Question

Draw all the structural isomers of ${\mathbf{\text{C}}}_{\mathbf{5}}{\mathbf{\text{H}}}_{\mathbf{10}}$. Ignore any cyclic isomers. Which of the structural isomers exhibit cis-trans isomerism?

Short answer

There are five structural isomers of ${\text{C}}_5{\text{H}}_{10}$.

Only pent-2-ene shows cis-trans isomerism.

Step-by-step solution

Definition of isomerism.

Isomerism refers to the presence of molecules with the same number of atoms of the same type (and hence the same formula) but different chemical and physical characteristics.

All the structural isomers of C5H10.

The structural isomersare the ones that have a similar number of atomsbut are completely different compounds.

Here, the compounds have atoms arranged differently and therefore they have all the physical and chemical properties different.

The image below depicts all structural isomers of ${\text{C}}_5{\text{H}}_{10}$.

All the geometric isomers of C5H10.

In geometric isomerism, the compounds have atoms bonded similarly. Here, the two bulky groups on the doubly-bonded carbon atoms are one time on the opposite side (TRANS) and one time on the similar side (CIS).

In pent-2-eneonly the cis-trans isomerism exists.

The terminal alkene with 2-H atoms cannot form cis-trans isomers as the condition for CIS-TANS isomerism is that both the doubly-bonded carbon atoms should be attacked to two different substituents. The example of the compounds that cannot show cis-trans isomerism is shown below.

The alkene with 2-methyl groups on one doubly bonded carbon atom also cannot form cis-trans isomers.