Cyclic Hydrocarbons: Cyclohexane & Benzene Structure Class 10

Quick answer: Cyclic hydrocarbons have carbon atoms joined in a ring. Cyclohexane ( $\mathrm{C_6H_{12}}$ ) is a saturated ring: six carbon atoms joined by single bonds, each carbon also bonded to two hydrogen atoms, giving 18 covalent bonds in all. Benzene ( $\mathrm{C_6H_6}$ ) is an unsaturated ring with alternating single and double bonds between its six carbon atoms, each carbon bonded to one hydrogen atom.

Carbon atoms can bond to each other to form long chains. This property is called catenation. The chains can be straight or branched, and they can also close into rings. Hydrocarbons with carbon atoms in a ring are called cyclic hydrocarbons.

In CBSE Class 10 Science, Chapter 4 (Carbon and its Compounds), you need to know the structures of cyclohexane and benzene, including the electron dot structure of cyclohexane.

What You Will Learn

What cyclic hydrocarbons are
Saturated rings (cycloalkanes) and their general formula $\mathrm{C_nH_{2n}}$
The structure and electron dot structure of cyclohexane
The structure of benzene
How to count the covalent bonds in cyclohexane and benzene
How cyclohexane differs from hexene, which has the same formula

1. What Are Cyclic Hydrocarbons?

Definition: Cyclic hydrocarbons are compounds of carbon and hydrogen in which the carbon atoms are joined in a ring.

Type	Bonds between carbon atoms	Example
Saturated	Only single bonds	Cyclohexane, $\mathrm{C_6H_{12}}$
Unsaturated	At least one double bond	Benzene, $\mathrm{C_6H_6}$

2. Saturated Rings: Cycloalkanes

When the two ends of an open chain of carbon atoms join to form a ring, two hydrogen atoms fewer are needed. So saturated rings have the general formula $\mathrm{C_nH_{2n}}$ , where $n$ is at least $3$ .

Number of carbon atoms	Name	Formula
3	Cyclopropane	$\mathrm{C_3H_6}$
4	Cyclobutane	$\mathrm{C_4H_8}$
5	Cyclopentane	$\mathrm{C_5H_{10}}$
6	Cyclohexane	$\mathrm{C_6H_{12}}$

Note: Cycloalkanes have the same general formula as alkenes ( $\mathrm{C_nH_{2n}}$ ), but they are saturated: they contain only single bonds.

3. Cyclohexane

Formula: $\mathrm{C_6H_{12}}$
Structure: Six carbon atoms in a ring, joined by single bonds. Each carbon atom is also bonded to two hydrogen atoms, so each carbon forms four bonds.

         H2C - CH2
        /         \
     H2C           CH2
        \         /
         H2C - CH2
       cyclohexane, C6H12

Counting the Covalent Bonds

Bond	Number
C–C single bonds in the ring	6
C–H single bonds	12
Total covalent bonds	18

Each covalent bond is a shared pair of electrons, so cyclohexane has $18 \times 2 = 36$ shared electrons.

Electron dot structure: Draw six carbon atoms in a ring. Show one shared pair of electrons between each pair of neighbouring carbon atoms, and one shared pair between each carbon atom and each of its two hydrogen atoms.

4. Benzene

Formula: $\mathrm{C_6H_6}$
Structure: Six carbon atoms in a ring with three single and three double bonds, arranged alternately. Each carbon atom is bonded to one hydrogen atom.

           CH
         //   \
       HC      CH
        |      ||
       HC      CH
         \\   /
           CH
        benzene, C6H6

Counting the Bonds

Bond	Number of shared pairs
3 C–C single bonds	3
3 C=C double bonds (2 pairs each)	6
6 C–H single bonds	6
Total	15 shared pairs (30 electrons)

Key point: Benzene is unsaturated, because it has carbon–carbon double bonds.

Extra facts: Benzene was discovered by Michael Faraday in 1825, and its ring structure was proposed by August Kekulé in 1865. Because benzene has a high proportion of carbon (about 92% by mass), it burns with a sooty flame.

5. Cyclohexane vs Hexene

Cyclohexane and hexene both have the formula $\mathrm{C_6H_{12}}$ , but they have different structures.

Feature	Cyclohexane	Hexene
Structure	Closed ring	Open chain
Bonds	Only single bonds (saturated)	One C=C double bond (unsaturated)
Type	Cycloalkane	Alkene

Compounds with the same molecular formula but different structures are called isomers.

6. Summary Table

Compound	Formula	Saturated or unsaturated	Bonds between carbon atoms
Cyclopropane	$\mathrm{C_3H_6}$	Saturated	Single only
Cyclohexane	$\mathrm{C_6H_{12}}$	Saturated	Single only; 18 covalent bonds in total
Benzene	$\mathrm{C_6H_6}$	Unsaturated	Alternating single and double bonds

7. Exam Tips and Common Mistakes

Exam Tip: If asked for the number of covalent bonds in cyclohexane, count both the C–C bonds (6) and the C–H bonds (12): the answer is 18.

Exam Tip: When drawing benzene, show the alternating single and double bonds and one hydrogen on each carbon.

Common Mistake: Mixing up cyclohexane ( $\mathrm{C_6H_{12}}$ , saturated, 2 H per carbon) and benzene ( $\mathrm{C_6H_6}$ , unsaturated, 1 H per carbon).

Common Mistake: Thinking every compound with formula $\mathrm{C_nH_{2n}}$ is an alkene. Cycloalkanes have the same formula but only single bonds.

Frequently Asked Questions

What is a cyclic hydrocarbon?

A hydrocarbon in which the carbon atoms are joined in a ring.

What is the molecular formula of cyclohexane?

C₆H₁₂. It is a saturated ring of six carbon atoms, each bonded to two hydrogen atoms.

How many covalent bonds are there in cyclohexane?

18: six C–C bonds and twelve C–H bonds.

What is the structure of benzene?

Benzene, C₆H₆, is a ring of six carbon atoms with alternating single and double bonds, and one hydrogen atom on each carbon.

Is benzene saturated or unsaturated?

Benzene is unsaturated, because it contains carbon–carbon double bonds.

Cyclohexane and Benzene Structure | Class 10 Science