Why do alkenes but not alkanes exhibit cis-trans isomerism?
Alkanes have free rotation around the single bond; the pi bond keeps the alkenes from rotating
alkane have no double bond just sigma bond.
Alkenes, unlike alkanes, exhibit cis-trans isomerism due to the presence of a carbon-carbon double bond. The double bond restricts rotation around the bond axis, which creates a rigid structure and allows the formation of different spatial arrangements of substituent groups around the double bond. These different spatial arrangements are known as cis and trans isomers.
To understand why alkenes can exhibit cis-trans isomerism, you need to understand the nature of the double bond and how it affects molecular geometry.
To start, alkenes have a general formula of CnH2n, indicating that they contain a carbon-carbon double bond (denoted by the "n" in Cn). The double bond consists of a sigma bond and a pi bond. The sigma bond is formed by the overlap of hybridized orbitals between the carbon atoms, while the pi bond is created by the sideways overlap of unhybridized p orbitals.
The presence of the pi bond results in the electrons being shared over a larger volume of space above and below the plane of the carbon atoms. As a result, there is a restricted rotation around the double bond, unlike in alkanes where the sigma bonds allow free rotation.
This restricted rotation is what gives rise to the possibility of cis-trans isomerism in alkenes. In a cis isomer, the substituent groups on each carbon atom of the double bonded carbons are on the same side of the molecule. In a trans isomer, the substituent groups are on opposite sides of the molecule.
Now, let's delve into how to determine whether a given alkene exhibits cis-trans isomerism.
To determine the possibility of cis-trans isomerism, examine the substituents attached to the double bonded carbons. If two substituents are different on each carbon, and the remaining two places on each carbon are occupied by hydrogen atoms, cis-trans isomerism is possible.
For example, consider the alkene but-2-ene. A possible substituent arrangement is one side of the double bond having a methyl group (CH3) and a hydrogen (H), while the other side has a hydrogen (H) and an ethyl group (CH2CH3). In this case, the two substituents are different on each carbon, and cis-trans isomerism is possible.
To summarize, alkenes exhibit cis-trans isomerism due to the presence of a carbon-carbon double bond that restricts rotation around the bond axis. This restriction allows for different spatial arrangements of substituent groups, creating the possibility of cis and trans isomers. Determining whether a given alkene exhibits cis-trans isomerism involves evaluating the arrangement of substituents around the double bond.