Classification of hydrocarbons

What are hydrocarbons and how are they classified?

In plain words: a hydrocarbon is built only from carbon and hydrogen, and it is sorted by how its carbons are joined. If every carbon–carbon bond is single, the hydrocarbon is saturated; an open chain of this kind holds the most hydrogens possible. A double or triple bond means fewer hydrogens, so the molecule is unsaturated. A ring also removes two hydrogens, but a ring with only single bonds (a cycloalkane) is still saturated. Benzene and its relatives are rings with a special, extra-stable arrangement of electrons, and they form a class of their own.

Hydrocarbons in daily life

Hydrocarbons are important sources of energy (NCERT page 295):

• LPG (liquefied petroleum gas) is a domestic fuel with the least pollution; kerosene is also a domestic fuel but causes some pollution.
• CNG (compressed natural gas) is natural gas, found in the upper strata while drilling oil wells, compressed for use in vehicles; LNG (liquefied natural gas) is obtained by liquefying natural gas.
• Petrol, diesel and kerosene come from the fractional distillation of petroleum; coal gas comes from the destructive distillation of coal.
• Petrol- and CNG-run vehicles cause less pollution.

Beyond the NCERT text: LPG is mainly butane and propane, and natural gas, and so CNG and LNG, is mainly methane.

Beyond fuels, hydrocarbons are used to make polymers such as polythene, polypropene and polystyrene; higher hydrocarbons are solvents for paints and starting materials for many dyes and drugs (page 295).

The three classes

Saturated hydrocarbons contain only C–C and C–H single bonds. With an open chain they are alkanes, general formula C_nH_2n+2 (NCERT page 296); with a closed chain, or ring, they are cycloalkanes. NCERT gives no cycloalkane formula, but a ring costs two hydrogens, so a cycloalkane with one ring is C_nH_2n. Each carbon is sp³ hybridised and tetrahedral.

Unsaturated hydrocarbons contain carbon–carbon multiple bonds:

• alkenes, with at least one C=C, general formula C_nH_2n for one double bond;
• alkynes, with at least one C≡C, C_nH_2n−2 for one triple bond;
• compounds with two or more multiple bonds, such as buta-1,3-diene.

Aromatic hydrocarbons (arenes) are a special type of ring compound, of which benzene, C₆H₆, is the parent. Their rings are unusually stable, and they react mainly by substitution, not addition.

NCERT suggests building models: toothpicks and plasticine balls for alkanes, and spring models for alkenes, alkynes and aromatic hydrocarbons, keeping in mind that carbon is tetravalent and hydrogen monovalent (page 296).

Counting rings and multiple bonds

Beyond the NCERT text, but very useful: compared with an open-chain alkane of the same number of carbons, each ring and each π bond means two fewer hydrogen atoms. The degree of unsaturation (also called the double bond equivalent) counts them from the molecular formula alone:

A C=C counts 1, a C≡C counts 2 and a ring counts 1. Benzene, C₆H₆, has (12 + 2 − 6)/2 = 4: one ring and three π bonds.

Solved examples

Common mistakes to avoid

Quick revision summary

• Hydrocarbons: compounds of carbon and hydrogen only; fuels (LPG, CNG, LNG, petrol, diesel, kerosene) and raw materials for polymers, dyes and drugs.
• Saturated: alkanes C_nH_2n+2, cycloalkanes C_nH_2n.
• Unsaturated: alkenes C_nH_2n for one C=C, alkynes C_nH_2n−2 for one C≡C.
• Aromatic: benzene and related rings.
• Degree of unsaturation = (2x + 2 − y)/2 = rings + π bonds.

Questions students ask

What is the difference between saturated and unsaturated hydrocarbons?

Saturated hydrocarbons have only single carbon–carbon bonds, as in ethane, while unsaturated ones have at least one C=C or C≡C bond, as in ethene, CH₂=CH₂, and so fewer hydrogens than the alkane with the same number of carbons. Saturated ones react mainly by substitution, under heat or light; unsaturated ones add H₂, halogens or HBr across their π bonds, so at room temperature they decolourise bromine in CCl₄ and alkanes do not. Benzene has π bonds too, but reacts mainly by substitution, so it is classed apart, as aromatic.

What is the difference between LPG and CNG?

