Chemistry 30 · Organic chemistry
Carbon compounds
Carbon bonds to itself four ways at once, which is why there are millions of organic compounds and only a handful of rules. Learn the families, the naming system, and the four reactions that keep coming back.
The families
The idea: a functional group is the reactive part of a molecule. Everything in a family behaves alike because they share that group, however long the carbon chain is.
| Family | Functional group | Name ends in | Example |
|---|---|---|---|
| Alkane | All single bonds (CₙH₂ₙ₊₂) | -ane | propane |
| Alkene | C=C double bond (CₙH₂ₙ) | -ene | propene |
| Alkyne | C≡C triple bond (CₙH₂ₙ₋₂) | -yne | propyne |
| Aromatic | Benzene ring | benzene, -benzene | methylbenzene |
| Alcohol | −OH | -ol | ethanol |
| Carboxylic acid | −COOH | -oic acid | ethanoic acid |
| Ester | −COO− between two carbons | -yl -oate | methyl ethanoate |
| Aldehyde | C=O at the end of the chain | -al | ethanal |
| Ketone | C=O within the chain | -one | propanone |
| Ether | −O− between two carbons | -yl -yl ether | diethyl ether |
| Amine | −NH₂ | -amine | ethylamine |
| Halide | −F, −Cl, −Br, −I | fluoro-, chloro-, bromo-, iodo- | chloroethane |
- Saturated means only single bonds — every carbon is holding as many hydrogens as it can. Unsaturated means there is a double or triple bond, which is where reactions happen.
- Aldehyde or ketone? Both have C=O. If the carbonyl carbon also carries a hydrogen, it is at the end of the chain and the compound is an aldehyde.
- Isomers share a molecular formula but not a structure. C₄H₁₀ is either butane or methylpropane, and they boil at different temperatures.
IUPAC naming
The idea: the name is a set of instructions for drawing the molecule. Longest chain, lowest numbers, alphabetical branches.
- Find the longest continuous carbon chain that contains the functional group. Its length gives the stem: meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-.
- Number the chain from the end that gives the functional group — or, failing that, the first branch — the lowest number.
- Name each branch with its position: methyl, ethyl, propyl, or a halogen prefix.
- Put branches in alphabetical order, using di-, tri- and tetra- for repeats (these prefixes do not count for alphabetizing).
- Add the ending for the family, with a number if the group could sit in more than one place.
Worked examples
- CH₃CH(CH₃)CH₂CH₃ — longest chain is four carbons (butane) with a methyl branch. Numbering from the nearer end puts it on carbon 2: 2-methylbutane.
- CH₃CH=CHCH₂CH₃ — five carbons with a double bond starting at carbon 2: 2-pentene.
- CH₃CH₂OH — two carbons and a hydroxyl group: ethanol.
Watch out: the longest chain is not always the one drawn in a straight line. Trace every path through the molecule before deciding, or you will name a branch that should have been part of the chain.
The reactions
The idea: what a molecule can do is decided by whether it has a double bond to open, a hydrogen to swap, or a group that can join with another group.
| Reaction | Happens to | What changes |
|---|---|---|
| Addition | Alkenes and alkynes | The multiple bond opens and atoms add to both carbons: C₂H₄ + Br₂ → C₂H₄Br₂ |
| Substitution | Alkanes and aromatics | An atom replaces a hydrogen: CH₄ + Cl₂ → CH₃Cl + HCl |
| Elimination | Alcohols and halides | A small molecule leaves and a double bond forms |
| Esterification | Carboxylic acid + alcohol | They join and water is released — a condensation reaction |
| Combustion | Any hydrocarbon | Complete burning gives CO₂ and H₂O; incomplete gives CO or soot |
Worked: balancing a combustion
C₃H₈ + O₂ → CO₂ + H₂O
Three carbons give 3CO₂, and eight hydrogens give 4H₂O. That totals 6 + 4 = 10 oxygen atoms, so 5 O₂ are needed:
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Carbon, then hydrogen, then oxygen last — always, because oxygen appears in both products.
Why alkenes react and alkanes mostly do not: the second bond in C=C is exposed and comparatively weak, so it is an easy target. Alkanes have nothing on offer but strong single bonds, which is why substitution needs UV light or heat to get started.
Polymers
The idea: a polymer is one small molecule repeated thousands of times. There are two ways to join the links, and they are told apart by whether anything is left over.
| Addition polymerization | Condensation polymerization | |
|---|---|---|
| Monomer needs | A double bond | Two reactive groups, such as −OH and −COOH |
| By-product | None — every atom ends up in the polymer | A small molecule, usually water, at each link |
| Examples | Polyethene, PVC, polystyrene | Nylon, polyester, and proteins in living things |
Ethene, CH₂=CH₂, becomes polyethene when many molecules open their double bonds and join end to end. Nothing is released, which is why the empirical formula of the polymer matches the monomer.