Hydrocarbon Families
Interactive 3D viewer of organic molecules — alkanes, alkenes, alkynes, and aromatics. Explore how the number of bonds between carbons changes molecular geometry, reactivity, and everyday applications.
Petroleum, natural gas, plastics, pharmaceuticals, DNA bases — a staggering fraction of chemistry and industry runs on molecules built from just two elements: carbon and hydrogen.
The reason carbon can do this is a coincidence of physics. Carbon has four valence electrons. It can form four bonds simultaneously — to other carbons, to hydrogen, to oxygen, nitrogen, or anything else. That single fact generates essentially unlimited molecular diversity. The simplest slice of this space is hydrocarbons: carbon and hydrogen only, no other elements.
Within hydrocarbons, the number of bonds between carbons determines everything: geometry, reactivity, what the molecule smells like, how it burns, what it polymerizes into. The viewer below lets you explore the four fundamental families.
Alkanes
Methane CH₄
The simplest hydrocarbon. One carbon, four hydrogens, perfect tetrahedral geometry. Natural gas is ~90% methane. Burns cleanly to CO₂ + H₂O.
The four families
A hydrocarbon's family is defined by its degree of unsaturation — how many double or triple bonds exist between carbons. Each extra bond removes two hydrogens and locks the carbons into a more constrained geometry.
| Family | Bond between C–C | Geometry | Rule |
|---|---|---|---|
| Alkane | Single bond only (σ) | Tetrahedral, 109.5° | All carbons sp³ hybridized. Maximum hydrogens. Freely rotating single bond. |
| Alkene | One double bond (σ + π) | Trigonal planar, 120° | The double bond locks two carbons and their substituents into a flat plane. The π bond breaks in addition reactions. |
| Alkyne | Triple bond (σ + 2π) | Linear, 180° | Two π bonds force perfect linearity. Highly reactive and energy-dense. |
| Aromatic | 1.5 bonds (delocalized) | Hexagonal planar, 120° | Electrons delocalized over the whole ring. Unusually stable — reactions tend to substitute rather than add to preserve the ring. |
Why structure dictates properties
The number of carbon-carbon bonds is not cosmetic. It changes:
Geometry. Switch from a single to a double bond, and the attached atoms jump from tetrahedral to planar. Propane bends at ~109°; ethylene is flat. This has immediate consequences for how molecules pack, fit into enzyme pockets, and interact with each other.
Reactivity. The π bonds in alkenes and alkynes are high-energy and exposed. They are prime targets for electrophilic addition: Br₂ adds across an alkene double bond in seconds at room temperature. Alkanes have no such weak point — they need high temperatures and radical initiators to react. This is why natural gas (methane, an alkane) needs a spark to ignite, while cooking oil fumes (with unsaturated fatty acid chains) can auto-ignite at lower temperatures.
Physical state. Each CH₂ added to an alkane chain raises the boiling point by roughly 30 °C, because larger molecules have stronger van der Waals dispersion forces. Methane (1 carbon) is a gas; pentane (5 carbons) is a liquid; octadecane (18 carbons) is a solid wax at room temperature. This predictable staircase is the homologous series.
Energy density. The C–H bond is one of the most energy-dense chemical bonds. More C–H bonds per kilogram = more fuel energy. This is why hydrogen-rich alkanes are the best hydrocarbon fuels by weight; the most saturated structure carries the most burnable hydrogen.
Why it matters
| Domain | The hydrocarbon story |
|---|---|
| Energy | Natural gas is ~90% methane. Refinery fractions (LPG, gasoline, diesel, jet fuel, heavy fuel oil) are sorted alkane chains by boiling point. Combustion of each releases CO₂ + H₂O. The energy economy is almost entirely alkane chemistry. |
| Polymers | Polyethylene (your plastic bags) is ethylene — the alkene — polymerized by opening its π bond and linking millions of carbons into a chain. Polypropylene, polyvinyl chloride (PVC), polystyrene, Teflon: all built from the π bond of an alkene or alkyne monomer. |
| Pharmaceuticals | Nearly every drug contains a benzene ring or a derivative. The aromatic ring is conformationally rigid, planar, flat, and hydrophobic — ideal for fitting into protein binding pockets. Over 60% of approved drugs contain at least one aromatic ring system. |
| Materials | Graphene, carbon fiber, and carbon nanotubes are all sp² aromatic carbon extended into sheets or cylinders. Their extraordinary stiffness comes from the same delocalized π electrons that stabilize benzene. (For how this same carbon packs into solid crystal lattices — diamond, graphite, graphene — see the Crystal Structure Explorer.) |
Ethylene is produced at ~200 million tonnes per year — the most-produced organic compound on Earth. Almost all of it goes to polyethylene.
Who should care, and how to think about it
Hydrocarbon families are the starting point of chemical process engineering.
