Difficulty: Introductory to Intermediate | Prerequisites: General chemistry (bonding, Lewis structures, electronegativity). Familiarity with molecular geometry and orbital hybridisation is helpful but not essential.
Organic chemistry is, at its core, the study of how carbon-based molecules react with one another. Before you can predict products, design a synthesis, or read a mechanism, you need a working mental model of the four reaction classes (addition, elimination, substitution, rearrangement) and the two broad ways bonds break and form (polar and radical). This material sits right at the start of most Organic Chemistry I courses and underpins everything that follows, from alkene chemistry through carbonyl reactions. If you are joining late, make sure your Lewis structures and electronegativity are solid first.
Organic reactions fall into four categories: addition, elimination, substitution, and rearrangement. Bonds break and form by either polar mechanisms (electrons move in pairs, driven by nucleophile-electrophile interactions) or radical mechanisms (electrons move singly, via chain reactions). Energy diagrams tell you whether a reaction is favourable (exergonic) or not (endergonic) and how fast it will go (activation energy).
Nucleophile (Nu⁻)
An electron-rich species that donates a pair of electrons to form a new bond with an electrophile. Think of it as the attacker in a polar reaction: it has electrons to spare and goes looking for a positive or electron-poor site.
Electrophile (E⁺)
An electron-poor species that accepts a pair of electrons from a nucleophile. In simple terms, this is the molecule (or atom) with a "help wanted" sign for electrons.
Addition reaction
A reaction in which two reactants combine to form a single product, increasing the number of bonds to one or more atoms. Think of two pieces snapping together into one.
Elimination reaction
A reaction in which a single reactant loses atoms or groups to form a new pi bond (typically a double bond) and a small molecule or ion leaves. In simple terms, the molecule sheds a piece and tightens up internally.
Substitution reaction
A reaction in which one atom or group in a molecule is replaced by another. Think of it as a straight swap: one group walks in, another walks out.
Rearrangement reaction
A reaction in which a molecule reorganises its own bonds to produce a structural isomer of the starting material. Nothing enters or leaves; the atoms just shuffle.
Leaving group (LG)
An atom or group that departs from a molecule during a reaction, taking a bonding pair of electrons with it. Think of it as the group that gets kicked out, electrons in hand.
Homolytic bond cleavage
Bond breaking in which each fragment takes one electron, producing two radicals (each with an unpaired electron). In simple terms, the bond splits evenly.
Heterolytic bond cleavage
Bond breaking in which one fragment takes both electrons, producing a cation and an anion. The bond splits unevenly: one side gets everything.
Radical
A species with an unpaired electron, typically very reactive. Think of it as a molecule fragment that desperately wants to pair up.
Activation energy (Eₐ)
The minimum energy required for reactants to reach the transition state and proceed to products. In simple terms, it is the energy hill the reaction must climb.
Transition state (TS)
The highest-energy, most unstable arrangement of atoms along the reaction pathway. It exists for an instant and cannot be isolated.
ΔG (Gibbs free energy change)
The difference in free energy between products and reactants. A negative ΔG means the reaction is exergonic (spontaneous under standard conditions); a positive ΔG means it is endergonic (non-spontaneous).
Exergonic reaction
A reaction in which the products are lower in energy than the reactants (negative ΔG). Energy is released overall.
Endergonic reaction
A reaction in which the products are higher in energy than the reactants (positive ΔG). Energy must be supplied for the reaction to proceed.
Polar reactions are the most common type in organic chemistry. They are driven by the interaction between electron-rich nucleophiles and electron-poor electrophiles. Electrons move as pairs (shown by curved arrows in mechanisms).
Electron flow in polar reactions
Nucleophile → electrophile: the nucleophile donates a pair of electrons to the electrophile, forming a new covalent bond.
Curved-arrow notation tracks these electron pairs. The arrow starts at the electron source (a lone pair or a bond) and points to the electron sink.
The four classes of organic reactions
1. Addition reactions (nucleophilic attack)
Two reactants combine into one product.
A pi bond (or ring) is broken, and new sigma bonds form.
Example: Br⁻ + CH₂=CH₂ → BrCH₂CH₂⁻
The nucleophile attacks the electrophilic carbon of the double bond.
2. Elimination reactions (loss of a leaving group)
One reactant splits into two products: a molecule with a new pi bond, plus a small leaving group.
