Why is geometric isomerism possible for alkenes but not for alkanes and alkynes?

Alkanes are hydrocarbons containing only single covalent bonds (C-C) between carbon atoms. They have the general formula \(C_nH {2n+2}\), where n is the number of carbon atoms in the molecule. Alkanes are saturated hydrocarbons and exhibit free rotation around the C-C single bonds. Alkenes are hydrocarbons containing a carbon-carbon double bond (C=C). Their general formula is \(C_nH {2n}\). The double bond in alkenes limits the rotation around the C=C bond, leading to the possibility of geometric isomerism. Alkynes are hydrocarbons with at least one carbon-carbon triple bond (C≡C). They have the general formula \(C_nH {2n-2}\). In alkynes, the triple bond holds the carbon atoms even closer together than the double bond in alkenes, leaving no room for geometric isomers.

For geometric isomerism to occur, the following conditions must be met: 1. The molecule must have restricted rotation, preventing the groups around the central bond from interconverting easily. 2. There must be two different groups (or atoms) bonded to each of the central atoms in the bond of interest.

Alkenes meet both conditions for geometric isomerism. The presence of a C=C double bond leads to restricted rotation around this bond. Additionally, each carbon atom of the double bond can have two different groups or atoms bonded to it, fulfilling the second condition. Therefore, geometric isomerism is possible for alkenes.

For example, consider 2-butene: - In the cis isomer, the two methyl groups (CH3) are on the same side of the double bond, while in the trans isomer, they are on opposite sides. - Notice that the difference in arrangement is only possible because of the restricted rotation around the double bond and the different groups bonded to the carbon atoms.

Alkanes do not exhibit geometric isomerism because they have only single bonds (C-C) between carbon atoms, which allow free rotation. This rotational freedom prevents the formation of stable geometric isomers in alkanes, as any different arrangement can interconvert easily.

Although alkynes have a restricted rotation due to the presence of a triple bond (C≡C), they still do not meet the second condition for geometric isomerism. Both carbon atoms in the triple bond are already bonded to two other carbon atoms (the triple bond itself counts as two bonds), leaving no room for two different groups or atoms on each carbon. Therefore, geometric isomerism is not possible for alkynes. In summary, geometric isomerism is possible for alkenes due to the presence of a double bond that both restricts rotation and allows for two distinct groups to fix their relative positions, while alkanes and alkynes lack the required structural characteristics for this type of isomerism.