Same atoms, different shapes: hydrocarbons and their isomers

How to use this simulation

1. Start with the slider on 4 and the bond type on alkane. The molecule in the middle is butane, C₄H₁₀, spinning slowly. Dark balls are carbon, pale balls are hydrogen, and the sticks are bonds.
2. On the right you see two thumbnails: butane and 2-methylpropane. Both are C₄H₁₀. Click the isomer buttons below the canvas to move the large model from one to the other and watch the name, the condensed formula and the shape change while the formula stays put.
3. Slide the carbons up to 6. The hydrogens follow the rule 2n + 2, and the number of isomers jumps to five.
4. Switch the bond type to alkene, then alkyne. Count the hydrogens in the readings panel: two fewer each time. The pi (π) bond counter goes up to 1, then 2.
5. Tick “Hide hydrogens” to see only the carbon skeleton. This is how organic chemists sketch structures in an exam.

The gas in the kitchen cylinder

The orange LPG cylinder in many kitchens is full of just two molecules, propane and butane. Petrol is a mix of larger ones. The candle wax, the plastic bag and the tar on a road are hydrocarbons too. All of them are built from the same two elements, carbon and hydrogen.

If it is only two elements, why are there thousands of compounds? Because carbon atoms can join to each other in chains, in branches and with single, double and triple bonds, and the same set of atoms can be arranged in more than one way. This page lets you build those arrangements and turn them around.

What a hydrocarbon is, and the three families

A hydrocarbon is a compound made of carbon and hydrogen only. If every carbon–carbon bond is single, it is saturated: each carbon is holding as many hydrogens as it can. These are the alkanes. If there is a double bond, it is an alkene, and if there is a triple bond, an alkyne. The last two are unsaturated, because they could take extra hydrogen atoms if the multiple bond were opened up.

The three families follow neat counting rules. For n carbons: alkanes have 2n + 2 hydrogens, alkenes have 2n, and alkynes have 2n − 2. Each extra bond in the carbon chain costs two hydrogens.

Reading the animation

The big model in the middle is built by the program from just the carbon skeleton. For each carbon it works out how many hydrogens are needed to make four bonds, then places everything in 3D using real bond lengths and angles. The scene is then tilted, given perspective, and sorted by depth so that near atoms hide far ones, and it spins slowly so you can see the shape from all sides.

The header shows the molecular formula and the IUPAC name. Underneath the model is the condensed formula, a one-line way to write the structure. The panel under the canvas lists the formula, the general formula, the number of carbons and hydrogens, the number of chain isomers, and the count of sigma and pi bonds.

Shapes and bonds: why the sticks look the way they do

A carbon atom with four single bonds points them to the corners of a tetrahedron, about 109.5° apart. That is why a chain of single bonds is a zigzag in 3D and not a flat line. When a carbon has a double bond, its three neighbours lie flat in a plane about 120° apart. With a triple bond, the carbon and its two neighbours line up straight, 180°.

The bonds also get shorter as they get multiple. In the model a C–C single bond is 1.54 Å, C=C is 1.34 Å and C≡C is 1.20 Å. A double bond is one sigma bond (σ, head-on overlap, the bond that holds the skeleton together) plus one pi bond (π, sideways overlap above and below). A triple bond is one σ plus two π. Look at the readings panel: every single connection is a σ bond, and the π counter only counts the extra ones.

Naming: the IUPAC recipe

The naming rules are short enough to memorise. Find the longest continuous carbon chain and use its stem. Number the chain from the end that gives the lowest numbers to the branches. Name each branch (one carbon is methyl, two is ethyl), put its number in front, and list the branches in alphabetical order. If the same branch appears twice, use di; three times, tri.

Take the branched C₄H₁₀ in the animation. The longest chain has three carbons, so the stem is propane, with one methyl branch on carbon 2: 2-methylpropane (written CH₃–CH(CH₃)–CH₃). The five-carbon one with a carbon at the centre holding four others is 2,2-dimethylpropane (CH₃–C(CH₃)(CH₃)–CH₃): the longest chain is again three carbons, and there are two methyl groups on carbon 2.

