Organic Chemistry · An Overview of Organic Reactions

Kinds of Organic Reactions

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

Every organic reaction can be assigned to one of four broad classes based on what happens to the molecular skeleton: addition, elimination, substitution, and . In an addition, two molecules join to form one; in an elimination, one molecule splits into two; in a substitution, two molecules swap partners; in a rearrangement, a single molecule reshuffles its own atoms. This classification is the first lens chemists use when they look at any reaction — before worrying about mechanisms, catalysts, or solvents, ask: what kind of bond change is happening?

The classes are not just academic labels. Each class has characteristic starting materials, typical conditions, and predictable products. An alkene under electrophilic conditions undergoes addition; an alkyl halide treated with a strong base may undergo elimination; a halide treated with a good may undergo substitution. Knowing the class tells you what to look for in the products and what kind of mechanism to expect — polar or radical — which the rest of this chapter develops.

Why this matters

Predicting products: If you recognize an alkene + HX as an addition, you can predict the haloalkane product; recognizing a suggests substitution or elimination.
Designing syntheses: Synthetic planning is organized around reaction classes — build a skeleton by additions and substitutions, introduce unsaturation by eliminations, and count carbons through rearrangements.
Reading mechanisms: Every mechanism step is itself an addition, elimination, substitution, or rearrangement; classifying steps makes curved-arrow mechanisms easier to follow.
Industrial and biological chemistry: Polymerization is a series of additions; metabolic pathways use eliminations (dehydration) and substitutions (methylation, acylation) continuously.
Exams: "What kind of reaction is this?" is a standard question format, and the answer drives everything that follows in the problem.

The college version

Core Concepts

Addition reactions

An combines two reactants into a single product: A + B → C. The typical substrates are compounds with π bonds — alkenes, alkynes, carbonyls — because a can break while two new σ bonds form. The classic example is addition of HBr to ethylene:

CH₂=CH₂ + HBr → CH₃CH₂Br

The π bond of the alkene is lost, and new C–H and C–Br bonds are formed; no atoms are lost. Addition reactions are common with electrophiles (HX, X₂, H₂O in acid) and with nucleophiles at carbonyl groups. When an addition uses the same reagent molecule twice (e.g., hydration of an alkyne), it is sometimes called an addition–elimination sequence, but the net class remains addition.

Elimination reactions

An is the reverse of addition: one reactant splits into two products, C → A + B. Typically, two adjacent atoms each lose a group, and a π bond forms between them. The classic example is dehydrohalogenation of an alkyl halide with a strong base:

CH₃CH₂Br + KOH → CH₂=CH₂ + KBr + H₂O

Note that the net event is loss of HBr from the haloalkane to form the alkene (the KOH supplies the base that removes the proton and the KBr is a byproduct salt). Eliminations are the standard way to make alkenes and alkynes from saturated precursors.

Substitution reactions

In a , two molecules exchange partners: A–B + C–D → A–C + B–D. The hallmark is that one atom or group on a molecule is replaced by another. The classic example is reaction of an alkyl halide with a nucleophile:

CH₃Br + OH⁻ → CH₃OH + Br⁻

The OH⁻ replaces Br⁻ on the methyl carbon. Substitutions dominate the chemistry of alkyl halides (Chapter 11) and of carbonyl compounds (acylation, alkylation). A substitution changes the functional group but keeps the carbon skeleton intact.

Rearrangement reactions

A rearrangement reaction changes the connectivity of a single molecule without changing its molecular formula: A → B (an ). Atoms move, but nothing is added or removed. The classic example is a carbocation rearrangement, where a hydride or alkyl group shifts to give a more stable carbocation — for example, a 1,2-hydride shift converts a secondary carbocation to a tertiary one. Rearrangements are common in addition reactions to alkenes (a carbocation intermediate may rearrange before the nucleophile attacks) and in some substitution reactions. Spotting a rearrangement matters because it changes the final skeleton — the product is an isomer of what you might naively predict.