Example: CH₃CH₂Br → CH₂=CH₂ + Br⁻
A leaving group departs, taking the bonding electrons. A pi bond forms where sigma bonds used to be.
3. Substitution reactions
One group on a molecule is replaced by another.
Two reactants produce two products.
Example: CH₃O⁻ + CH₃Cl → CH₃OCH₃ + Cl⁻
The nucleophile (CH₃O⁻) replaces the leaving group (Cl⁻) at the electrophilic carbon.
4. Rearrangement reactions
A single reactant reorganises its bonds to form an isomer.
No atoms are added or removed; the connectivity changes.
Example: (CH₃)₃C⁺ → CH₃C⁺(CH₃)CH₃ (a 1,2-methyl shift stabilising the carbocation)
Often occurs alongside other reaction types, particularly when a more stable carbocation can form.
Radical reactions involve species with unpaired electrons. Bonds break homolytically (one electron to each side), and the resulting radicals are highly reactive. These reactions typically proceed as chain reactions with three distinct phases.
1. Initiation
A bond is broken homolytically, usually by heat or UV light, to generate two radicals.
Example: Cl₂ → 2 Cl· (UV light splits the Cl–Cl bond)
This step consumes energy and produces the reactive species that set the chain in motion.
2. Propagation
A radical reacts with a stable molecule, breaking one bond and forming another. This produces a new radical, keeping the chain going.
Each propagation step is typically exothermic.
Example: Cl· + CH₄ → HCl + ·CH₃, then ·CH₃ + Cl₂ → CH₃Cl + Cl·
The regenerated Cl· radical feeds back into the cycle.
3. Termination
Two radicals combine to form a stable, non-radical product.
This ends the chain.
Example: Cl· + ·CH₃ → CH₃Cl
Termination steps are less frequent because they require two radicals to meet.
Fishhook arrows
Radical mechanisms use single-barbed (fishhook) arrows to show the movement of one electron at a time, unlike the double-barbed curved arrows used in polar mechanisms.
Energy diagrams (also called reaction coordinate diagrams) plot free energy on the y-axis against reaction progress on the x-axis. They tell you two things at a glance: whether a reaction is thermodynamically favourable and how fast it will be.
Activation energy (Eₐ)
The energy gap between the reactants and the transition state (the peak of the curve).
A large Eₐ means a slow reaction; a small Eₐ means a fast one.
Catalysts lower Eₐ without changing ΔG.
Gibbs free energy change (ΔG)
ΔG = G(products) − G(reactants).
Negative ΔG: exergonic, products are more stable than reactants, reaction is thermodynamically favourable.
Positive ΔG: endergonic, products are less stable, reaction is thermodynamically unfavourable under standard conditions.
Transition state vs intermediate
A transition state sits at an energy maximum. It cannot be isolated and exists only fleetingly.
An intermediate sits at a local energy minimum between two transition states. It can sometimes be detected or even isolated.
A one-step reaction has one transition state and no intermediate. A two-step reaction has two transition states and one intermediate.
Reading an energy diagram
Height of the highest peak above the reactants = overall Eₐ (determines rate).
Difference between the starting level and the final level = ΔG (determines equilibrium position).
The rate-determining step is the step with the highest-energy transition state.
Key relationship
\Delta G = G_{\text{products}} - G_{\text{reactants}}A negative value means the reaction releases free energy (exergonic). A positive value means it absorbs free energy (endergonic).
Diagrams to know
Energy diagram for a one-step reaction: single peak (one transition state), with Eₐ measured from reactant level to peak, and ΔG measured from reactant level to product level.
Energy diagram for a two-step reaction: two peaks separated by a valley (the intermediate). The higher peak corresponds to the rate-determining step.
Fishhook arrows (single-barbed) for radical mechanisms vs curved arrows (double-barbed) for polar mechanisms.
The polar and radical mechanisms covered here are not just exam material. Radical chain reactions are exactly how polyethylene (the plastic in carrier bags and cling film) is manufactured: an initiator generates radicals that add to ethylene monomers in a propagation chain. Substitution reactions are central to pharmaceutical synthesis, where chemists swap functional groups on a molecular scaffold to tune a drug's activity, solubility, and metabolic stability.
Students often confuse the transition state with an intermediate. A transition state is an energy maximum that cannot be isolated; an intermediate is an energy minimum that (in principle) can be.
