
- (b) Methyl shift
- (c) Phenyl shift
- (d) Alkyl shift
Arrange in increasing order of acidity: HC≡CH, CH₂=CH₂, CH₃-CH₃
Assertion (A): Benzyl cation is more stable than ethyl cation. Reason (R): Benzyl cation shows resonance stabilization.
- (a) Both true, R explains A
- (b) Both true, R doesn’t explain A
- (c) A true, R false
- (d) Both false
In the reaction CH₃CH=CH₂ + HBr (with peroxide), the intermediate formed is:
- (a) CH₃CH⁺CH₂Br
- (b) CH₃CHCH₂Br· (radical)
- (c) CH₃CH⁻CH₂Br
- (d) CH₃CH(Br)CH₂·
The stability order of following carbanions is: CH₃⁻, CF₃CH₂⁻, CH₃CH₂⁻, (CH₃)₃C⁻
Memory Tricks
“CPR-F” for Intermediates
- Carbocation: Positive, planar, sp²
- Carbanion: Negative, pyramidal, sp³ (like NH₃)
- Radical: Neutral, planar, unpaired electron
- Free radical stability = Carbocation stability
Stability Orders
“3-2-1 for Cations & Radicals”
- 3° > 2° > 1° (carbocations and radicals)
“1-2-3 for Anions” (reverse!)
- 1° > 2° > 3° (carbanions)
Hyperconjugation
“Count Alpha H - More is Better (except anions!)”
- Carbocations: More α-H → More stable
- Radicals: More α-H → More stable
- Carbanions: More α-H → LESS stable (reverse!)
Peroxide Effect
“Only Bromine Reverses”
- Peroxide effect: Only with HBr
- Anti-Markovnikov addition
- NOT with HCl or HI
Related Topics
Within Organic Principles
- Hybridization - sp², sp³ in intermediates
- Electronic Effects - +I, resonance effects
- Reaction Types - Mechanisms involving intermediates
- Isomerism Types - Chirality in carbocations
Other Chemistry Topics
- Hydrocarbons - Alkenes - Carbocation rearrangements in addition
- Hydrocarbons - Alkynes - Vinyl cation stability
- Halogen Compounds - SN1/SN2 - Carbocation formation in SN1
- Elimination Reactions - E1 mechanism intermediates
- Alcohols - Carbocation formation in dehydration
- Benzene - Arenium ion intermediate in EAS
Foundation Topics
- Covalent Bonding - Formal charge and stability
- VSEPR Theory - Geometry of intermediates
- Molecular Orbital Theory - Electron distribution