What is a reaction mechanism?
In an organic reaction, the main molecule is the substrate and the attacking species is the reagent. A mechanism shows each step: bonds breaking, short-lived reactive intermediates, and new bonds forming. Electron movement is drawn with curved arrows: a full arrow ( ↷ ) moves a pair of electrons; a half-headed 'fish-hook' arrow moves one electron.
Substrate + Reagent → [Intermediate] → Product(s)
Fission of a covalent bond
Homolytic fission (homolysis)
The bond pair is shared equally: each atom takes one electron. Products are free radicals (atoms or groups with an unpaired electron). It needs heat or light and is common in non-polar bonds and in the gas phase. Example: Cl–Cl → 2 Cl• under UV light.
Heterolytic fission (heterolysis)
The bond pair goes to the more electronegative atom. Ions form. If carbon loses the pair it becomes a carbocation; if carbon keeps the pair it becomes a carbanion. It is favoured by polar bonds and polar solvents. Example: (CH₃)₃C–Br → (CH₃)₃C⁺ + Br⁻.
Reactive intermediates: carbocation, carbanion, free radical
| Intermediate | Charge / electrons | Hybrid, shape | Stability order |
|---|---|---|---|
| Carbocation C⁺ | +, 6 electrons around C | sp², trigonal planar (empty p orbital) | 3° > 2° > 1° > CH₃⁺ |
| Carbanion C⁻ | −, 8 electrons (one lone pair) | sp³, pyramidal | CH₃⁻ > 1° > 2° > 3° |
| Free radical C• | neutral, 7 electrons | sp², planar (nearly) | 3° > 2° > 1° > CH₃• |
Why 3° carbocations are most stable: alkyl groups push electrons (+I effect) and share σ electrons by hyperconjugation, spreading the positive charge. Alkyl groups do the opposite for carbanions, adding more negative charge, so the order reverses. Carbocations next to a C=C or benzene ring (allyl, benzyl) are extra stable by resonance.
Electrophiles and nucleophiles
Nucleophile ('nucleus-loving', Nu:) is electron-rich and donates a pair: OH⁻, CN⁻, Cl⁻, RO⁻, and neutral ones with lone pairs like H₂O, NH₃, R–NH₂.
Electrophile ('electron-loving', E⁺) is electron-poor and accepts a pair: H⁺, NO₂⁺, Br⁺, carbocations, and neutral ones like BF₃, AlCl₃ and the carbon of C=O.
A nucleophile attacks an electron-poor centre; an electrophile attacks an electron-rich centre such as a C=C or benzene ring. The curved arrow goes from the nucleophile to the electrophile.
Types of organic reactions (overview)
Substitution (one group replaces another), addition (atoms add across a multiple bond), elimination (a small molecule leaves, forming a multiple bond) and rearrangement (atoms reorganise in the same molecule).
Inductive effect
When a σ bond joins atoms of different electronegativity, the electron pair shifts towards the more electronegative atom. This shift passes along the chain but dies out after about 3 carbons. It is permanent.
- −I groups pull electrons: –NO₂ > –CN > –COOH > –F > –Cl > –Br > –I > –OH > –C₆H₅.
- +I groups push electrons: alkyl groups, (CH₃)₃C– > (CH₃)₂CH– > CH₃CH₂– > CH₃–.
Use: chloroacetic acid (ClCH₂COOH) is stronger than acetic acid because –Cl pulls electrons and stabilises the anion.
Resonance effect (mesomeric effect)
When π electrons or lone pairs are next to π bonds (conjugation), they spread over several atoms. One Lewis structure is not enough; the real molecule is a resonance hybrid of several canonical structures. The hybrid is more stable than any single structure; the extra stability is the resonance energy. Benzene is the classic case: all six C–C bonds are equal (139 pm).
- +R effect: group gives electrons into the π system: –OH, –OR, –NH₂, –Cl (lone pair). Ring gets richer at ortho and para positions.
- −R effect: group pulls π electrons: –NO₂, –CHO, –COOH, –CN, >C=O.
Rules for canonical structures: same positions of atoms, same number of unpaired electrons; more covalent bonds and less charge separation means a bigger contribution.
Electromeric effect
A temporary effect: when an attacking reagent comes near a multiple bond, the whole π pair moves to one atom. It disappears when the reagent is removed.
