Source: Chapter 5 lecture slides, Organic Chemistry I (University of Minnesota Twin Cities) | LOs 5a–5h
Tags: organic reactions, reaction mechanisms, functional groups, addition, elimination, substitution, rearrangement, homolytic cleavage, heterolytic cleavage, radical reactions, CHEM 2301, orgo 1
Difficulty: Intermediate | Prerequisites: Chapters 1–4 (bonding, Lewis structures, electronegativity, functional group basics from Ch. 2)
Chapter 5 is where organic chemistry shifts from "what are these molecules" to "what do these molecules do." You are moving from structure and nomenclature into reactivity, which is the central theme of the rest of the course. This chapter introduces the four major reaction classes, explains how bonds break and form at the electron level, and gives you the vocabulary (nucleophile, electrophile, leaving group) you will use in every mechanism from here on. If you are comfortable with Lewis structures, electronegativity trends, and the functional groups from Chapters 2 and 5, you are ready for this material.
Organic reactions fall into four types: addition, elimination, substitution, and rearrangement. Bonds break either homolytically (one electron each, giving radicals) or heterolytically (both electrons to one side, giving ions). Most of the reactions you will study in this course are polar reactions driven by electron-rich nucleophiles attacking electron-poor electrophiles, and understanding that flow of electrons is the key to drawing mechanisms.
Functional group
A specific arrangement of atoms within a molecule that determines its chemical reactivity and properties. Think of it as the reactive "hot spot" on an otherwise inert carbon skeleton.
Addition reaction
A reaction in which two reactants combine to form a single product (A + B → C). In simple terms, two things join together and nothing leaves.
Elimination reaction
A reaction in which a single reactant splits into two products (A → B + C). The molecule sheds atoms or groups to form a new pi bond.
Substitution reaction
A reaction in which two reactants exchange parts to generate two new products (A + B → C + D). One group walks in, another walks out.
Rearrangement reaction
A reaction in which a single reactant reorganises its bonds to form an isomeric product (A → B). The molecular formula stays the same, but the connectivity changes.
Homolytic cleavage (homolysis)
Bond breaking in which each fragment keeps one electron from the shared pair, producing two radicals. Think of it as an even split.
Heterolytic cleavage (heterolysis)
Bond breaking in which both electrons from the shared pair go to one fragment, producing a cation and an anion. One side takes everything.
Radical
A species with an unpaired electron, formed by homolytic cleavage. Radicals are reactive and short-lived.
Nucleophile (Nu⁻)
An electron-rich species that donates a pair of electrons to form a new bond. In simple terms, the nucleophile is the "electron giver."
Electrophile (E⁺)
An electron-poor species that accepts a pair of electrons to form a new bond. The electrophile is the "electron taker."
Leaving group (LG)
An atom or group that departs with the bonding electrons during a reaction, typically forming a stable anion or neutral molecule.
Reaction intermediate
A species that forms during a multi-step reaction but is consumed before the final product appears. It is less stable than either starting material or product and exists only transiently.
Curved arrow (double-headed)
A notation showing the movement of two electrons from a source (nucleophile or bond) to a destination (electrophile). Used in polar (ionic) mechanisms.
Fishhook arrow (single-headed)
A notation showing the movement of one electron. Used in radical mechanisms.
Carbocation
A positively charged carbon with an empty p orbital. Stability increases with substitution: methyl < 1° < 2° < 3°.
Functional groups were introduced in Chapter 2 and expanded in Chapter 5. They will not be re-taught in lecture, but they appear on homework and exams. You need to recognise these on sight.
Key classes to know from Table 5.1:
Alcohol (ROH) – e.g. ethanol (CH₃CH₂OH)
Alkyl halide (RCl, RBr, etc.) – e.g. chloroethane
Amine (RNH₂) – e.g. ethanamine
Epoxide (three-membered ring with oxygen) – e.g. oxirane
Ether (ROR) – e.g. diethyl ether
Nitrile (RC≡N) – e.g. propanenitrile
Thiol (RSH) – e.g. ethanethiol
Sulfide (RSR) – e.g. dimethyl sulfide
Aldehyde (RCHO) – e.g. ethanal
Ketone (RCOR) – e.g. 2-butanone
Carboxylic acid (RCOOH) – e.g. ethanoic acid
The pattern: each functional group is defined by a specific heteroatom arrangement or bond type attached to the carbon framework. Learn to spot the group and name the class.
Every organic reaction you meet in this course falls into one of four categories. Learn to classify a reaction by counting how many reactants and products there are and whether the atoms have rearranged.
1. Addition (A + B → C)
Two reactants combine into a single product. A pi bond (or ring) in the starting material is typically consumed to form new sigma bonds. You will see this constantly with alkenes and alkynes.
2. Elimination (A → B + C)
A single reactant loses atoms or groups to split into two products. This usually creates a new pi bond (a double or triple bond forms where there was none).
3. Substitution (A + B → C + D)
Two reactants exchange parts to give two new products. One group on the substrate is replaced by another. The example from the lecture slides: an alkyl chloride reacting with hydroxide to replace Cl with OH is a substitution.
