Describe the shapes of C2H6 and C2H4. In doing so, explain why one molecule is planar and why the other cannot be planar.
The Correct Answer and Explanation is:
The molecules C2H6 (ethane) and C2H4 (ethene) differ significantly in terms of their molecular shapes and bonding, leading to differences in their planarity.
- C2H6 (Ethane):
Ethane has a tetrahedral geometry around each carbon atom. This is because each carbon is bonded to three hydrogen atoms and one other carbon atom. The bonding in ethane involves single bonds (sigma bonds), which allow for free rotation around the C-C bond. Each carbon atom has four regions of electron density (one for each bond), and these regions arrange themselves as far apart as possible, giving the carbon atoms a tetrahedral shape. The bond angles in ethane are approximately 109.5 degrees. Because of the tetrahedral geometry, ethane is not planar; the atoms are arranged in three-dimensional space, and the molecule can rotate freely around its C-C bond. - C2H4 (Ethene):
Ethene, on the other hand, has a different bonding arrangement. The two carbon atoms in ethene are double-bonded, meaning each carbon shares one sigma bond and one pi bond with the other carbon. The presence of the double bond restricts rotation around the C-C bond, making the molecule planar. The carbon atoms in ethene also have two regions of electron density from the bonds with the other carbon and the hydrogen atoms, giving each carbon a trigonal planar geometry. The bond angles around each carbon atom are approximately 120 degrees. The double bond in ethene, consisting of both a sigma and a pi bond, prevents the free rotation found in ethane, so the molecule remains flat, with all the atoms in the same plane.
Why One Is Planar and the Other Is Not:
Ethene is planar because the double bond creates a rigid structure, and the trigonal planar geometry of the carbon atoms forces the molecule to lie in a single plane. Ethane, however, cannot be planar due to the tetrahedral geometry around each carbon atom and the ability to rotate around the C-C bond, which results in a three-dimensional structure.
