Organic Chemistry · An Overview of Organic Reactions
How Organic Reactions Occur: Mechanisms
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A reaction Mechanism The step-by-step sequence of bond breaking and forming in a reaction Full entry → is the step-by-step description of how reactants become products: which bonds break, which bonds form, in what order, and how the electrons move at each step. Chemists do not observe mechanisms directly — they infer them from evidence such as reaction rates, product distributions, isotope labeling, and the detection of intermediates. A good mechanism is consistent with all of that evidence and predicts what new experiments will show.
The central question: how do bonds actually break and form? There are two answers. In Homolytic cleavage Bond breaking where each atom keeps one electron Full entry →, a bond breaks so each fragment keeps one of the shared electrons, producing two radicals (species with unpaired electrons). In Heterolytic cleavage Bond breaking where one atom keeps both electrons Full entry →, one fragment keeps both electrons, producing an ion pair — a Carbocation A positively charged carbon with six valence electrons Full entry → (electron-poor) and a Carbanion A negatively charged carbon with a lone pair Full entry → (electron-rich). Reactions built from homolytic steps are Radical A species with an unpaired electron Full entry → reactions; those built from heterolytic steps are polar reactions. Everything else in this chapter — curved arrows, energy diagrams, intermediates, the HBr/ethylene example — is detail on top of these two pictures.
Why this matters
- Prediction over memorization: A mechanism tells you why a reaction gives a product, so you can predict outcomes for new substrates instead of memorizing each reaction.
- Selectivity and side products: Mechanisms explain regioselectivity (which position reacts), stereoselectivity, and why rearranged or racemized products sometimes appear.
- Rate and conditions: Knowing the rate-determining step tells you what to change (temperature, solvent, catalyst) to speed a reaction up or slow a side reaction.
- Biological chemistry: Enzyme catalysis is mechanism in action — every metabolic step is a polar or radical mechanism with a known sequence of bond changes.
- Exams: Mechanism questions are core to organic chemistry assessment: draw the steps, identify intermediates, justify arrows.
The college version
Core Concepts
Homolytic vs heterolytic bond cleavage
When a covalent bond breaks, the two electrons of the bond must go somewhere:
- Homolytic cleavage: each atom keeps one electron. The products are two radicals, each with an unpaired electron. Homolysis requires energy — the bond dissociation energy — and is typical of nonpolar bonds (Cl–Cl, C–H, Br–Br) under heat or light.
- Heterolytic cleavage: one atom keeps both electrons. The products are ions: the atom that took the electrons becomes negatively charged (a carbanion or anion), and the one that lost them becomes positively charged (a carbocation or cation). Heterolysis is typical of polar bonds (C–Br, C–O, O–H) and is assisted by polar solvents that stabilize ions.
Shorthand: homolysis uses a fishhook (single-barbed) arrow showing one electron moving; heterolysis uses a double-headed curved arrow showing an electron pair moving. The arrow always points from the electron source to the electron sink.
Radical reactions
A radical is a species with an unpaired electron — for example, Cl• from homolysis of Cl₂, or •CH₃ from a C–H bond. Radicals are highly reactive because they seek to pair that electron. Radical reactions typically proceed in chains: initiation creates radicals, propagation steps consume and regenerate them, and termination destroys them. Halogenation of alkanes is the prototype: chlorine radicals abstract hydrogen atoms, and the resulting alkyl radicals react with Cl₂ to give the alkyl chloride and a new chlorine radical. Radical stability increases with substitution (tertiary > secondary > primary > methyl), which explains where halogenation occurs.
Polar reactions
A polar reaction proceeds by heterolytic steps in which an electron-rich species (a nucleophile) donates an electron pair to an electron-poor species (an electrophile) — a Lewis base and a Lewis acid. Polar reactions dominate the chemistry of alkenes, carbonyls, and alkyl halides (next topic). The families are complementary: radical reactions dominate when bonds are nonpolar and conditions energetic (heat, light); polar reactions dominate when bonds are polar and conditions favor ions.
Reaction intermediates and their stability
Intermediates are transient species formed in one step and consumed in the next. The three classics:
- Carbocations (R₃C⁺): trivalent carbon with six valence electrons — electron-poor, electrophilic, planar. Stability: tertiary > secondary > primary > methyl, because alkyl groups donate electron density (hyperconjugation and inductive effects).
- Carbanions (R₃C⁻): trivalent carbon with a lone pair — electron-rich, nucleophilic, pyramidal. Stability is the reverse: methyl > primary > secondary > tertiary; electron-withdrawing groups stabilize carbanions.
