Difficulty: Introductory to Intermediate | Prerequisites: basic bonding (covalent, ionic), Lewis structures, electronegativity, familiarity with functional groups.
Organic reactions are classified by what happens to the bonds: do they break evenly (radicals) or unevenly (polar)? Does a group migrate within the molecule (rearrangement)? These three mechanism families cover most of what you will see in Organic Chemistry I beyond substitution and elimination. Understanding how electrons move in each family is the single most transferable skill in the course, because it lets you predict products for reactions you have never seen before. If you can draw curved arrows for polar reactions and fishhook arrows for radicals, you are ready for this material.
Bonds break in two ways: homolytically (one electron each, producing radicals) or heterolytically (both electrons to one atom, producing ions in polar reactions). Rearrangement reactions shuffle groups within a molecule to reach a more stable carbocation. Energy diagrams map the energy cost of each step, and curved-arrow notation tracks where the electrons go.
Homolytic cleavage (homolysis)
Bond breaking in which each atom retains one electron from the shared pair, producing two radicals.
Think of it as a fair split: one electron goes left, one goes right.
Heterolytic cleavage (heterolysis)
Bond breaking in which both electrons go to one atom, producing a cation and an anion.
In simple terms, one atom takes everything and the other is left empty-handed.
Radical (free radical)
A species with an unpaired electron. Radicals are highly reactive because they want to pair that lone electron.
Think of it as an atom with one electron looking for a partner.
Initiation
The first stage of a radical chain reaction, where radicals are generated, usually by heat or UV light breaking a weak bond.
In simple terms, this is the spark that starts the chain.
Propagation
The stage in which a radical reacts with a stable molecule, forming a new radical and continuing the chain.
Think of it as each radical creating the next one, like dominoes.
Termination
The stage in which two radicals combine and the chain stops.
In simple terms, two unpaired electrons find each other and form a bond, ending the cycle.
Nucleophile
An electron-rich species that donates electrons to an electron-poor centre. (See SN1/SN2 notes for full detail.)
Electrophile
An electron-poor species that accepts electrons from a nucleophile.
Hydride shift
The migration of a hydrogen atom together with its bonding pair of electrons from one carbon to an adjacent carbocation.
Think of it as a hydrogen hopping next door to stabilise a positive charge.
Alkyl (methyl) shift
The migration of an alkyl group (often a methyl group) with its bonding electrons to an adjacent carbocation.
In simple terms, a whole carbon group moves over when there is no hydrogen available to shift.
Activation energy (ΔG‡)
The energy difference between the reactants and the transition state. It determines how fast a reaction proceeds.
Think of it as the height of the hill the reaction must climb.
Overall energy change (ΔG)
The energy difference between products and reactants. It determines whether the reaction is thermodynamically favourable.
In simple terms, negative ΔG means the reaction releases energy and is spontaneous; positive ΔG means it costs energy.
Transition state
The highest-energy point along a reaction pathway, where bonds are partially broken and partially formed.
Think of it as the peak of the energy hill. It cannot be isolated.
Reaction intermediate
A species that forms between two steps of a multi-step reaction. It sits in an energy valley between two transition states.
In simple terms, it exists briefly but, unlike a transition state, it has a real (if short) lifetime.
Radical reactions involve species with unpaired electrons. The bond-breaking event is homolytic: each atom gets one electron, shown by fishhook (single-barbed) arrows.
Three stages of a radical chain reaction
Initiation: A weak bond (commonly Cl₂ or Br₂) breaks homolytically, usually triggered by UV light or heat. Each atom leaves with one electron, producing two radicals. Example: Cl₂ → 2 Cl•
Propagation: A radical abstracts an atom (typically hydrogen) from a stable molecule, generating a new radical that continues the chain. Each propagation step consumes one radical and produces another. The chain continues until radicals run out.
Termination: Two radicals collide and combine their unpaired electrons into a covalent bond. This removes radicals from the system and ends the chain. Termination can produce the desired product, a by-product, or regenerate starting material.
Radical stability
Follows the same trend as carbocation stability: 3° > 2° > 1° > methyl.
More substituted radicals are more stable because of hyperconjugation.
Allylic and benzylic radicals are stabilised by resonance.
