What Is a Hydrocarbon
The term hydrocarbon is self-explanatory: a compound built from carbon and hydrogen only. Despite this austere composition, the family is vast, because carbon is tetravalent and bonds readily to itself, generating open chains, branched chains and closed rings of almost unlimited size. Methane, $\ce{CH4}$, is the simplest member; benzene, $\ce{C6H6}$, sits at the opposite stylistic pole.
Because so many distinct molecules share only these two elements, chemists need an organising principle. NCERT supplies it at the very start of Unit 9: hydrocarbons are sorted by the types of carbon–carbon bonds present. That one criterion partitions the family into three main categories, and almost every physical and chemical property — reactivity, the tests that distinguish one class from another, even the appropriate general formula — follows from which category a compound belongs to.
A hydrocarbon contains C and H only
A molecule such as ethanol $\ce{C2H5OH}$ or chloromethane $\ce{CH3Cl}$ is not a hydrocarbon, because it carries a third element. These belong to other functional families. The classification in this article applies strictly to compounds of carbon and hydrogen.
Rule: no oxygen, no halogen, no nitrogen — if a third element appears, the compound has left the hydrocarbon family.
The Classification Tree
Depending on the carbon–carbon bonds it contains, a hydrocarbon falls into one of three main categories — saturated, unsaturated and aromatic. Saturated hydrocarbons hold only single bonds; unsaturated hydrocarbons contain at least one carbon–carbon multiple bond; aromatic hydrocarbons are a special type of cyclic compound. The schematic below maps the whole family at a glance before we walk each branch in turn.
Caption. The three-way classification of hydrocarbons. Saturated hydrocarbons split into open-chain alkanes and ring cycloalkanes; unsaturated hydrocarbons into alkenes and alkynes; aromatic hydrocarbons form a special cyclic class typified by benzene.
Saturated Hydrocarbons
Saturated hydrocarbons contain carbon–carbon and carbon–hydrogen single bonds only. The word "saturated" signals that every carbon valence is occupied by a single bond — the molecule cannot take up any more hydrogen. When the carbon atoms are joined in an open chain by single bonds, the compound is an alkane; when the carbons close into a ring, it is a cycloalkane.
Methane $\ce{CH4}$ is the first alkane, followed by ethane $\ce{C2H6}$, propane $\ce{C3H8}$ and butane $\ce{C4H10}$. Ethane can be regarded as methane with one hydrogen replaced by a $\ce{-CH3}$ group, and the series is generated by repeating that substitution. Cyclohexane $\ce{C6H12}$ and cyclopropane $\ce{C3H6}$ are familiar cycloalkanes; their carbons are saturated, but the ring closure costs them two hydrogens relative to the open-chain alkane of the same carbon count.
| Saturated type | Carbon skeleton | General formula | Example |
|---|---|---|---|
| Alkane | Open chain, single bonds | CnH2n+2 | Ethane $\ce{C2H6}$ |
| Cycloalkane | Closed ring, single bonds | CnH2n | Cyclohexane $\ce{C6H12}$ |
Unsaturated Hydrocarbons
Unsaturated hydrocarbons contain carbon–carbon multiple bonds — double bonds, triple bonds, or both. Because a multiple bond can be opened to admit additional atoms, these compounds are "unsaturated" with respect to hydrogen and react readily by addition. The two principal sub-classes are alkenes and alkynes.
An alkene carries at least one carbon–carbon double bond; ethene $\ce{C2H4}$ (ethylene) is the simplest, followed by propene $\ce{C3H6}$. An alkyne carries at least one carbon–carbon triple bond; ethyne $\ce{C2H2}$ (acetylene) heads the series, with propyne $\ce{C3H4}$ next. The multiple bond is the seat of reactivity: it is what allows bromine water and dilute alkaline $\ce{KMnO4}$ to act as tests for unsaturation, since the multiple bond adds the reagent and the colour is discharged.