Structural isomers: same formula, different skeleton

Structural isomers are compounds with the same molecular formula but different connectivity. Butane and 2-methylpropane are both C₄H₁₀. In butane the four carbons are in a row; in 2-methylpropane one carbon sits in the middle with three others attached. They are different compounds with different properties, even though a formula would not tell them apart.

The number of alkane isomers climbs fast: 2 for C₄H₁₀, 3 for C₅H₁₂, 5 for C₆H₁₄, and then 9 for heptane and 18 for octane. Pick an alkane from 4 to 6 carbons in the simulation and click through the isomers. The six-carbon family is hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane.

In general, the more branched an isomer is, the more compact and spherical the molecule becomes, so it has less surface to stick to its neighbours. That is why a branched isomer usually boils at a lower temperature than its straight-chain cousin.

Try it yourself in the simulation

Do these in order. Guess first, then check.

• Experiment 1: set 1 carbon and watch methane. Count the hydrogens (4) and notice that all four bonds point to the corners of a tetrahedron.
• Experiment 2: for the alkane, step n from 1 to 6 and write down the hydrogen count each time. You should get 4, 6, 8, 10, 12, 14: it climbs by 2 per carbon.
• Experiment 3: set 2 carbons and switch between alkane, alkene and alkyne. Count the hydrogens (6, 4, 2). Ethane points its hydrogens in tetrahedral directions, ethene is flat, and ethyne is a straight line with the C–C bond getting shorter each time.
• Experiment 4: set 5 carbons, alkane. Flip through the 3 isomers. Which one looks most like a ball? (It is 2,2-dimethylpropane.)
• Experiment 5: set 6 carbons. Find all 5 isomers, and for each one write its longest chain length. Hexane keeps all six in a row; the others have a longest chain of five or four.

Solved problems

Every figure below is computed from the same rules as the animation.

Mistakes everybody makes

These cost marks every year.

• Counting hydrogens by eye instead of using the rule: every carbon needs four bonds in total.
• Choosing the wrong main chain. The longest continuous chain counts even when it bends around the page, so draw the skeleton carefully.
• Thinking a rotated drawing is a new isomer. If you can turn one structure into the other without breaking any bond, it is the same molecule.
• Forgetting that methane cannot have a double or triple bond, so the first alkene is ethene and the first alkyne is ethyne.
• Saying a double bond is two σ bonds. It is one σ and one π.
• Numbering the chain from the wrong end: the branches must get the lowest possible numbers.

Where you meet these molecules

Hydrocarbons are everywhere in everyday life.

• Cooking gas: methane, propane and butane burn cleanly and release a lot of heat.
• Petrol and diesel: mixtures of chains from about five to over a dozen carbons. A branched isomer burns more smoothly in an engine, which is why branching raises the octane number.
• Plastics: ethene is joined into long chains to make polythene, and propene to make polypropylene.
• Fruit ripening: ethene is a plant hormone, and a ripe banana in a bag speeds up the unripe ones around it.
• Welding: ethyne (acetylene) burns in oxygen with a very hot flame.

Exam corner

Try these without looking, then compare.

• Write the molecular formula of an alkane with 8 carbons (answer: C₈H₁₈) and an alkene with 5 carbons (answer: C₅H₁₀).
• Draw all 2 isomers of C₄H₁₀ and name them (answer: butane and 2-methylpropane).
• How many structural isomers does C₆H₁₄ have? Name them (answer: 5; hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane).
• Why are alkenes more reactive than alkanes? (The π bond is weaker and exposed, so it breaks easily to add other atoms.)
• What is the difference between a saturated and an unsaturated hydrocarbon? Give an example of each.

Revision: the whole page in a minute

Read this before the exam.

• Hydrocarbons contain only C and H. Alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂.
• Stems: meth, eth, prop, but, pent, hex. Endings: -ane, -ene, -yne.
• Carbon always makes four bonds. Single bond is tetrahedral (109.5°), double is planar (120°), triple is linear (180°).
• Double bond = 1 σ + 1 π. Triple bond = 1 σ + 2 π.
• Structural isomers share a formula but differ in how the carbons are joined: 2 for C₄H₁₀, 3 for C₅H₁₂, 5 for C₆H₁₄.
• IUPAC: longest chain, lowest numbers for branches, branches in alphabetical order.

Frequently asked questions