Selectivity
Bromine radicals are more selective than chlorine radicals. Bromination preferentially abstracts the most substituted hydrogen (producing the most stable radical), while chlorination is less discriminating and gives a statistical mixture more closely reflecting the number of each type of hydrogen.
Polar reactions involve heterolytic cleavage: both bonding electrons go to one atom. The result is a pair of oppositely charged species (or partial charges). These are the most common reactions in organic chemistry.
Nucleophilic attack
The nucleophile (electron-rich) donates a pair of electrons to the electrophile (electron-poor), forming a new bond.
Shown with a full curved arrow from the electron source to the electron sink.
This is the fundamental pattern behind substitution, addition to carbonyls, and many other reaction types.
Loss of leaving group
A leaving group departs with both bonding electrons, forming a stable anion (or neutral molecule, e.g. water).
Good leaving groups are weak bases: halides (I⁻ > Br⁻ > Cl⁻ >> F⁻), tosylate, water.
This pattern is critical in SN1, SN2 and elimination reactions.
Proton transfer
A Brønsted base abstracts a proton from a Brønsted acid.
Often the first or last step of a multi-step mechanism.
Example
Cl-CH₃ + HO⁻ → HO-CH₃ + Cl⁻ (nucleophilic attack by hydroxide on the electrophilic carbon, displacing chloride).
Rearrangement reactions involve the migration of a group (hydrogen or alkyl) within a molecule to form a more stable carbocation. They occur in reactions that pass through carbocation intermediates (SN1, E1, certain additions).
Hydride shift (1,2-hydride shift)
A hydrogen atom migrates with its bonding electron pair from one carbon to the adjacent positively charged carbon.
Preferred over alkyl shifts because hydrogen is smaller and migrates more easily.
Result: the carbocation moves one carbon over, often from a less stable position (2°) to a more stable one (3°).
Alkyl (methyl) shift (1,2-alkyl shift)
An alkyl group (commonly methyl) migrates with its bonding electrons to the adjacent carbocation.
Occurs when no hydrogen is available on the neighbouring carbon for a hydride shift.
Same driving force: forming a more stable carbocation.
Carbocation stability order
Tertiary (3°) > Secondary (2°) > Primary (1°) > Methyl
Stability increases with substitution because of hyperconjugation and inductive electron donation from alkyl groups.
Resonance-stabilised carbocations (allylic, benzylic) can be exceptionally stable.
When to expect rearrangements
Whenever a reaction produces a carbocation that is adjacent to a carbon bearing a hydrogen or alkyl group that could shift to give a more stable cation.
If the product of a reaction has a different carbon skeleton from the starting material, a rearrangement has occurred.
Reading an energy diagram
The x-axis is the reaction coordinate (progress from reactants to products). The y-axis is free energy.
Each peak is a transition state. Each valley between peaks is a reaction intermediate.
The highest peak relative to the starting materials determines the overall rate (this is the rate-determining step).
The difference between products and reactants is ΔG. Negative ΔG = exergonic (thermodynamically favourable). Positive ΔG = endergonic.
One-step reactions (e.g. SN2)
Show a single peak. No intermediate.
Two-step reactions (e.g. SN1)
Show two peaks with a valley in between (the carbocation intermediate). The first peak is typically higher because leaving-group departure is the slow step.
Curved-arrow notation (polar reactions)
A full curved arrow shows the movement of an electron pair from a nucleophile (or lone pair) to an electrophile.
Arrows always flow from electron-rich to electron-poor.
Each arrow represents a bond forming, a bond breaking, or both simultaneously.
Fishhook arrows (radical reactions)
A single-barbed (fishhook) arrow shows the movement of one electron.
Used in homolytic cleavage and radical propagation steps.
Two fishhook arrows leaving a bond = homolysis.
Homolytic cleavage of Cl₂
\text{Cl}_2 \xrightarrow{h\nu} 2\text{Cl}\bulletCarbocation stability order
3^\circ > 2^\circ > 1^\circ > \text{methyl}Energy diagram key relationships
ΔG‡ (activation energy) = E(transition state) minus E(reactants). Controls rate.
ΔG (overall energy change) = E(products) minus E(reactants). Controls thermodynamic favourability.
A catalyst lowers ΔG‡ without changing ΔG.