Students sometimes think a negative ΔG means a reaction is fast. It does not. ΔG tells you about thermodynamic favourability (will it go?), while Eₐ tells you about kinetics (how fast?).
Students often assume all bond breaking is heterolytic. Homolytic cleavage (one electron to each side) is equally important and is the defining feature of radical reactions.
Students frequently forget that rearrangement reactions do not require a second reactant. The molecule reorganises itself.
⚠️ You will almost certainly be asked to classify a reaction as addition, elimination, substitution, or rearrangement. Count the reactants and products: two become one = addition, one becomes two = elimination, two become two = substitution, one becomes one = rearrangement.
⚠️ Expect a question that asks you to draw the energy diagram for a given reaction and label Eₐ, ΔG, transition state, and (if present) intermediate.
⚠️ Know the difference between thermodynamic control (which product is more stable, governed by ΔG) and kinetic control (which product forms faster, governed by Eₐ).
⚠️ Be ready to identify nucleophiles and electrophiles in a reaction. Look for lone pairs and negative charges (nucleophile) vs empty orbitals, positive charges, and polarised bonds (electrophile).
⚠️ Radical chain-reaction questions love the three-step framework: if you can label initiation, propagation, and termination, you are most of the way there.
True or false: A transition state can be isolated in the laboratory. (False. It is an energy maximum and exists only momentarily.)
Fill in the blank: In a polar reaction, the ______ donates electrons to the ______. (nucleophile; electrophile)
True or false: A reaction with a large negative ΔG is always fast. (False. ΔG describes favourability, not speed. Speed depends on Eₐ.)
Fill in the blank: The three stages of a radical chain reaction are ______, ______, and ______. (initiation; propagation; termination)
True or false: In homolytic bond cleavage, one atom gets both electrons. (False. Each atom gets one electron, producing two radicals.)
Q: Classify the following reaction: CH₃OH + HBr → CH₃Br + H₂O. Is this addition, elimination, substitution, or rearrangement?
A: Substitution. Two reactants exchange parts to give two products: Br replaces OH on the carbon.
Q: In the reaction Br⁻ + CH₂=CH₂ → BrCH₂CH₂⁻, identify the nucleophile and the electrophile.
A: The nucleophile is Br⁻ (electron-rich, donating electrons). The electrophile is the carbon of the C=C double bond (relatively electron-poor when attacked).
Q: A reaction has a negative ΔG and a very high Eₐ. Will it proceed quickly at room temperature? Explain.
A: No. Although the reaction is thermodynamically favourable (negative ΔG), the high activation energy means it will be very slow at room temperature. It would need heating or a catalyst to proceed at a practical rate.
Q: Write out the three stages of the radical chlorination of methane (CH₄ + Cl₂ → CH₃Cl + HCl) and label each stage.
A: Initiation: Cl₂ → 2 Cl· (homolytic cleavage by UV light). Propagation step 1: Cl· + CH₄ → HCl + ·CH₃. Propagation step 2: ·CH₃ + Cl₂ → CH₃Cl + Cl·. Termination: any two radicals combine (e.g. Cl· + ·CH₃ → CH₃Cl).
Q: What is the difference between homolytic and heterolytic bond cleavage?
A: In homolytic cleavage, the bond breaks so each atom retains one electron, forming two radicals. In heterolytic cleavage, one atom takes both electrons, forming a cation and an anion.
This material connects directly to the study of substitution mechanisms (SN1 and SN2), which you will see in the companion notes. Understanding nucleophile and electrophile roles here is essential for those mechanisms.
Elimination reactions (E1 and E2) compete with substitution reactions, and your exam will likely ask you to predict which pathway dominates for a given substrate. The energy diagram concepts reappear throughout the course, particularly when comparing reaction pathways and understanding catalysis.
organic reactions, reaction types, addition reaction, elimination reaction, substitution reaction, rearrangement reaction, nucleophile, electrophile, polar reaction, radical reaction, free radical, chain reaction, initiation, propagation, termination, homolytic cleavage, heterolytic cleavage, energy diagram, reaction coordinate diagram, activation energy, Eₐ, Gibbs free energy, ΔG, exergonic, endergonic, transition state, intermediate, rate-determining step, curved arrow mechanism, fishhook arrow, electron flow, leaving group, organic chemistry I, OChem 1