Reaction Types & Mechanisms

Most organic reactions are variations on four moves. Learn the moves and you can read almost any reaction.

High schoolIntro OrganicUni Year 1
⏱️ About 20 min

Organic chemistry can look like thousands of unrelated reactions. It is not. Almost everything is a version of four basic moves: swap a group, add across a double bond, kick a group out to make a double bond, or join two molecules and spit out water. Learn the four and the fog clears.

💡
The big idea: Organic reactions fall into a few recurring patterns — substitution, addition, elimination and condensation. Whether a molecule is saturated or unsaturated largely decides which pattern it can undergo, so the reaction type follows directly from the structure.
🎯 By the end, you'll be able to
  • Describe substitution, addition, elimination and condensation reactions
  • Link addition to unsaturated molecules and substitution to saturated ones
  • Explain condensation as joining two molecules while losing a small molecule (often water)
  • Predict which reaction type a given structure is likely to undergo
📎 Helpful to know first

Four moves to learn

A mechanism is the step-by-step story of which bonds break and form during a reaction. You do not need every mechanism at first — you need the four broad reaction types they belong to. Each one is a distinct kind of bookkeeping for atoms.

🔑 The four types at a glance
Substitution: one atom/group is swapped for another. Addition: atoms add across a double or triple bond, removing the unsaturation. Elimination: a small molecule leaves and a new double bond forms (the reverse idea of addition). Condensation: two molecules join and expel a small molecule, usually water.

Addition: the signature move of unsaturated molecules

Because alkenes and alkynes have reactive multiple bonds, their characteristic reaction is addition: a molecule adds across the C=C, and the double bond becomes a single bond. Adding hydrogen to ethene (hydrogenation) turns it into ethane — the very reaction that hardens vegetable oils into margarine.

\[ \ce{CH2=CH2 + H2 -> CH3CH3} \]
Addition: H₂ adds across the C=C double bond of ethene, saturating it to ethane.

Substitution: the move for saturated molecules

Saturated molecules have no multiple bond to add across, so instead one atom is substituted for another. When methane reacts with chlorine, a hydrogen is replaced by a chlorine atom to give chloromethane. Substitution swaps a group; the number of bonds on the carbon stays the same.

\[ \ce{CH4 + Cl2 -> CH3Cl + HCl} \]
Substitution: a hydrogen on methane is swapped for a chlorine atom.
✨ Structure predicts the reaction
This is the payoff: unsaturated → addition (there is a multiple bond to add across), saturated → substitution (no multiple bond, so a group must be swapped). Knowing whether a molecule is saturated already tells you its most likely reaction.

Elimination and condensation

Elimination is addition run backwards: a small molecule (such as water) is removed from a single-bonded molecule and a new double bond appears. Dehydrating an alcohol to an alkene is a classic elimination.

Condensation joins two molecules into a larger one while expelling a small molecule — very often water. When a carboxylic acid and an alcohol combine into an ester, a water molecule is released. This same join-and-release-water step builds proteins and other biological polymers.

\[ \ce{CH3COOH + CH3CH2OH <=> CH3COOCH2CH3 + H2O} \]
Condensation: an acid and an alcohol join to form an ester, releasing a molecule of water.
⚠️ Addition and condensation are not the same
Both make a bigger molecule, so they are easy to confuse. The tell: addition adds across a double bond and loses nothing — every atom ends up in the product. Condensation joins two separate molecules and expels a small molecule such as water.
📝 Worked example: Propene (CH₃CH=CH₂) reacts with hydrogen gas. What type of reaction is this, and what is the product?
  1. Look at the structure: propene has a C=C double bond, so it is unsaturated.
  2. Unsaturated molecules characteristically undergo addition.
  3. H2 adds across the C=C, converting the double bond to a single bond.
  4. The product is propane, CH3CH2CH3 — now fully saturated.
✓ An addition reaction; the product is propane (CH₃CH₂CH₃).
✏️ Practice: In the condensation of ethanoic acid with ethanol to form an ester, how many water molecules are released per ester formed?
H₂O molecule
Solution
  1. Condensation joins the two molecules and expels one small molecule.
  2. The –OH from the acid and an –H from the alcohol combine and leave as water.
  3. That releases exactly 1 water molecule for each ester formed.

Check your understanding

1. An unsaturated molecule (with a C=C bond) most characteristically undergoes which reaction?
The reactive C=C double bond lets atoms add across it, so addition is the signature reaction of unsaturated molecules. Saturated molecules react by substitution instead.
2. What defines a condensation reaction?
Condensation joins two molecules into a larger one while expelling a small molecule, usually water. This is how esters and biological polymers such as proteins form.
3. Why does methane (CH₄) react by substitution rather than addition?
Methane is saturated (only single bonds). With no C=C or C≡C to add across, it can only react by swapping one atom for another — substitution.
🔎 Related lessons
Previous: Isomerism Next: Biomolecules Functional Groups
✅ Key takeaways
  • Most organic reactions are substitution, addition, elimination or condensation.
  • Addition adds atoms across a C=C/C≡C and removes unsaturation — the move for unsaturated molecules.
  • Substitution swaps one group for another — the move for saturated molecules.
  • Elimination removes a small molecule to form a new double bond (addition in reverse).
  • Condensation joins two molecules and expels a small molecule, usually water.
➡️ One of these moves — condensation — is how life builds its giant molecules. Next, we scale up from small organics to the biomolecules that run every cell: carbohydrates, lipids, proteins and nucleic acids.
Want to test yourself on this? Try the Chemistry practice test →