Radical chain reactions are the basis of polymerisation (how polyethylene and polystyrene are manufactured) and are also responsible for ozone depletion by CFCs in the atmosphere. Polar reactions underpin nearly all biological chemistry: enzyme-catalysed reactions in your body rely on nucleophilic attack, proton transfer and leaving-group departure. Carbocation rearrangements explain unexpected products in industrial processes such as catalytic cracking of petroleum.
Students often confuse transition states with intermediates. A transition state is the peak of the energy diagram and cannot be isolated. An intermediate sits in a valley and, while short-lived, has a real existence.
Students sometimes draw full curved arrows for radical steps. Radical mechanisms use fishhook (single-barbed) arrows because only one electron moves at a time.
Students frequently assume that a rearrangement always occurs when a carbocation forms. A rearrangement only happens if a more stable carbocation can be reached by a 1,2-shift. If the carbocation is already tertiary, there is no driving force to rearrange.
Students often think that a negative ΔG means the reaction is fast. It does not. ΔG tells you about thermodynamic favourability (whether the reaction will happen). ΔG‡ tells you about rate (how fast it happens). A reaction can be thermodynamically favoured but kinetically slow.
⚠️ Drawing correct arrow-pushing mechanisms is one of the most commonly tested skills. Know when to use full curved arrows (polar) versus fishhook arrows (radical).
⚠️ Be able to sketch and interpret energy diagrams for one-step and two-step reactions. Identify the rate-determining step, the transition states and any intermediates.
⚠️ Radical halogenation problems frequently appear. Know the three stages, be able to write out the propagation steps, and understand why bromine is more selective than chlorine.
⚠️ Rearrangement questions often give you an unexpected product and ask you to explain it. Look for a carbon skeleton that differs from the starting material, then identify the hydride or alkyl shift.
True or false: Homolytic cleavage produces ions. (False, it produces radicals.)
Fill in the blank: The three stages of a radical chain reaction are initiation, ______ and termination. (propagation)
True or false: A transition state can be isolated and characterised. (False, it exists only at the energy maximum and cannot be isolated.)
True or false: A hydride shift moves a hydrogen without its electrons. (False, the hydrogen migrates with its bonding electron pair.)
Fill in the blank: In an energy diagram, the height of the tallest peak above the reactants is the ______. (activation energy / ΔG‡)
Q: Write the initiation and two propagation steps for the radical chlorination of methane (CH₄ + Cl₂ → CH₃Cl + HCl).
A: Initiation: Cl₂ → 2 Cl• (UV light). Propagation step 1: Cl• + CH₄ → HCl + CH₃•. Propagation step 2: CH₃• + Cl₂ → CH₃Cl + Cl•. The Cl• produced in step 2 feeds back into step 1, continuing the chain.
Q: A secondary carbocation forms during an SN1 reaction. The carbon adjacent to the cationic centre bears three methyl groups and one hydrogen. Will a rearrangement occur? If so, what kind?
A: Yes. A hydride shift will move the hydrogen (with its electron pair) from the adjacent carbon to the carbocation, converting the secondary carbocation into a more stable tertiary carbocation. Hydride shifts are preferred over methyl shifts when a hydrogen is available.
Q: On an energy diagram for a two-step reaction, how do you identify the rate-determining step?
A: The rate-determining step corresponds to the highest-energy transition state (the tallest peak relative to the starting materials). The activation energy for that step is the largest energy barrier the reaction must overcome.
Q: Explain the difference between a full curved arrow and a fishhook arrow, and state when each is used.
A: A full curved arrow represents the movement of an electron pair and is used in polar (heterolytic) mechanisms. A fishhook (single-barbed) arrow represents the movement of a single electron and is used in radical (homolytic) mechanisms.
Radical reactions connect to alkene chemistry later in the course: radical addition to alkenes follows anti-Markovnikov regiochemistry, which is the opposite of the ionic (polar) addition that follows Markovnikov's rule.
Polar reaction patterns (nucleophilic attack, leaving-group loss, proton transfer) recur in nearly every topic from here on, including carbonyl chemistry, amine reactions and carbohydrate chemistry.
Carbocation rearrangements tie directly to SN1 and E1 mechanisms, and also appear in electrophilic aromatic substitution (Friedel-Crafts alkylation) and in terpene biosynthesis.
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