Difficulty: Introductory-Intermediate | Prerequisites: General chemistry (Lewis structures, electronegativity, bonding), CHEM 2301 Ch. 2 functional groups.
Organic chemistry is ultimately about understanding how and why molecules react. Chapter 5 introduces the language you will use for the rest of the course: the four broad classes of organic reactions (addition, elimination, substitution, rearrangement) and the two ways bonds can break (homolytic and heterolytic cleavage). If general chemistry taught you what molecules look like, this chapter teaches you what they do. You should already be comfortable with Lewis structures, electronegativity, and the idea that electrons are not shared equally in polar bonds, because every mechanism in this chapter builds on that foundation.
Organic reactions fall into four types: addition, elimination, substitution, and rearrangement. Bonds break either homolytically (one electron to each side, producing radicals) or heterolytically (both electrons to one side, producing ions). Most of this chapter focuses on polar (heterolytic) reactions, where electron-rich nucleophiles attack electron-poor electrophiles.
Addition reaction
Two reactants combine to form a single product. A + B → C. In simple terms, two molecules join together and nothing leaves.
Elimination reaction
A single reactant splits into two products. A → B + C. Think of it as: the molecule sheds a piece of itself.
Substitution reaction
Two reactants exchange parts to generate two new products. A + B → C + D. In simple terms, one group swaps in while another group leaves.
Rearrangement reaction
A single reactant reorganises its bonds to form a single isomeric product. A → B. Think of it as: the same atoms, just rearranged into a different structure.
Homolytic cleavage (homolysis)
Bond breaking in which one bonding electron goes to each fragment, producing two radicals. In simple terms, the electrons in the bond split evenly, one to each side.
Heterolytic cleavage (heterolysis)
Bond breaking in which both bonding electrons go to one fragment, producing a cation and an anion. Think of it as: one side takes both electrons and walks away charged.
Radical
A species with an unpaired electron, formed by homolytic cleavage. In simple terms, a highly reactive fragment with a lone, unpaired electron.
Curved arrow formalism
A notation system that tracks electron movement in reaction mechanisms. Double-headed (full) arrows show movement of two electrons (polar reactions). Single-headed (fishhook) arrows show movement of one electron (radical reactions). Think of it as: a map showing where the electrons go.
Addition: Two reactants combine into one product (A + B → C). The product has more bonds than either starting material alone.
Elimination: One reactant splits into two products (A → B + C). A bond within the molecule breaks and fragments depart.
Substitution: Two reactants exchange parts to give two new products (A + B → C + D). One group comes in, another leaves.
Rearrangement: A single reactant reshuffles its bonds to give an isomeric product (A → B). No atoms are gained or lost, just repositioned.
Every organic reaction involves breaking and/or forming bonds. The two ways a bond can break define two broad categories of mechanism.
Homolytic cleavage splits the bonding pair evenly: one electron to each fragment. This produces radicals (species with unpaired electrons). Shown with single-headed (fishhook) curved arrows, each representing one electron.
Heterolytic cleavage sends both bonding electrons to one fragment. This produces a cation (the electron-poor fragment) and an anion (the electron-rich fragment). Shown with standard double-headed curved arrows, each representing two electrons.
Radical reactions proceed through three stages:
Initiation: Energy input (typically UV light or heat) breaks a bond homolytically to generate radicals. Example: Cl₂ + UV light → 2 Cl·
Propagation: A radical reacts with a stable molecule, forming a new bond and generating a new radical. The chain continues because each propagation step produces a fresh radical. Example: Cl· + CH₄ → HCl + ·CH₃, then ·CH₃ + Cl₂ → CH₃Cl + Cl·
Termination: Two radicals combine, destroying both unpaired electrons and ending the chain. Example: Cl· + Cl· → Cl₂. Termination is not a frequent product-forming step.
The overall reaction Cl₂ + CH₄ → HCl + CH₃Cl is a radical chain reaction. The propagation steps are where the useful products form. Radicals will be covered in greater depth later in the semester.