Caption. The defining bond order rises across the unsaturated classes: a single C–C bond in alkanes, a C=C double bond in alkenes, and a C≡C triple bond in alkynes. Each added bond removes two hydrogens from the saturated formula.
Once the double bond is in place, geometry and reactivity follow. See Alkenes for cis–trans isomerism, Markovnikov addition and ozonolysis.
Aromatic Hydrocarbons
Aromatic hydrocarbons are a special type of cyclic compound. Benzene $\ce{C6H6}$ is the parent: a planar six-membered ring whose carbons share a delocalised system of $\pi$ electrons, giving the ring an unusual stability that sets it apart from ordinary unsaturated compounds. Aromatic hydrocarbons that contain a benzene ring are described as benzenoid; toluene $\ce{C6H5CH3}$ and naphthalene $\ce{C10H8}$ belong here.
Although benzene contains formal double bonds, it does not behave like a simple alkene. Its stability means it prefers substitution to addition, and it does not readily decolourise bromine water under ordinary conditions — a contrast that NEET tests directly. The criteria for aromatic character (planarity, full delocalisation, and a $(4n+2)\,\pi$-electron count) are explored in the dedicated benzene topic, but the classification point here is simply that aromatic hydrocarbons constitute their own branch of the family tree.
Not every cyclic hydrocarbon is aromatic
Cyclohexane and cyclopentane are cyclic, but they are saturated alicyclic compounds, not aromatic. Aromaticity is a special property that depends on a delocalised $(4n+2)\,\pi$-electron ring — a far stricter requirement than merely being a ring. Treating "cyclic" and "aromatic" as synonyms is a classic source of errors.
Rule: ring + delocalised $(4n+2)\,\pi$ electrons + planarity = aromatic; a plain ring is only alicyclic.
Two Cross-Cutting Splits
The three-way bond classification is the primary scheme, but NCERT layers two further descriptive splits on top of it. The first is aliphatic versus aromatic. Aliphatic hydrocarbons are the open-chain and non-aromatic ring compounds — alkanes, alkenes, alkynes and the alicyclic rings. Aromatic hydrocarbons are the benzene-type ring compounds. The second split is open-chain (acyclic) versus cyclic, which simply asks whether the carbon skeleton is an open chain or closes into a ring.
These splits intersect rather than compete. A given molecule can be placed on every axis at once: ethene is unsaturated, aliphatic and open-chain; cyclohexane is saturated, aliphatic and cyclic; benzene is aromatic and cyclic. Holding all three descriptors in mind keeps the categories from blurring together.
| Compound | Bond type | Aliphatic / Aromatic | Open-chain / Cyclic |
|---|---|---|---|
| Ethene $\ce{C2H4}$ | Unsaturated (double) | Aliphatic | Open-chain |
| Propyne $\ce{C3H4}$ | Unsaturated (triple) | Aliphatic | Open-chain |
| Butane $\ce{C4H10}$ | Saturated (single) | Aliphatic | Open-chain |
| Cyclohexane $\ce{C6H12}$ | Saturated (single) | Aliphatic (alicyclic) | Cyclic |
| Benzene $\ce{C6H6}$ | Delocalised $\pi$ | Aromatic | Cyclic |
General Formulae and Degree of Unsaturation
Each homologous series carries a characteristic general formula that lets you write any member from its carbon count alone. An open-chain alkane is $\ce{C_nH_{2n+2}}$, an alkene with one double bond is $\ce{C_nH_{2n}}$, and an alkyne with one triple bond is $\ce{C_nH_{2n-2}}$. The pattern is governed by a single idea: each "degree of unsaturation" — whether a ring or a multiple bond — removes two hydrogen atoms from the fully saturated count.
This is why a cycloalkane shares the alkene formula $\ce{C_nH_{2n}}$: a ring and a double bond each subtract one $\ce{H2}$ from the alkane reference. A triple bond counts as two degrees, removing $\ce{2 H2}$, which is why alkynes are $\ce{C_nH_{2n-2}}$. Benzene, with its ring and three formal double bonds, sits at $\ce{C6H6}$ — four degrees of unsaturation below the alkane $\ce{C6H14}$.