- +E: π electrons move towards the atom where the electrophile attaches (e.g. H⁺ adding to an alkene).
- −E: π electrons move away from the atom where the nucleophile attaches (e.g. CN⁻ attacking C=O; electrons go to O).
If inductive and electromeric effects act in opposite directions, the electromeric effect wins while the reagent is present.
Hyperconjugation
The σ electrons of a C–H bond on the carbon next to a positive carbon (or a C=C) can spread into the empty p orbital or the π bond. It is also called no-bond resonance. More α-hydrogens mean more hyperconjugation.
(CH₃)₃C⁺ has 9 α-H, (CH₃)₂CH⁺ has 6, CH₃CH₂⁺ has 3, CH₃⁺ has 0. This matches the carbocation stability order 3° > 2° > 1° > methyl. It also explains why more substituted alkenes are more stable.
Try it
Pencil snap game (homolysis vs heterolysis). Then count α-hydrogens for each carbocation on paper and rank them; check with the 3D step 3.
Key formulas and definitions
- Homolysis: A–B → A• + B• (fish-hook arrows)
- Heterolysis: A–B → A⁺ + B:⁻ (B more electronegative)
- Carbocation / radical stability: 3° > 2° > 1° > CH₃
- Carbanion stability: CH₃⁻ > 1° > 2° > 3°
- Number of hyperconjugation structures = number of α-H atoms
- −I: –NO₂ > –CN > –COOH > –F > –Cl > –Br > –I; +I: alkyl groups
Worked examples
1. Show the fission of CH₃–CH₃ by UV light and name the products.
Line 1: C–C is non-polar and light supplies energy → homolysis. Line 2: CH₃–CH₃ → CH₃• + CH₃•. Line 3: Products are two methyl free radicals, each with 7 electrons around carbon.
2. Give the products of heterolysis of (CH₃)₃C–Cl.
Line 1: Cl is more electronegative, so it takes the bond pair. Line 2: (CH₃)₃C–Cl → (CH₃)₃C⁺ + Cl⁻. Line 3: A tert-butyl (3°) carbocation forms; it is fairly stable.
3. Arrange in order of stability: CH₃⁺, (CH₃)₂CH⁺, (CH₃)₃C⁺, CH₃CH₂⁺.
Line 1: Count α-H: 0, 6, 9, 3. Line 2: More α-H and more +I alkyl groups = more stable. Line 3: (CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ > CH₃⁺.
4. Classify as electrophile or nucleophile: NH₃, BF₃, CN⁻, NO₂⁺, H₂O, AlCl₃.
Line 1: Lone pair / negative charge → nucleophile: NH₃, CN⁻, H₂O. Line 2: Electron-poor (incomplete octet or +) → electrophile: BF₃, NO₂⁺, AlCl₃. Line 3: Tip: count electrons around the central atom.
5. Which is the stronger acid: CH₃COOH or ClCH₂COOH? Why?
Line 1: Cl has a −I effect; it pulls electrons towards itself. Line 2: In ClCH₂COO⁻ the negative charge is spread and stabilised. Line 3: A more stable anion means a stronger acid → ClCH₂COOH.
6. How many hyperconjugation structures do (CH₃)₃C⁺ and CH₃CH₂⁺ have?
Line 1: Each α-C–H bond gives one structure. Line 2: (CH₃)₃C⁺: 3 CH₃ × 3 H = 9. Line 3: CH₃CH₂⁺: 1 CH₃ × 3 H = 3.
7. Show the electromeric effect when H⁺ attacks propene CH₃–CH=CH₂.
Line 1: The π pair moves towards the carbon that H⁺ will attach to (+E). Line 2: H⁺ joins the end CH₂ carbon, giving CH₃–CH⁺–CH₃. Line 3: This forms the more stable 2° carbocation (not the 1° one).
Common mistakes
- Drawing the curved arrow from the positive atom to the electrons. Arrows always start at the electrons.
- Using the carbocation order for carbanions. For carbanions it reverses: CH₃⁻ is the most stable.
- Thinking the inductive effect reaches far down the chain. It becomes negligible after about 3 carbons.
- Calling the electromeric effect permanent. It lasts only while the attacking reagent is present.