- Radicals (R₃C•): trivalent carbon with an unpaired electron. Stability: tertiary > secondary > primary > methyl, like carbocations.
Carbocations rearrange by 1,2-shifts (hydride or alkyl migration) to more stable forms — a major source of "unexpected" products.
Bond dissociation energies and reaction energetics
The Bond dissociation energy (BDE) Energy to break one mole of a bond homolytically Full entry → is the energy required to break a particular bond homolytically, reported in kcal/mol or kJ/mol. Typical values: H–H 104, CH₃–H 105, Cl–Cl 58, H–Cl 103, CH₃–Cl 85, C–C 90, C=C 174 kcal/mol (approximate textbook values). BDEs let you estimate reaction enthalpy by comparing bonds broken and formed:
ΔH°= ∑BDE(bonds broken) - ∑BDE(bonds formed)
A negative ΔH° means the reaction is exothermic (releases heat); a positive value means endothermic. BDE estimates ignore gas-phase vs solution energetics, so they are a first approximation, not a precise measurement.
Mechanisms as evidence-based models
Mechanisms are models, not photographs, judged by how well they explain: (1) the rate law (which step is slowest), (2) product identity and stereochemistry, (3) detected intermediates, and (4) isotope effects and labeling studies. New evidence can revise a mechanism — textbook mechanisms are the ones that have survived the evidence so far.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Homolytic cleavage | Heterolytic cleavage | Homolytic: one electron to each atom → radicals; heterolytic: both electrons to one atom → ions |
| Radical reactions | Polar reactions | Radical: unpaired-electron species, chain steps, fishhook arrows; polar: nucleophile/electrophile pairs, curved arrows |
| Carbocation stability order | Carbanion stability order | Carbocations: tertiary > primary (alkyl groups stabilize); carbanions: primary > tertiary (alkyl groups destabilize the anion) |
| Bond dissociation energy | Bond strength in a specific molecule | BDE is defined for homolysis of a particular bond in a particular molecule; "bond strength" is the same idea averaged across contexts |
| ΔH° sign convention | Which bonds are "broken" | ΔH°= BDE(broken) - BDE(formed); breaking bonds always costs energy (positive terms), forming bonds always releases it (subtracted) |
| Intermediate | Transition state | An intermediate is a real, detectable species with a finite lifetime; a transition state is the fleeting energy maximum, not a molecule |
| kcal/mol | kJ/mol | 1 kcal = 4.184 kJ; forgetting the conversion factor misstates energies by ~4× |
| Mechanism being "the truth" | Mechanism being a model | Mechanisms are evidence-based models and are revised when new evidence appears |
Eli explains
The same idea, in plain words
Explain it like I’m 10
A reaction mechanism is like a recipe written as a step-by-step dance: first this atom lets go of its partner, then those two atoms hold hands, then one pair spins around. Two kinds of breakups happen: sometimes both atoms walk away with one toy each (homolytic — two radicals), and sometimes one atom grabs both toys (heterolytic — an ion pair). When you know the dance steps, you can predict what the dancers will look like at the end — without watching every single performance.
Worked example
Example 1: Classifying bond cleavages
Question: Classify each cleavage as homolytic or heterolytic and name the products.
Case A — Cl–Cl in Cl₂ under UV light: the bond is nonpolar and breaks with one electron to each atom → homolytic, giving two chlorine radicals: Cl₂ → Cl• + Cl•.
Case B — CH₃–Br in CH₃Br in polar solvent: bromine is more electronegative than carbon and takes the electron pair → heterolytic, giving a methyl cation and bromide: CH₃Br → CH₃⁺ + Br⁻.
Case C — C–H in CH₄ at high temperature: the C–H bond is nearly nonpolar and breaks with one electron each → homolytic, giving a methyl radical and a hydrogen atom: CH₄ → •CH₃ + H•.
Method: compare electronegativities. Very different electronegativities → heterolytic (ion formation); similar electronegativities → homolytic (radical formation).
Example 2: Estimating ΔH° for methane chlorination with dimensional analysis
Question: Estimate the enthalpy change for CH₄ + Cl₂ → CH₃Cl + HCl using BDEs (kcal/mol): CH₃–H 105, Cl–Cl 58, CH₃–Cl 85, H–Cl 103. Then convert the result to kJ/mol.
Step 1 — Write the enthalpy formula (bonds broken minus bonds formed):
ΔH°= ∑BDE(broken) - ∑BDE(formed)
Step 2 — Substitute the values:
ΔH°= (105 + 58) - (85 + 103) = 163 - 188 = -25 kcal/mol
Step 3 — Convert units with the factor 1 kcal = 4.184 kJ:
ΔH°= -25 kcal/mol × 4.184 kJ1 kcal = -105 kJ/mol
Interpretation: the reaction is exothermic — consistent with the fact that methane chlorination proceeds readily once initiated. The dimensional analysis (multiplying by the unit conversion factor) is what carries the units from kcal to kJ.
