## Molecular Structure of Ethene Ethene ($\mathrm{C_2H_4}$) is an alkene consisting of two carbon atoms joined by a double covalent bond, with two hydrogen atoms singly bonded to each carbon. - **Hybridization:** Each carbon atom in ethene is **$sp^2$ hybridized**. - **Geometry:** The $sp^2$ hybridization results in a **trigonal planar** arrangement around each carbon atom. - **Planarity:** The entire ethene molecule is planar, meaning all six atoms (2 carbons and 4 hydrogens) lie in the same plane. - **Bond Angles:** The H-C-H and H-C-C bond angles are approximately **$120^{\circ}$**. ### Bonding in Ethene The double bond between the two carbon atoms is a combination of one sigma ($\sigma$) bond and one pi ($\pi$) bond. #### Sigma ($\sigma$) Bonds Sigma bonds are formed by the direct, head-on overlap of atomic or hybrid orbitals. They are strong and hold electrons tightly. - **Carbon-Carbon ($\sigma$) Bond:** Formed by the head-on overlap of two **$sp^2$ hybridized orbitals**, one from each carbon atom ($sp^2 - sp^2$ overlap). - **Carbon-Hydrogen ($\sigma$) Bonds:** Each of the four C-H bonds is formed by the overlap of an **$sp^2$ orbital** from a carbon atom and the **s-orbital** of a hydrogen atom ($sp^2 - s$ overlap). #### Pi ($\pi$) Bond Pi bonds are formed by the parallel (sideways) overlap of unhybridized p-orbitals. For more on orbital shapes, see . - **Carbon-Carbon ($\pi$) Bond:** After $sp^2$ hybridization, each carbon atom has one unhybridized p-orbital perpendicular to the molecular plane. These two parallel p-orbitals overlap sideways to form the $\pi$ bond. - **Characteristics:** The electrons in a $\pi$ bond are held less tightly and are located above and below the plane of the sigma bonds. This makes the $\pi$ bond **weaker** and more **exposed** than a sigma bond. ### Reactivity of Ethene The presence of the weak and accessible $\pi$ bond makes ethene significantly more reactive than alkanes like ethane. - **Ethene vs. Ethane:** Ethene is more reactive than ethane because the $\pi$ bond can be easily broken to form new single bonds (addition reactions). Ethane only contains strong $\sigma$ bonds, which are difficult to break. - **Ethene vs. Ethyne:** Ethene is more reactive than ethyne. The $\pi$ electrons in ethene are more exposed and accessible for reaction. The shorter C≡C triple bond in ethyne holds its $\pi$ electrons more tightly between the carbon nuclei, making them less available for attack compared to the more diffused $\pi$ cloud of the C=C double bond in ethene.

16.5 Structure And Reactivity Of Ethene | Hydrocarbons | Chemistry 11