Class 10 Chemistry – Chapter 4: Hydrocarbons (Quick Revision)

This page covers structure of hydrocarbons, saturated and unsaturated compounds, alkanes, alkenes, alkynes, cyclo-compounds, root words, functional groups and isomers for Class 10 students.[web:23][web:31]

1. Hydrocarbon Basics

Hydrocarbons are compounds made of carbon (C) and hydrogen (H) only.[web:23][web:30]

Molecular formula

Example: Methane → CH₄
Ethane → C₂H₆

Condensed formula

Methane → CH₄
Propane → CH₃–CH₂–CH₃

Teacher tip: Ask students to convert molecular formula ⇄ condensed structure for first 4–5 members.

2. Electron Dot Structures

Electron dot (Lewis) structures show valence electrons as dots around symbols, helping students see sharing of electrons in covalent bonds.[web:16][web:23]

Methane (CH₄)

•C• surrounded by 4 H atoms sharing one pair each
(Tetravalency of carbon clearly visible)

Ethene (C₂H₄)

C = C double bond with each C attached to 2 H atoms
(Shows double bond = unsaturation)

3. Saturated and Unsaturated Hydrocarbons

Type	Definition	Bond type	Family	Example
Saturated	All C–C single bonds.[web:16][web:31]	Single bonds only.	Alkanes.	CH4, C2H6
Unsaturated	At least one C=C or C≡C.[web:16][web:31]	Double or triple bond present.	Alkenes, Alkynes.	C2H4, C2H2

Saturated → Single bond Unsaturated → Double/Triple bond

4. Alkanes, Alkenes and Alkynes

Family	Type	General formula	Characteristic bond	1st three members
Alkanes	Saturated.[web:16][web:31]	CnH2n+2	All C–C single bonds.	Methane, Ethane, Propane
Alkenes	Unsaturated.[web:16][web:19]	CnH2n	At least one C=C double bond.	Ethene, Propene, Butene
Alkynes	Unsaturated.[web:16][web:19]	CnH2n−2	At least one C≡C triple bond.	Ethyne, Propyne, Butyne

Ask: “Which formula fits C₄H₁₀? What family is it?” to test concept.[web:16][web:31]

5. Cyclo (Ring) Hydrocarbons

Cyclo-hydrocarbons have carbon atoms joined in a ring instead of an open chain (for example, cyclohexane).[web:22][web:30]

• General idea: “Cyclo” + alkane name for simple saturated rings (e.g., cyclopropane, cyclobutane).[web:22]
• Ring structure can still be saturated (all single bonds) or unsaturated (with double bonds in the ring).

Example: Cyclopropane → triangle of 3 C atoms, each attached to 2 H atoms (C₃H₆)

6. Root Word Concept

The root word (word root) tells the number of carbon atoms in the longest continuous chain of the molecule.[web:18][web:21]

C atoms	Root word	Example (alkane)
1	meth	methane
2	eth	ethane
3	prop	propane
4	but	butane
5	pent	pentane

Activity: Give condensed formulas and ask students to identify root word and family (ane/ene/yne).

7. Common Functional Groups

Functional groups are specific atoms or groups of atoms that decide the main chemical properties of an organic compound.[web:19][web:21]

Functional group	Formula	Family name	Example
Alcohol	–OH	Alkanol	CH3–OH (methanol)
Aldehyde	–CHO	Alkanal	CH3–CHO (ethanal)
Carboxylic acid	–COOH	Alkanoic acid	CH3–COOH (ethanoic acid)
Halogen	–Cl, –Br, –I	Haloalkane	CH3Cl (chloromethane)

8. Isomers (Same formula, different structure)

Isomerism occurs when compounds have the same molecular formula but different structures and properties.[web:16][web:24]

• Example: C₄H₁₀ can be arranged as a straight chain (n-butane) or branched chain (isobutane).[web:16]
• Position isomerism appears when a functional group or double bond occupies different positions (like but-1-ene and but-2-ene).[web:19][web:24]

Task for students: Draw two different structures of C₄H₁₀ and name them.