Stereoisomerism is caused by a double or triple bond. Optical isomers are also a form of stereoisomerism. They are compounds that have the same structural formula but the arrangement of their atoms in space is different. Atoms can rotate around a single bond. However, tis rotation is restricted around a double or triple bond. For example, ethane C2H4 has a double C=C bond. If a methyl group is added to each carbon, replacing a hydrogen atom, stereoisomers form. These are called cis and trans-isomers. E/Z and cis/trans are two different methods of classifying geometric isomers. If something is the cis-isomer, then its name is cis-but-2-ene; is it is the E isomer, then its name is (E)-but-2-ene. E/Z isomerism is different as it applies a priority to each group attached to the double-bonded carbon and compares the position of those with highest priority. Alkenes showing cis/trans isomerism will also show E/Z isomerism, but the opposite is not true. For example, 1-bromo-1-chloropropene willl have E/Z isomerism but not cis/trans isomerism.
Carbon has four electrons in its outer shell. These form four covalent bonds by pairing with electrons on other atoms. The electron clouds from each electron in a bond overlap. This is called a sigma bond. C-C bonds and C-H bonds are examples of sigma bonds. The overlap of two s orbitals or two p orbitals forms sigma bonds. In alkanes such as methane, the carbon 2s and 2p atomic orbitals make four hybrid orbitals called sp3 orbitals. One sp3 orbital from each carbon overlap to produce a C-C bond. The others overlap with 1s orbitals in each of the hydrogen atoms, forming C-H sigma bonds. The bonding in the double bond in alkenes is slightly different. A sigma bond forms between the two carbons in the same was as in a single bond. A pi-bond if formed by the electrons in adjacent p orbitals overlapping above and below the carbon atoms. The pi-bond can only form is a sigma bond has already formed. The pi-bond restricts movement around the double bond. This means the region around the double bond in a molecule is flat. Double bonds are ‘planar’. The pi-bond is the reactive part of the molecule because there is a high electron density around it.