LPG (liquefied petroleum gas) is mainly propane and butane, liquefied under moderate pressure in cylinders for cooking, while CNG (compressed natural gas) is mainly methane, which cannot be liquefied by pressure at room temperature, so it is kept as a gas at about 200 bar and used mostly in vehicles. Propane (44 g mol^⁻¹) and butane (58 g mol^⁻¹) are heavier than air (about 29 g mol^⁻¹), so leaked LPG collects near the floor, while methane (16 g mol^⁻¹) rises and disperses. PNG (piped natural gas) is natural gas sent to kitchens through pipes, and LNG is natural gas cooled to about 111 K (−162 °C), methane’s boiling point, so that it can be shipped as a liquid.

What is the difference between hydrocarbons and carbohydrates?

Hydrocarbons contain only carbon and hydrogen, such as methane, CH₄, while carbohydrates contain carbon, hydrogen and oxygen, usually with hydrogen and oxygen in water’s 2 : 1 ratio, as in glucose, C₆H₁₂O₆, and sucrose, C₁₂H₂₂O₁₁. Carbohydrates are polyhydroxy aldehydes or ketones, or give them on hydrolysis, and their many –OH groups make sugars soluble in water, whereas hydrocarbons are non-polar and practically insoluble. Both give CO₂ and H₂O and release energy when oxidised, hydrocarbons burnt as fuels and carbohydrates respired as food; carbohydrates are a Class 12 topic (Biomolecules).

How do you find the degree of unsaturation of a compound containing oxygen, nitrogen or a halogen?

With oxygen, nitrogen or halogens present, the degree of unsaturation is (2n_C + 2 + n_N − n_H − n_X)/2, counting the carbon, nitrogen, hydrogen and halogen atoms: oxygen and sulphur are ignored, a halogen counts as a hydrogen, and each nitrogen adds 1 inside the bracket. Ethanal, C₂H₄O, gives (4 + 2 − 4)/2 = 1, its C=O; chloroethene, C₂H₃Cl, gives (4 + 2 − 3 − 1)/2 = 1, its C=C; and pyridine, C₅H₅N, gives (10 + 2 + 1 − 5)/2 = 4, one ring and three π bonds. The result is still rings + π bonds, now counting C=O and C=N too.

What does a degree of unsaturation of 4 mean?

A degree of unsaturation of 4 means that the rings and π bonds of the molecule add up to four, in any combination: a benzene ring alone accounts for exactly 4 (one ring and three C=C), and so do two C≡C bonds. For example, C₉H₁₂, with (18 + 2 − 12)/2 = 4, is most often a benzene compound, such as isopropylbenzene (cumene), while buta-1,3-diyne, HC≡C–C≡CH (C₄H₂), also has 4 with no ring at all. The same number is called the double bond equivalent or the index of hydrogen deficiency, and only the structure decides which combination is present.

Why are hydrocarbons used as fuels?

Hydrocarbons are used as fuels because they burn in enough air to carbon dioxide and water with a large release of heat: burning methane releases 890 kJ mol^⁻¹, about 55 kJ g^⁻¹, and butane, a main part of LPG, about 2877 kJ mol^⁻¹, nearly 50 kJ g^⁻¹. They are stable until lit, so they can be stored and carried safely, and as gases or liquids they mix well with air and leave little ash. They are also abundant: petroleum and natural gas supply them in large amounts, which refining sorts into fuels for homes, vehicles and industry.

Is the degree of unsaturation in the JEE Main 2027 and NEET (UG) 2026 syllabuses?

The degree of unsaturation is not named in the JEE Main 2027 (NTA) or NEET (UG) 2026 (NMC) syllabus, whose Unit 15, Hydrocarbons, lists ‘Classification, isomerism, IUPAC nomenclature, general methods of preparation, properties and reactions’. NCERT uses the phrase only in passing, for benzene in Section 9.5.2, and gives no formula. It is still a useful shortcut for those listed topics, since it turns a molecular formula into the number of rings and π bonds, the first step in drawing the isomers of a formula or in working out a structure from clues.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Which fuel does NCERT describe as a domestic fuel with the least pollution, and which as causing some pollution?

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   LPG causes the least pollution as a domestic fuel; kerosene causes some pollution.

2. Q2. Find the degree of unsaturation of C₄H₆ and give two structures that fit.

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   (8 + 2 − 6)/2 = 2. For example but-1-yne, HC≡CCH₂CH₃ (one triple bond), and buta-1,3-diene, CH₂=CH–CH=CH₂ (two double bonds). Cyclobutene (one ring, one double bond) also fits.

3. Q3. Why were alkanes earlier called paraffins?

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   Because they are inert under normal conditions and do not react with acids, bases and most other reagents; the name comes from the Latin parum, little, and affinis, affinity (NCERT page 296).