16.3 Shapes of Ethane and Cyclopropane | Hydrocarbons | Chemistry 11

1. Shape of Ethane Molecules ( $C_{2} H_{6}$ )

Bonding Structure

The ethane molecule consists of two carbon atoms bonded to each other by a single covalent bond. Each carbon atom is also bonded to three hydrogen atoms by single covalent bonds. A covalent bond formed by the direct, end-on overlap of atomic orbitals is called a sigma ( $σ$ ) bond. All single covalent bonds in ethane are sigma bonds.

Hybridization

Each carbon atom in ethane undergoes $s p^{3}$ hybridization. Consequently, all orbitals involved in bonding, both C-C and C-H bonds, are $s p^{3}$ hybridized orbitals. For more on how these orbitals are formed, see Shapes of Orbitals→.

Molecular Geometry

The ethane molecule is non-planar. The covalent bonds around each carbon atom are arranged in a tetrahedral geometry. This arrangement results in bond angles of approximately $109. 5^{\circ}$ between any two bonds on the same carbon atom.

Visual Representation

2. Shape of Cyclopropane Molecules ( $C_{3} H_{6}$ )

Bonding Structure

The cyclopropane molecule consists of three carbon atoms bonded by single covalent bonds to form a triangular ring. Each carbon atom is also bonded to two hydrogen atoms.

Hybridization

Like ethane, each carbon atom in cyclopropane exhibits $s p^{3}$ hybridization. The orbitals involved in C-C and C-H bonds are $s p^{3}$ hybridized, which ideally leads to a tetrahedral arrangement.

Molecular Geometry and Bond Angles

The molecule has a three-dimensional, non-planar structure. The C-C-C bond angle within the ring is constrained to $6 0^{\circ}$ . This is a significant deviation from the ideal $109. 5^{\circ}$ angle for $s p^{3}$ hybridized orbitals, causing substantial angle strain (also called ring strain).

Because the orbitals cannot overlap head-on effectively due to the geometric constraints of the ring, the C-C bonds are often described as 'bent bonds' or 'banana bonds'.

The H-C-H and C-C-H bond angles are closer to the ideal tetrahedral angle of $109. 5^{\circ}$ .

Reactivity of Cyclopropane

Due to the high angle strain from the $6 0^{\circ}$ C-C-C bond angle (vs. ideal $109. 5^{\circ}$ ), the bent bonds in cyclopropane are weaker than normal $σ$ bonds. This makes cyclopropane significantly more reactive than other alkanes such as ethane, and it can undergo ring-opening reactions.

Visual Representation

Figure 16.3b: Shape of cyclopropane molecule

References

Derived from FBISE Textbook for Class 11 Chemistry, Chapter 16: Hydrocarbons.

1. Shape of Ethane Molecules (

C_{2} H_{6}

)

Each carbon atom in ethane undergoes $s p^{3}$ hybridization. Consequently, all orbitals involved in bonding, both C-C and C-H bonds, are

s p^{3}

hybridized orbitals. For more on how these orbitals are formed, see Shapes of Orbitals→.

Figure 16.3a: Shape of ethane molecule

2. Shape of Cyclopropane Molecules (

C_{3} H_{6}

)

The cyclopropane molecule consists of three carbon atoms bonded by single covalent bonds to form a triangular ring. Each carbon atom is also bonded to two hydrogen atoms.

Like ethane, each carbon atom in cyclopropane exhibits $s p^{3}$ hybridization. The orbitals involved in C-C and C-H bonds are

s p^{3}

hybridized, which ideally leads to a tetrahedral arrangement.

The molecule has a three-dimensional, non-planar structure. The C-C-C bond angle within the ring is constrained to $6 0^{\circ}$ . This is a significant deviation from the ideal

109. 5^{\circ}

angle for

s p^{3}

hybridized orbitals, causing substantial angle strain (also called ring strain).

Because the orbitals cannot overlap head-on effectively due to the geometric constraints of the ring, the C-C bonds are often described as 'bent bonds' or 'banana bonds'.

The H-C-H and C-C-H bond angles are closer to the ideal tetrahedral angle of

109. 5^{\circ}

Due to the high angle strain from the

6 0^{\circ}

C-C-C bond angle (vs. ideal

109. 5^{\circ}

), the bent bonds in cyclopropane are weaker than normal

σ

bonds. This makes cyclopropane significantly more reactive than other alkanes such as ethane, and it can undergo ring-opening reactions.

Figure 16.3b: Shape of cyclopropane molecule