# 16.3 Shapes of Ethane and Cyclopropane ## 1. Shape of Ethane Molecules ($C_2H_6$) ### 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 ($\sigma$) bond**. All single covalent bonds in ethane are sigma bonds. ### Hybridization Each carbon atom in ethane undergoes **$sp^3$ hybridization**. Consequently, all orbitals involved in bonding, both C-C and C-H bonds, are $sp^3$ hybridized orbitals. For more on how these orbitals are formed, see . ### 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 ($C_3H_6$) ### 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 **$sp^3$ hybridization**. The orbitals involved in C-C and C-H bonds are $sp^3$ 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 **$60^\circ$**. This is a significant deviation from the ideal $109.5^\circ$ angle for $sp^3$ hybridized orbitals, causing substantial **angle 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 $109.5^\circ$. ### Visual Representation --- ## Possible Questions and Answers **Q:** What is the hybridization of the carbon atoms in an ethane ($C_2H_6$) molecule? **A:** Each carbon atom in ethane is $sp^3$ hybridized. **Q:** What is the geometry around each carbon atom in ethane? **A:** The geometry is **tetrahedral**, with bond angles of approximately $109.5^\circ$. **Q:** Why is the cyclopropane molecule considered to be "strained"? **A:** The carbon atoms in cyclopropane are $sp^3$ hybridized, which prefers a bond angle of $109.5^\circ$. However, the triangular ring structure forces the C-C-C bond angle to be $60^\circ$. This large deviation from the ideal angle creates significant **angle strain**, making the molecule highly reactive compared to other alkanes. --- ## References Derived from FBISE Textbook for Class 11 Chemistry, Chapter 16: Hydrocarbons.