Organic molecules can look complicated, but reactions almost always happen at the functional group while the rest of the carbon skeleton comes along unchanged. That means you do not have to memorize thousands of reactions. Learn a small set of reaction types, learn which functional groups undergo each one, and you can predict the product of a reaction you have never seen. For every reaction, ask three questions: which functional group is reacting, what reagent or condition is present, and which bonds break and form?
| Reaction type | What happens | Example |
|---|---|---|
| Addition | Atoms add across a C=C or C≡C; the multiple bond becomes a single bond; one product | |
| Substitution | One atom or group replaces another; two products | |
| Elimination | Atoms are removed from neighbouring carbons, forming a C=C | |
| Oxidation | A carbon gains bonds to O or loses H | |
| Esterification | Carboxylic acid + alcohol form an ester and water | |
| Hydrolysis | Water splits a molecule (the reverse of esterification) | |
| Combustion | Burning in oxygen |
Combustion is the most familiar organic reaction: hydrocarbons and most other organic compounds burn. With plenty of oxygen, complete combustion turns all the carbon into carbon dioxide and all the hydrogen into water, releasing a lot of energy, which is why alkanes are fuels: . With too little oxygen, incomplete combustion also forms carbon monoxide (a toxic, colourless, odourless gas) and soot (carbon). That is why fuel-burning appliances need ventilation and homes need carbon monoxide alarms. To balance a combustion equation, balance C first, then H, then O last, doubling everything if you end up needing half an .
Addition Reactions
- Hydrogenation: an alkene plus hydrogen, with a metal catalyst such as Pt, Pd or Ni, gives an alkane: An alkyne can add two .
- Halogenation: an alkene plus or gives a dihaloalkane: ethene + bromine gives 1,2-dibromoethane. Orange-brown bromine water turns colourless when it reacts, so this is a quick test for C=C or C≡C bonds.
- Hydrohalogenation: an alkene plus HCl or HBr gives a haloalkane: ethene + HBr gives bromoethane.
- Hydration: an alkene plus water, with an acid catalyst, gives an alcohol: ethene + water gives ethanol.
- Addition only happens at multiple bonds. Alkanes are already saturated, and benzene resists addition because it would lose its stable ring of shared electrons.
Substitution and Elimination
In a substitution, one atom or group swaps places with another, so there are two reactants and two products. Alkanes react with chlorine or bromine only when UV light supplies energy: gives chloromethane. Benzene also undergoes substitution rather than addition: with bromine and an iron(III) bromide catalyst it forms bromobenzene and HBr, keeping its ring intact. A haloalkane heated with aqueous hydroxide swaps its halogen for –OH: An elimination is the reverse of an addition: a small molecule is removed from two neighbouring carbons and a double bond forms between them. Heating an alcohol with concentrated sulfuric acid removes (dehydration): ethanol gives ethene. Heating a haloalkane with a strong base dissolved in ethanol removes H and the halogen, giving an alkene.
Oxidation of Alcohols
| Alcohol type | The –OH carbon is bonded to | Example | Product with an oxidizing agent |
|---|---|---|---|
| Primary | one other carbon (two H atoms on it) | ethanol, | aldehyde (ethanal, ), then carboxylic acid (ethanoic acid, ) |
| Secondary | two other carbons (one H on it) | propan-2-ol, | ketone (propanone, ); no further oxidation |
| Tertiary | three other carbons (no H on it) | 2-methylpropan-2-ol, | no reaction |
Common oxidizing agents are acidified potassium dichromate, which changes from orange to green as it reacts, and acidified potassium permanganate, which changes from purple to colourless. Chemists write the oxidizing agent as [O]. A primary alcohol first forms an aldehyde; if the aldehyde stays in the mixture it is oxidized further to a carboxylic acid, so to collect the aldehyde it is distilled off as it forms (it boils lower than the alcohol because it cannot hydrogen bond). A tertiary alcohol leaves dichromate orange, which is how you can tell the three alcohol types apart in the lab.
Esterification and Hydrolysis
Esterification is a condensation reaction: two molecules join and a small molecule (water) is released. It needs heat and a few drops of concentrated sulfuric acid as a catalyst, and it is reversible, so the mixture never turns completely into ester. Hydrolysis runs the reaction backward: water splits the ester into its carboxylic acid and alcohol, helped by an acid catalyst. Hydrolysis with a base such as NaOH goes to completion and gives the salt of the acid instead: Base hydrolysis of fats, which are esters, is how soap is made (saponification). Amides can be hydrolyzed in the same way, giving a carboxylic acid and an amine or ammonia.