How to classify a reaction

Count the reactant and product molecules and watch what happens to the skeleton:

Feature	Class
Two reactants → one product	Addition
One reactant → two products	Elimination
Two reactants → two products, partners exchanged	Substitution
One reactant → one product, same formula, new connectivity	Rearrangement

This atom-counting approach works at the net-reaction level. Within a mechanism, individual steps are classified the same way — a proton transfer is a substitution, a π-bond attack is an addition, and so on.

A note on reaction polarity and radicals

Class (addition/elimination/substitution/rearrangement) is about what the skeleton does; the mechanism (polar vs radical) is about how electrons move. Chapter 6 develops this distinction: polar reactions involve electron-rich and electron-poor partners, while radical reactions involve species with unpaired electrons. Every one of the four classes can proceed by either mechanism — you will meet radical additions, polar substitutions, and both flavors of elimination.

Common Confusions

Do Not Confuse	With	Difference
Addition	Elimination	Addition: 2 reactants → 1 product (π bond lost); elimination: 1 reactant → 2 products (π bond formed)
Substitution	Elimination	Both start from alkyl halides; substitution swaps the leaving group (skeleton intact), elimination removes H + X and makes a π bond
Rearrangement	Any isomer change	All rearrangements are isomerizations, but not all isomerizations are called rearrangements in the mechanistic sense (cis/trans isomerization is not a rearrangement)
Reaction class	Reaction mechanism	Class = skeleton change (addition/elimination/substitution/rearrangement); mechanism = electron movement (polar/radical). Same class can run by different mechanisms
Addition polymerization	Condensation polymerization	Addition polymers lose no small molecule per step; condensation polymers eliminate water or another small molecule
"One product, so it's not a substitution"	Substitutions with byproducts	Substitutions are 2 → 2 at the molecular level (the salt byproduct is often omitted in shorthand)
Hydration of an alkene	Hydration of an alkyne	Both are additions, but alkyne hydration can add twice or tautomerize to a carbonyl — read the conditions

Eli explains

The same idea, in plain words

Explain it like I’m 10

Reactions are like rearranging LEGO bricks. Addition: two small towers snap together into one bigger tower. Elimination: one tower breaks into two smaller ones. Substitution: two towers swap one brick each, so you get two new towers. Rearrangement: one tower stays the same size but you move a brick to a different spot. Every LEGO build is just these four moves repeated.

Worked example

Example 1: Classifying five reactions

For each reaction, state the class and justify it with an atom count.

Reaction A: CH₂=CH₂ + Cl₂ → ClCH₂CH₂Cl

Two reactants, one product, π bond consumed → addition.

Reaction B: CH₃CH₂OH → CH₂=CH₂ + H₂O (acid-catalyzed dehydration)

One reactant, two products, π bond formed → elimination.

Reaction C: CH₃CH₂Br + NaCN → CH₃CH₂CN + NaBr

Two reactants, two products, Br replaced by CN → substitution.

Reaction D: a 1,2-hydride shift in a carbocation — the cation (CH₃)₂CH–CH₂⁺ rearranges to (CH₃)₃C⁺

One reactant, one product, same formula, connectivity changed → rearrangement (a hydride moves from the adjacent carbon to the electron-poor center).

Reaction E: n CH₂=CH₂ → –(CH₂CH₂)ₙ– (polymerization)

Many identical addition steps, each 2 → 1 → addition (chain-growth polymerization).

Example 2: Stoichiometry of an addition with dimensional analysis

Question: How many grams of HBr are required to react completely with 28.0 g of ethylene in the addition CH₂=CH₂ + HBr → CH₃CH₂Br? Molar masses: C₂H₄ = 28.05 g/mol, HBr = 80.91 g/mol.

Step 1 — Write the mole ratio from the balanced equation. The reaction consumes ethylene and HBr in a 1:1 molar ratio:

$1 mol HBr 1 mol C 2 H 4 = 1$

Step 2 — Convert grams of ethylene to moles (formula: n = m/M):

$n(C 2 H 4) = 28.0 g 28.05 g/mol = 0.998 mol$

Step 3 — Convert to moles of HBr using the mole ratio, then to grams (formula: m = n · M):

$m(HBr) = 0.998 mol C 2 H 4 \times 1 mol HBr 1 mol C 2 H 4 \times 80.91 g HBr 1 mol HBr = 80.8 g$

Answer: about 80.8 g of HBr. This chain of conversion factors — g → mol → mol → g — is dimensional analysis in action.