If you work with fuels, lubricants, polymers, or solvents, you are working with specific fractions of the alkane family. Cracking (breaking long chains into short ones) and reforming (converting alkanes to aromatics) are the two core refinery operations. Understanding why a C12 alkane has a higher boiling point than a C8 alkane — van der Waals forces scaling with molecular contact area — tells you why atmospheric distillation separates crude oil at all.
If you work with polymers, the alkene double bond is your workhorse. Addition polymerization needs a π bond to open. The difference between HDPE (linear polyethylene, tight packing) and LDPE (branched, loose packing) comes down to the same structural argument: chain geometry determines crystallinity, which determines density, stiffness, and permeability.
Cheminformatics and drug discovery pipelines are built around exactly these structural representations.
SMILES notation (the standard string encoding for molecules) encodes atom types and bond orders: CC is ethane, C=C is ethylene, C#C is acetylene, c1ccccc1 is benzene (lowercase c signals aromatic). Molecular fingerprinting methods like Morgan / ECFP encode circular neighborhoods of bonds — alkyl chains produce very different fingerprints from aromatic rings, even if the molecular weight is similar.
Graph neural networks treat molecules as graphs where nodes are atoms and edges are bonds. Bond order (1, 2, 3, aromatic) is a standard edge feature. The same architecture you use for message-passing on social networks is what protein structure prediction and molecular property prediction pipelines use. The bond types you see in this viewer map directly to those edge features.
The four hydrocarbon families span three separate multi-trillion-dollar markets.
Alkanes underpin the global energy system — oil and gas is a $3+ trillion annual revenue industry. The long-run thesis question is about the rate at which electrification displaces alkane combustion, not whether it does. Short-chain alkanes (methane, ethane, propane) have a longer tail as chemical feedstocks even after fuel displacement.
Alkenes are the feedstock for the entire polymer industry. Ethylene → polyethylene; propylene → polypropylene. The olefin-to-polymer chain is one of the highest-margin segments of petrochemicals. Biobased ethylene (from ethanol dehydration) is growing as a drop-in replacement that preserves the downstream polymer value chain with lower carbon footprint.
Aromatics (benzene, toluene, xylene — the BTX stream) flow into plastics, synthetic fibers, and pharmaceuticals. Paraxylene → PET (plastic bottles, polyester clothing). The BTX market is roughly $100B/year and is structurally tied to refinery economics.
The four families are the grammar of organic chemistry.
Carbon is the only atom that reliably chains to itself in long strings at normal temperatures. Silicon is below carbon on the periodic table and forms a similar valence, but Si–Si bonds are much weaker than C–C bonds and the Si–O bond is so strong that silicon naturally ends up as oxide minerals instead of chains. Life is carbon-based, not silicon-based, because carbon chains are thermodynamically stable in the presence of water; silicon chains would hydrolyze.
The reason benzene is special is that its 6 π electrons fit exactly into three bonding molecular orbitals — a consequence of a quantum mechanical rule called Hückel's rule (4n + 2 π electrons, n = 1 for benzene). Every other number of delocalized electrons would either leave an antibonding orbital occupied (destabilizing) or leave an orbital half-filled (reactive). Six is the magic number for stability. This is why nature's most stable organic skeleton is the six-carbon ring.
Reading the viewer
| Control | What it does |
|---|---|
| Family tabs | Filter molecules by the number of C–C bonds (Alkanes / Alkenes / Alkynes / Aromatics) |
| Molecule buttons | Load the selected molecule from PubChem (3D conformer data; may take 1–2 seconds) |
| Space-filling / Ball & stick | Space-filling mode scales atom spheres to van der Waals radii; ball & stick reduces them to show bond geometry more clearly |
| Auto-rotate toggle | Pause rotation to inspect a specific orientation |
| Drag | Orbit around the molecule |
| Scroll / pinch | Zoom in and out |
| Top / Front / Corner | Jump to axis-aligned views |
| ↺ | Reset camera to default position |
| Color legend | CPK convention: dark gray = carbon, white = hydrogen, red = oxygen, blue = nitrogen |
| Bond colors | Light gray = single bond; blue = double bond; cyan = triple bond; amber = aromatic |
Bond color and bond order
The viewer draws multiple cylinders for double and triple bonds (parallel offset lines), and uses a warm amber color for aromatic bonds to indicate that benzene's bonds are intermediate — not quite single, not quite double — due to electron delocalization.
