What Is a Hydrocarbon? Carbon Chains Made Simple

Organic chemistry looks intimidating from the outside: thousands of compounds, strange names, mechanisms with curly arrows everywhere. But almost all of it is built on one small idea, and the simplest molecules in the whole subject are the place to meet it. Those molecules are the hydrocarbons.

The short answer: a hydrocarbon is a compound made of carbon and hydrogen only — nothing else. They matter because carbon can bond to itself over and over, so a handful of atoms builds an endless family of chains, branches and rings that forms the skeleton of every organic molecule.

Why carbon, of all elements

Carbon sits in group 14 with four valence electrons, which means it forms four covalent bonds — the most it can make while staying neutral and stable. Two things follow from that, and together they explain organic chemistry.

First, four bonds is enough to build outward in three dimensions rather than just forming a short chain that runs out of room. Second, the carbon–carbon single bond is exceptionally strong compared with the equivalent bond in neighbouring elements — about 347 kJ/mol, against roughly 222 for Si–Si, 163 for N–N and 146 for O–O — so long carbon chains don't spontaneously fall apart. This self-linking ability has a name: catenation. Silicon sits directly below carbon and has the same four valence electrons, but Si–Si bonds are much weaker, which is why there is no silicon-based organic chemistry to speak of.

Hydrogen's job is simpler. It forms exactly one bond, so it caps off every carbon valence that isn't used for another carbon. A hydrocarbon is, in effect, a carbon skeleton with hydrogen filling all the gaps.

The four families

Family	Bonding	General formula	Simplest member
Alkanes	C–C single bonds only	CₙH₂ₙ₊₂	Methane, CH₄
Alkenes	contains a C=C double bond	CₙH₂ₙ (one C=C, no ring)	Ethene, C₂H₄
Alkynes	contains a C≡C triple bond	CₙH₂ₙ₋₂ (one C≡C, no ring)	Ethyne, C₂H₂
Aromatics	a benzene-type delocalised ring	varies	Benzene, C₆H₆

Cyclic alkanes are the odd one out: closing a ring costs two hydrogens, so cycloalkanes share the formula CₙH₂ₙ with the alkenes. Cyclohexane (C₆H₁₂) and hex-1-ene (C₆H₁₂) are different compounds with identical formulas — your first encounter with isomerism.

The first four alkanes are worth memorising, because their roots appear in thousands of names: methane (1 carbon), ethane (2), propane (3), butane (4). After that the roots are mostly Greek number-words: pentane, hexane, heptane, octane, nonane (this one's Latin), decane.

How hydrocarbons behave

Because carbon and hydrogen have very similar electronegativities (2.55 and 2.20), C–H bonds are barely polar and hydrocarbons are nonpolar molecules. Three consequences you can predict from that alone:

• They don't dissolve in water. Water molecules hydrogen-bond strongly to each other and gain nothing by mixing with a nonpolar chain. This is why oil and water separate instead of mixing. (Oil ending up on top is a separate fact — that's density, not polarity.)
• Essentially their only intermolecular attraction is London dispersion. Weak per contact, but it adds up along a chain, so boiling point climbs steadily with size. Methane boils at −162 °C, butane at −0.5 °C, octane at +126 °C. That climb is why the first four alkanes are gases, the middle ones are liquid fuels, and the long ones are waxes.
• They burn. Complete combustion in plenty of oxygen always gives carbon dioxide and water:

CH₄ + 2O₂ → CO₂ + 2H₂O

Balancing combustion equations is a standard exam question, so practise the pattern: balance carbon first, then hydrogen, then oxygen last.

Worked examples

Try each before reading on.

1. What is the molecular formula of heptane? Heptane is an alkane with 7 carbons. CₙH₂ₙ₊₂ gives C₇H₁₆.

2. Is methanol, CH₃OH, a hydrocarbon? No. It contains oxygen, so it fails the carbon-and-hydrogen-only test. It is an organic compound and an alcohol, but not a hydrocarbon.

3. A hydrocarbon has 5 carbons and 8 hydrogens. What can it be? C₅H₈ is CₙH₂ₙ₋₂, so it has two degrees of unsaturation worth of structure — one triple bond, or two double bonds, or one double bond plus a ring, or two rings. Pent-1-yne and cyclopentene both fit.

4. Which boils higher, propane or decane? Decane. It's a far longer chain, so it has much more surface area for dispersion forces to act over.

Common mistakes to avoid

• Assuming every organic compound is a hydrocarbon. Ethanol, glucose and acetic acid are all organic; none is a hydrocarbon, because each contains oxygen. "Hydrocarbon" is the strictest possible label.
• Reading CₙH₂ₙ as "definitely an alkene". A ring uses up the same two hydrogens a double bond does, so cycloalkanes fit the alkene formula exactly.
• Thinking nonpolar means unreactive. Alkanes are sluggish toward polar reagents, but they burn readily and react with halogens under UV light. Low polarity is not the same as low energy.

FAQ

What is the simplest hydrocarbon?
Methane, CH₄ — one carbon with four hydrogens, arranged as a tetrahedron with bond angles of 109.5°. It is the main component of natural gas.

Where do hydrocarbons come from?
Mostly from crude oil and natural gas, which formed from buried marine organisms over millions of years. Fractional distillation separates crude oil into fractions by boiling point, which in practice means by chain length.

Are hydrocarbons soluble in water?
Essentially not. They are nonpolar and cannot hydrogen-bond, so they separate out rather than mixing — the basis of the phrase "like dissolves like".

Is benzene a hydrocarbon?
Yes. Benzene is C₆H₆ — carbon and hydrogen only. It belongs to the aromatic family, where six electrons are delocalised around a flat ring rather than being tied to individual bonds.

The takeaway

A hydrocarbon is the simplest kind of organic molecule: carbon and hydrogen, nothing else. Carbon's four bonds and its willingness to link to itself let those two elements build chains, branches and rings of any size, and the way the carbons are joined — all single bonds, a double bond, a triple bond, or a delocalised ring — is what sorts them into families. Learn the skeleton first; every functional group you meet later is just something hung onto it.

Build on this with Sigma vs Pi Bonds: What's the Difference?, Ionic vs Covalent Bonds: What's the Difference? and What Is a Valence Electron? Shells and Bonding. Next in this series: [Alkanes vs Alkenes vs Alkynes] and [What Is a Functional Group?].

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