Example 3: Tracing a radical chain mechanism in words
Question: Outline the radical chlorination of methane, CH₄ + Cl₂ → CH₃Cl + HCl, and identify each phase.
Step 1 — Initiation: the Cl–Cl bond homolyzes under UV light, Cl₂ → Cl• + Cl•. Each step after this is a chain step.
Step 2 — Propagation (first step): a chlorine radical abstracts a hydrogen from methane, Cl• + CH₄ → HCl + •CH₃ (the C–H bond breaks homolytically; the new H–Cl bond forms).
Step 3 — Propagation (second step): the methyl radical reacts with Cl₂, •CH₃ + Cl₂ → CH₃Cl + Cl•, regenerating the chlorine radical that continues the chain.
Step 4 — Termination: two radicals combine, e.g., Cl• + •CH₃ → CH₃Cl or Cl• + Cl• → Cl₂, ending the chain.
Check: the two propagation steps sum to the overall reaction CH₄ + Cl₂ → CH₃Cl + HCl, and the chlorine radical acts as a catalyst-like chain carrier. This mechanism predicts that chlorination can proceed at many equivalent C–H positions, which is why methane chlorination gives only one monochloro product but higher alkanes give mixtures.
Key takeaways
- Mechanism = step-by-step bond-breaking/forming sequence, inferred from evidence (rates, products, intermediates, labeling).
- Homolytic cleavage: each atom keeps one electron → radicals (fishhook arrow). Heterolytic: one atom keeps both → ions (double-headed curved arrow).
- Radical reactions: chain processes (initiation, propagation, termination); favored for nonpolar bonds under heat/light.
- Polar reactions: nucleophile + electrophile via heterolytic steps; favored for polar bonds and polar solvents.
- Intermediate stability: carbocations and radicals tertiary > secondary > primary > methyl; carbanions follow the reverse order.
- Carbocations undergo 1,2-hydride/alkyl shifts to more stable forms (rearrangements).
- ΔH°= ∑BDE(broken) - ∑BDE(formed); negative = exothermic. BDE values: H–H 104, Cl–Cl 58, H–Cl 103, CH₃–H 105, CH₃–Cl 85 kcal/mol (approx).
- Mechanisms are falsifiable models — revised when evidence demands it.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What is the difference between homolytic and heterolytic bond cleavage, and what species does each produce?
Show answer
Homolytic: each atom keeps one electron → two radicals. Heterolytic: one atom keeps both electrons → a cation and an anion (ion pair).
Order these carbocations by stability: methyl, tertiary, secondary, primary. Why?
Show answer
Tertiary > secondary > primary > methyl — alkyl groups stabilize the electron-poor carbon by hyperconjugation and inductive electron donation.
Why does carbanion stability follow the opposite order?
Show answer
Alkyl groups push electron density, which destabilizes an already electron-rich carbanion; carbanions are therefore most stable when least substituted, and are further stabilized by electron-withdrawing groups.
Using BDEs (CH₃–H 105, Cl–Cl 58, CH₃–Cl 85, H–Cl 103 kcal/mol), is methane chlorination exothermic or endothermic, and by how much in kJ/mol?
Show answer
ΔH°= (105 + 58) - (85 + 103) = -25 kcal/mol × 4.184 kJ/kcal = -105 kJ/mol → exothermic.
List the three phases of a radical chain reaction.
Show answer
Initiation (radicals created), propagation (radicals consumed and regenerated), termination (radicals combine; chain ends).
Why are mechanisms considered models rather than proven facts?
Show answer
Mechanisms are inferred from indirect evidence (rates, products, intermediates, labeling) and are revised when new evidence conflicts — consistent models, not direct observations.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Mechanism
- The step-by-step sequence of bond breaking and forming in a reaction
- Homolytic cleavage
- Bond breaking where each atom keeps one electron
- Heterolytic cleavage
- Bond breaking where one atom keeps both electrons
- Radical
- A species with an unpaired electron
- Carbocation
- A positively charged carbon with six valence electrons
- Carbanion
- A negatively charged carbon with a lone pair
- Bond dissociation energy (BDE)
- Energy to break one mole of a bond homolytically
- Initiation / propagation / termination
- The three phases of a radical chain reaction
- Nucleophile / electrophile
- Electron-pair donor / acceptor
Sources & references
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