Describe the pi and sigma bonds in ethene.

In ethene, there is one sigma bond and one pi bond between the two carbon atoms.

Ethene, also known as ethylene, is a hydrocarbon with the formula C2H4. It is the simplest alkene, characterised by the presence of a carbon-carbon double bond. This double bond is composed of one sigma (σ) bond and one pi (π) bond.

The sigma bond is the result of the end-to-end or head-on overlap of two sp2 hybrid orbitals, one from each carbon atom. This overlap creates a region of high electron density between the two carbon atoms, forming a strong sigma bond. Sigma bonds allow free rotation because the overlapping orbitals maintain the same degree of overlap, regardless of the rotation about the bond axis. To further understand this concept, see the section on Hybridization Basics.

The pi bond, on the other hand, is formed by the sideways overlap of two unhybridised 2p orbitals, one from each carbon atom. This overlap creates two regions of electron density, one above and one below the plane of the molecule. Unlike sigma bonds, pi bonds do not allow free rotation. This is because rotating the atoms would disrupt the parallel alignment of the 2p orbitals, breaking the pi bond. For more details on the molecular structure of alkenes like ethene, visit Alkenes.

The presence of the pi bond in ethene gives the molecule its planar shape, with bond angles of approximately 120 degrees. This is due to the electron repulsion in the three sp2 hybrid orbitals of each carbon atom, which push each other as far apart as possible. The pi bond also gives ethene its reactivity, as the electron-rich region above and below the plane of the molecule can easily be attacked by electrophiles. Learn more about the role of pi bonds in Functional Groups in Organic Chemistry.

IB Chemistry Tutor Summary: In ethene, there's a sigma bond and a pi bond between the two carbon atoms. The sigma bond forms from direct orbital overlap, allowing rotation, while the pi bond comes from sideways overlap of p orbitals, stopping rotation. This setup makes ethene flat and reactive, with a pi bond that attracts substances looking to react, giving ethene its distinct properties.