What hydrocarbons can't tell you
| What you want to know | Why hydrocarbons alone don't tell you | What you actually need |
|---|---|---|
| Biological activity | Most biologically active molecules contain nitrogen, oxygen, or sulfur heteroatoms. The aromatic rings in drugs are platforms; the heteroatoms are the pharmacophores that bind proteins. | Functional group analysis: amines, carbonyls, alcohols, thiols |
| Polymer properties | Chain length and branching matter as much as monomer type. Linear C₁₀₀₀ polyethylene and branched C₁₀₀₀ LDPE have different crystallinity, melting points, and mechanical properties. | Molecular weight distribution, tacticity, degree of branching |
| Combustion products | Complete combustion of pure alkanes gives CO₂ + H₂O. Real fuels have sulfur, nitrogen, and aromatic components. Their combustion gives NOₓ, SO₂, and particulate matter — the actual emission problems. | Fuel composition, combustion modeling |
| Aqueous behavior | Hydrocarbons are hydrophobic. To understand how molecules behave in biological systems (which are mostly water), you need polarity, hydrogen bonding, and amphiphilicity. | Log P (partition coefficient), pKa, solubility |
From hydrocarbons to functional groups
Adding one non-hydrocarbon atom to a hydrocarbon changes the chemistry completely. Each class of atom (O, N, S, halogens) at specific positions produces a functional group — a reactive site with predictable behavior regardless of what hydrocarbon scaffold it is attached to.
| Functional group | Atom(s) added | Example | Key chemistry |
|---|---|---|---|
| Alcohol (–OH) | Oxygen (single bond) | Ethanol | Hydrogen bonding, oxidation to aldehyde/acid, dehydration to alkene |
| Ether (–O–) | Oxygen (bridging) | Diethyl ether | Inert to most reagents, good solvent, flammable |
| Aldehyde (–CHO) | Oxygen (double bond, terminal) | Formaldehyde | Highly reactive, oxidized to carboxylic acid, reduced to alcohol |
| Carboxylic acid (–COOH) | O and OH | Acetic acid | Acidic, forms esters with alcohols, forms amides with amines |
| Amine (–NH₂) | Nitrogen | Methylamine | Basic, forms amides, pharmaceutical relevance everywhere |
This is covered in depth in the next page of the series: Hydrocarbon Functional Groups.
Glossary
| Term | Definition |
|---|---|
| Hydrocarbon | A molecule containing only carbon and hydrogen |
| Alkane | Hydrocarbon with only single C–C bonds; general formula CₙH₂ₙ₊₂; also called saturated hydrocarbon |
| Alkene | Hydrocarbon with at least one C=C double bond; general formula CₙH₂ₙ |
| Alkyne | Hydrocarbon with at least one C≡C triple bond; general formula CₙH₂ₙ₋₂ |
| Aromatic | Hydrocarbon with a ring of delocalized π electrons satisfying Hückel's rule; benzene is the archetype |
| Hybridization | Mixing of atomic orbitals to form equivalent bonding orbitals. sp³ = 4 bonds, tetrahedral; sp² = 3 bonds, trigonal planar (one π); sp = 2 bonds, linear (two π) |
| Degree of unsaturation | Count of rings + multiple bonds in a molecule. Each double bond = 1; each triple bond = 2; each ring = 1 |
| Homologous series | A family of compounds differing by CH₂ units, with smoothly varying properties (boiling point, density, etc.) |
| π bond | Bond formed by side-on overlap of p orbitals. Weaker than a σ bond, more reactive, and the site of addition reactions |
| CPK coloring | Standard molecular color convention: H=white, C=dark gray, O=red, N=blue, S=yellow |
| Delocalization | Electrons spread over more than two atoms, as in benzene. Stabilizes the molecule beyond what localized bond models predict |
| Van der Waals forces | Weak, temporary dipole–dipole interactions between all molecules. Scale with molecular surface area — larger molecules have higher boiling points |
| sp³ | Carbon bonded to four atoms via single bonds; tetrahedral geometry |
| sp² | Carbon involved in one double bond; three σ bonds in a plane, one p orbital free for π system |
| sp | Carbon in a triple bond; two σ bonds, linear geometry, two p orbitals form two π bonds |
Sources and further reading
- IUPAC nomenclature for hydrocarbons: goldbook.iupac.org
- PubChem 3D conformer data: pubchem.ncbi.nlm.nih.gov — all molecule coordinates in this viewer are fetched from PubChem's 3D SDF endpoint
- Clayden, Greeves, Warren — Organic Chemistry (2nd ed.) — the standard undergraduate text; chapters 4–7 cover hybridization, alkenes, and alkynes systematically
- Atkins, de Paula — Physical Chemistry — chapter on molecular orbital theory explains why aromatic systems are stable
- Ethylene production statistics: IHS Markit Petrochemicals report; also mirrored in various World Bank and IEA datasets
Crystal Structure Explorer
Interactive 3D viewer of common crystal lattice structures — FCC, BCC, diamond cubic, rock salt, and silicon. Explore how atomic arrangement determines a material's properties.
Hydrocarbon Functional Groups
Interactive 3D viewer of hydrocarbon derivatives organized by functional group — alcohols, carbonyls, acids, amines, and commercial chemicals. See how adding oxygen, nitrogen, or sulfur to a hydrocarbon scaffold changes everything.