Substitution
One atom or group is replaced by another. In nucleophilic substitution, a nucleophile replaces a leaving group on a saturated carbon, such as hydroxide replacing bromide. It can happen in one step (SN2) or through a carbocation (SN1).
Electrophilic substitution happens on benzene rings: an electrophile replaces an H, so the ring stays aromatic. Nitration is the classic example. Free-radical substitution is how alkanes react with chlorine or bromine in UV light.
Addition
Two groups add across a double bond, and the double bond becomes single. Alkenes undergo electrophilic addition, such as with HBr, Br₂ or water. Carbonyl compounds undergo nucleophilic addition, such as with NaBH₄ or HCN.
Elimination
The reverse of addition: a small molecule (HBr, H₂O) is removed from neighbouring carbons and a C=C forms. Strong bases and heat favour elimination over substitution. See SN1 vs SN2 vs E1 vs E2.
Condensation and hydrolysis
A condensation joins two molecules and releases a small one, usually water. Making an ester from an acid and an alcohol is a condensation, and so is making a polyamide. Hydrolysis is the reverse: water (often with acid or base) breaks a bond, such as an ester being split back into an acid and an alcohol.
Oxidation and reduction
In organic chemistry, oxidation usually means gaining bonds to oxygen or losing bonds to hydrogen: primary alcohol → aldehyde → carboxylic acid, with acidified dichromate. Reduction is the reverse: NaBH₄ reduces aldehydes and ketones to alcohols, and LiAlH₄ also reduces esters and acids.
Acid–base reactions
A proton moves from an acid to a base. These are often the first step of a bigger mechanism, such as protonating an alcohol so water can leave.
Quick summary
| Type | What happens | Typical example |
|---|---|---|
| Substitution | One group replaces another | CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻ |
| Addition | Groups add across C=C or C=O | CH₂=CH₂ + HBr → CH₃CH₂Br |
| Elimination | Small molecule removed, C=C forms | CH₃CH₂Br + base → CH₂=CH₂ |
| Condensation | Two molecules join, water released | acid + alcohol → ester + H₂O |
| Hydrolysis | Water breaks a bond | ester + H₂O → acid + alcohol |
| Oxidation | Gain O or lose H | ethanol → ethanal → ethanoic acid |
| Reduction | Gain H or lose O | propanone + NaBH₄ → propan-2-ol |
| Acid–base | Proton transfer | RCOOH + OH⁻ → RCOO⁻ + H₂O |
For the reagents and conditions of each reaction, see the reagents cheat sheet. For all the mechanisms, see the reaction library.
Quick answers
What are the main types of organic reactions?
Substitution, addition, elimination, condensation, hydrolysis, oxidation, reduction and acid–base reactions. Most organic reactions are one of these.
What is the difference between substitution and addition?
In a substitution, one group replaces another, so the number of groups stays the same. In an addition, two groups add across a double bond and nothing is lost.
What is the difference between addition and elimination?
They are opposites. Addition adds atoms across a double bond and turns it into a single bond. Elimination removes atoms from neighbouring carbons and forms a double bond.
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