Radical chain reactions are how polyethylene (plastic bags, bottles) is manufactured: an initiator generates radicals that add to ethylene monomers in a propagation chain thousands of units long. Substitution reactions are central to pharmaceutical synthesis, where chemists swap functional groups on drug molecules to fine-tune their activity and solubility.
Students often confuse elimination with substitution. The key: in elimination, one reactant breaks apart (A → B + C). In substitution, two reactants exchange parts (A + B → C + D). Count the reactants and products.
Fishhook (single-headed) arrows and full (double-headed) curved arrows are frequently mixed up. Fishhook arrows move one electron (radical mechanisms). Full curved arrows move two electrons (polar mechanisms). Using the wrong arrow type will cost you marks.
Students sometimes think radicals are charged. They are not. A radical is neutral but has an unpaired electron. Ions (cations, anions) result from heterolytic cleavage, not homolytic.
Termination steps in radical reactions are often incorrectly treated as the main product-forming steps. The products of interest form during propagation. Termination simply ends the chain.
⚠️ You must be able to classify any reaction as addition, elimination, substitution, or rearrangement by counting reactants and products.
⚠️ Know the difference between homolytic and heterolytic cleavage, and which arrow type represents each.
⚠️ Be able to write out the three stages of a radical chain reaction (initiation, propagation, termination) with correct fishhook arrows.
⚠️ Know all functional groups in Table 5.1. They will appear on homework and exams even though they are not re-taught in lecture (LOs 5a-5e).
True or false: A substitution reaction produces one product from two reactants. False. That describes addition. Substitution gives two products from two reactants.
Fill in the blank: Homolytic cleavage produces two ______, while heterolytic cleavage produces a ______ and an ______. Radicals; cation; anion.
True or false: A double-headed curved arrow represents the movement of one electron. False. A double-headed arrow represents two electrons. A single-headed (fishhook) arrow represents one.
Fill in the blank: In a radical chain reaction, useful products are formed during the ______ step. Propagation.
True or false: Rearrangement reactions always involve at least two different reactant molecules. False. A rearrangement involves a single reactant reorganising its own bonds.
Q: Classify the following reaction: CH₂=CH₂ + HBr → CH₃CH₂Br. What type of reaction is this?
A: Addition. Two reactants combine to form a single product.
Q: In the radical chlorination of methane (Cl₂ + CH₄ → HCl + CH₃Cl), write the two propagation steps and use the correct arrow type.
A: Step 1, Cl· + H-CH₃ → HCl + ·CH₃. Step 2, ·CH₃ + Cl-Cl → CH₃Cl + Cl·. Both steps use single-headed (fishhook) arrows because they involve movement of one electron at a time.
Q: A molecule R-Br breaks its C-Br bond. Both bonding electrons leave with the bromine. What type of bond cleavage is this, and what products form?
A: Heterolytic cleavage. The products are R⁺ (a carbocation) and Br⁻ (bromide anion).
Q: What is the difference between a reaction intermediate and a transition state?
A: A reaction intermediate is a real (though short-lived) species that sits in an energy minimum between two transition states. A transition state is the highest-energy point along a single reaction step and cannot be isolated.
Q: Give one example each of a nucleophile and an electrophile from the lecture material.
A: Nucleophile examples include HO⁻, Br⁻, NH₂ (with a lone pair), or a pi bond. Electrophile examples include a carbocation (6 electrons, empty p orbital), a carbon bonded to a more electronegative atom (partial positive charge), or H-Cl (H is the electrophilic site).
This material connects directly to Ch. 2 functional groups, which you need to recognise on sight for every reaction in this course. The nucleophile/electrophile framework introduced here underpins substitution (SN1, SN2) and elimination (E1, E2) reactions you will study in detail later. Radical reactions will return later in the semester with more complex examples, including radical addition to alkenes.
Organic reactions, reaction types, addition reaction, elimination reaction, substitution reaction, rearrangement reaction, bond cleavage, homolytic cleavage, homolysis, heterolytic cleavage, heterolysis, radical, free radical, radical chain reaction, initiation, propagation, termination, curved arrow formalism, fishhook arrow, double-headed arrow, electron flow, CHEM 2301, organic chemistry I, Chapter 5, functional groups, Table 5.1