Predict the molecular formula of a 5-carbon open-chain alkane, a 5-carbon alkene with one double bond, and a 5-carbon alkyne with one triple bond.
Alkane: $\ce{C_nH_{2n+2}}$ with $n=5$ gives $\ce{C5H12}$ (pentane). Alkene: $\ce{C_nH_{2n}}$ gives $\ce{C5H10}$ (a pentene). Alkyne: $\ce{C_nH_{2n-2}}$ gives $\ce{C5H8}$ (a pentyne). Each multiple bond drops the hydrogen count by two relative to the previous class.
Master Classification Table
The single reference below ties together class, general formula, characteristic bonding and a representative example. Committing this grid to memory gives you the spine of the entire chapter, because preparation methods and reactions are organised class by class along exactly these lines.
| Class | General formula | Characteristic C–C bonding | Representative example |
|---|---|---|---|
| Alkane (saturated, open-chain) | CnH2n+2 | All single bonds | Ethane $\ce{C2H6}$ |
| Cycloalkane (saturated, cyclic) | CnH2n | Single bonds in a ring | Cyclohexane $\ce{C6H12}$ |
| Alkene (unsaturated) | CnH2n | One C=C double bond | Ethene $\ce{C2H4}$ |
| Alkyne (unsaturated) | CnH2n-2 | One C≡C triple bond | Ethyne $\ce{C2H2}$ |
| Aromatic (benzenoid) | — | Delocalised $\pi$ ring | Benzene $\ce{C6H6}$ |
$\ce{C_nH_{2n}}$ is shared by two classes
A formula such as $\ce{C4H8}$ does not pin down the class — it could be a butene (alkene, one double bond) or methylcyclopropane / cyclobutane (a cycloalkane). The shared formula is the reason a question may ask you to also consider the ring possibility when counting isomers or interpreting a molecular formula.
Rule: $\ce{C_nH_{2n}}$ → one degree of unsaturation, satisfied by either a double bond or a ring.
Sources of Hydrocarbons
Classification is not a purely academic exercise — the categories map onto the fuels and feedstocks of everyday life. Petroleum and natural gas are the dominant natural reservoirs. Petrol, diesel and kerosene are obtained by the fractional distillation of petroleum found beneath the earth's crust, while coal gas comes from the destructive distillation of coal. Natural gas, found in the upper strata during the drilling of oil wells, yields compressed natural gas (CNG) on compression and liquified natural gas (LNG) on liquefaction; liquified petroleum gas (LPG) is the familiar low-pollution domestic fuel.
Every one of these is a mixture of hydrocarbons and the chief source of energy in the modern economy. Beyond combustion, hydrocarbons are the starting materials for polymers such as polythene, polypropene and polystyrene, and for many dyes and drugs. The classes you have just learned are therefore the vocabulary in which the petrochemical industry is written.
Classification of Hydrocarbons in one screen
- Hydrocarbons contain carbon and hydrogen only and are classified by the type of carbon–carbon bonds.
- Three main categories: saturated (single bonds), unsaturated (double or triple bonds), aromatic (special cyclic, delocalised $\pi$ ring).
- Saturated splits into open-chain alkanes ($\ce{C_nH_{2n+2}}$) and ring cycloalkanes ($\ce{C_nH_{2n}}$); unsaturated into alkenes ($\ce{C_nH_{2n}}$) and alkynes ($\ce{C_nH_{2n-2}}$).
- Two cross-cutting splits: aliphatic vs aromatic, and open-chain (acyclic) vs cyclic.
- Each degree of unsaturation — a ring or a multiple bond — removes one $\ce{H2}$ from the alkane reference; this is why $\ce{C_nH_{2n}}$ is shared by alkenes and cycloalkanes.
- Petroleum, natural gas and coal supply hydrocarbon mixtures used as fuels (LPG, CNG, LNG, petrol, diesel) and feedstocks for polymers, dyes and drugs.