Example 3: Predicting the class from the functional group

Question: Predict the class and likely product for each pair.

Alkene + H₂O (acid catalyst): alkenes have a π bond that can open, so expect addition → ethanol, CH₃CH₂OH (hydration, following Markovnikov's rule for the H placement on unsymmetrical alkenes).
2-Bromopropane + KOH (strong base, heat): an alkyl halide with β-hydrogens can lose HBr → elimination → propene, CH₃CH=CH₂.
2-Bromopropane + NaOH in water (nucleophile, no heat): OH⁻ can replace Br → substitution → 2-propanol, CH₃CH(OH)CH₃.
A secondary carbocation in an addition: check for a possible hydride shift to a more stable tertiary cation → rearrangement before the nucleophile adds.

The same starting material (2-bromopropane) can give elimination or substitution depending on conditions — class is determined by the reaction conditions as much as by the functional group.

Key takeaways

Four classes: addition (A + B → C), elimination (C → A + B), substitution (A–B + C–D → A–C + B–D), rearrangement (A → B, isomer).
Addition consumes a π bond; elimination creates one; substitution swaps a group; rearrangement changes connectivity only.
CH₂=CH₂ + HBr → CH₃CH₂Br is the prototype addition; CH₃CH₂Br + KOH → CH₂=CH₂ is the prototype elimination; CH₃Br + OH⁻ → CH₃OH + Br⁻ is the prototype substitution.
Carbocation 1,2-hydride/alkyl shifts are the classic rearrangements; watch for them in additions to alkenes.
Classify by counting: 2 → 1 addition; 1 → 2 elimination; 2 → 2 substitution; 1 → 1 rearrangement.
Class (skeleton change) is independent of mechanism (polar vs radical).
Polymers form by repeated addition (chain-growth) — the same addition class at every step.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

Name the four classes of organic reactions and give the net equation pattern for each.
Show answer
Addition (A + B → C), elimination (C → A + B), substitution (A–B + C–D → A–C + B–D), rearrangement (A → B, same formula).
Classify: CH₃CH=CH₂ + HBr → CH₃CHBrCH₃.
Show answer
Addition — one alkene plus HBr gives one haloalkane product (π bond consumed).
Classify: CH₃CH₂Cl + NaOH → CH₃CH₂OH + NaCl.
Show answer
Substitution — Cl is replaced by OH; two reactants, two products.
What distinguishes a rearrangement from the other three classes?
Show answer
A rearrangement changes connectivity within a single molecule without changing the molecular formula (nothing added or removed).
Why can the same alkyl halide undergo both substitution and elimination?
Show answer
The base/nucleophile can either remove a β-hydrogen (elimination) or attack the carbon bearing the leaving group (substitution); temperature, base strength, and solvent bias the competition.
How many grams of Cl₂ are needed to react with 56.0 g of ethylene (CH₂=CH₂ + Cl₂ → ClCH₂CH₂Cl)? Molar masses: C₂H₄ = 28.05 g/mol, Cl₂ = 70.90 g/mol.
Show answer
56.0 g C₂H₄ × (1 mol/28.05 g) × (1 mol Cl₂/1 mol C₂H₄) × (70.90 g/1 mol) = 141.6 g Cl₂.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Addition reaction: Two reactants combine into one product
Elimination reaction: One reactant splits into two products
Substitution reaction: Two molecules exchange groups
Rearrangement: One molecule changes connectivity, formula unchanged
π bond: A bond formed by side-by-side orbital overlap, part of a double or triple bond
Leaving group: A group that departs with its electron pair in substitution/elimination
Nucleophile: An electron-rich species that donates a pair of electrons
Isomerization: Conversion of one isomer into another

Sources & references

openstax.org — Organic Chemistry

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