4. Rearrangement (A → B)
A single reactant reshuffles its bonds to produce an isomeric product. The molecular formula does not change, but the connectivity does. Carbocation rearrangements (hydride shifts and methyl shifts) are the main examples in this chapter.
A reaction mechanism describes exactly which bonds break and form and in what order. The first distinction to make is how the bond breaks.
Homolytic cleavage → Radical reactions
The two bonding electrons split evenly: one electron goes to each fragment.
Produces two radicals (species with unpaired electrons).
Shown with single-headed (fishhook) curved arrows, each arrow representing the movement of one electron.
Requires energy input to start (often UV light or heat).
Heterolytic cleavage → Polar (ionic) reactions
Both bonding electrons go to one fragment.
Produces a cation (electron-poor) and an anion (electron-rich).
Shown with double-headed curved arrows, each arrow representing the movement of two electrons.
Favoured in polar solvents and with polar bonds.
The type of cleavage determines the entire mechanism pathway. Homolysis leads to radical chain reactions. Heterolysis leads to polar reactions involving nucleophiles and electrophiles.
Radical reactions proceed through a three-stage chain mechanism. The lecture example is the chlorination of methane:
Cl₂ + CH₄ → (UV light) → HCl + CH₃Cl
1. Initiation: radical formation
Energy input (UV light or heat) breaks a weak bond homolytically.
Example: Cl–Cl → 2 Cl·
This step generates the radicals that start the chain.
2. Propagation: the chain continues
A radical reacts with a stable molecule to form a new radical.
Step A: Cl· + CH₄ → HCl + ·CH₃ (the chlorine radical abstracts a hydrogen)
Step B: ·CH₃ + Cl₂ → CH₃Cl + Cl· (the methyl radical abstracts a chlorine)
Each propagation step produces a radical that feeds back into the next step. This is what makes it a chain reaction.
The products of the overall reaction (HCl and CH₃Cl) are formed during propagation, not termination.
3. Termination: radicals recombine
Two radicals collide and form a bond, ending the chain.
Example: Cl· + Cl· → Cl₂
Termination is statistically rare because radical concentrations are low.
This step does not contribute meaningfully to product formation.
Students often think the termination step of a radical reaction is where the products form. It is not. The products (e.g. HCl and CH₃Cl) are formed during the propagation steps.
Students confuse single-headed and double-headed curved arrows. Single-headed (fishhook) arrows move one electron and are used in radical mechanisms. Double-headed arrows move two electrons and are used in polar mechanisms. Using the wrong arrow type on an exam will cost marks.
Students sometimes believe that a substitution reaction involves only one reactant. A substitution requires two reactants (A + B → C + D), with one group entering and one leaving.
Students occasionally assume that rearrangement reactions change the molecular formula. They do not. A rearrangement produces an isomer with the same formula but different connectivity.
⚠️ Functional groups will appear on the exam even though they are not re-taught in lecture. Know them cold.
⚠️ Be able to classify any reaction as addition, elimination, substitution, or rearrangement by inspecting reactants and products.
⚠️ Curved arrow notation is the language of mechanisms. You will be asked to draw arrows showing electron flow, and using the wrong arrow type (single-headed vs double-headed) is a common error that examiners penalise.
⚠️ Know all three stages of a radical chain reaction and be able to write out each step for the chlorination of methane.
True or False: Homolytic cleavage produces ions. (False, it produces radicals.)
Fill in the blank: A reaction in which A + B → C is classified as an ______ reaction. (Addition.)
True or False: Double-headed curved arrows represent the movement of one electron. (False, they represent two electrons.)
Fill in the blank: In a radical chain reaction, the products are formed during the ______ step. (Propagation.)
True or False: A rearrangement reaction changes the molecular formula of the reactant. (False, it produces an isomer with the same formula.)
Q: Classify the following reaction: tert-butyl chloride + HO⁻ → tert-butyl alcohol + Cl⁻
A: This is a substitution reaction. Two reactants exchange parts to give two products: the Cl leaving group is replaced by the OH nucleophile.
Q: In the radical chlorination of methane, write the two propagation steps.
A: Step 1: Cl· + CH₄ → HCl + ·CH₃. Step 2: ·CH₃ + Cl₂ → CH₃Cl + Cl·. Each step consumes one radical and produces another.
Q: What type of curved arrow would you use to show the movement of electrons in a polar mechanism, and why?
A: A double-headed curved arrow, because polar mechanisms involve the movement of electron pairs (two electrons at a time), flowing from the nucleophile toward the electrophile.
Q: Explain the difference between homolytic and heterolytic bond cleavage.
A: Homolytic cleavage splits the bonding pair evenly, giving one electron to each fragment and producing two radicals. Heterolytic cleavage gives both electrons to one fragment, producing a cation and an anion.
Q: Why is the termination step of a radical reaction not the main product-forming step?
A: Radicals are present in very low concentrations, so the probability of two radicals colliding is small. The products form during propagation, where radicals react with abundant stable molecules.
This material connects directly to Chapters 6–10, where you will apply these reaction classes to specific functional groups (alkenes, alkynes, alkyl halides). The nucleophile/electrophile framework introduced here is the basis for understanding SN1, SN2, E1, and E2 mechanisms. Radical reactions return in Chapter 10 with radical halogenation of alkanes and allylic systems.
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