Question

Draw two constitutional isomers of \(c i s-1,2\) -dibromocyclopentane.

Short answer

Cis-1,2-dibromocyclopentane has isomers with Br at 1,3 and 1,2 positions on the ring, but the second has "trans" stereochemistry.

Step-by-step solution

Understand the problem

We need to draw two constitutional isomers of cis-1,2-dibromocyclopentane. Constitutional isomers have the same molecular formula but differ in the connectivity of their atoms.

Analyze the given compound

Cis-1,2-dibromocyclopentane suggests a cyclopentane ring with two bromine atoms attached. These bromine atoms are on adjacent carbon atoms (positions 1 and 2) and on the same side, due to the 'cis' configuration.

Create the first isomer

Draw the cyclopentane ring, a five-membered carbon ring. Position the two bromine atoms on the adjacent carbon atoms, ensuring both bromines lie on the same side (either above or below the plane of the ring).

Draw a constitutional isomer

To get a constitutional isomer, change the position of the bromine atoms while keeping the cyclopentane backbone intact. For example, place the bromine atoms at carbon 1 and carbon 3. It must be ensured that the new position changes the connectivity, not just the stereochemistry.

Verify isomerism

Verify that both structures have the same molecular formula, \(C_5H_8Br_2\), but are not identical, confirming they are indeed constitutional isomers.

Key concepts

Cis Configuration

A cis configuration refers to a specific arrangement of atoms in a molecule where two similar or identical substituents are located on the same side of a particular plane or bond. In the context of cyclic compounds, like cyclopentane, this configuration means that both substituents, say bromine atoms in dibromocyclopentane, are positioned on the same side of the ring. This is crucial for determining the molecule’s properties, especially its interactions with light and other compounds.
Cis configuration often impacts the physical properties such as boiling and melting points compared to its trans counterpart, where substituents are on opposite sides. For learners, remember:

• "Cis" indicates proximity of similar groups on the same plane.
• Important for identifying molecular isomers that differ in spatial orientation, not just atom connectivity.

Cyclopentane

Cyclopentane is an organic compound comprised of a ring of five carbon atoms, each saturated and single-bonded, forming a closed loop. Its structural formula can be represented as a pentagon, with implied hydrogen atoms bonded to each carbon. The ring shape of cyclopentane introduces unique chemical and physical properties compared to its acyclic counterparts.
Understanding the cyclopentane ring is crucial when dealing with derivatives like dibromocyclopentane, where additional atoms, such as bromines, are attached. This compound has a straightforward formula of \( C_5H_{10} \), but when bromines are added, it evolves into more complex structures.

• Five-membered carbon ring.
• Forms the framework for various substituted compounds.
• Flexible and less strained compared to smaller cycloalkanes.

Molecular Formula

The molecular formula of a compound simply provides the number of each type of atom present in a molecule. For 'cis-1,2-dibromocyclopentane', the molecular formula is \( C_5H_8Br_2 \). Every isomer, including constitutional isomers, retains the same molecular formula but may have different structures.
While the molecular formula shows what types and how many atoms are in a molecule, it doesn't indicate how those atoms are arranged. This is crucial when identifying isomers, as isomers share the same molecular formula but differ in either connectivity or spatial arrangement.Points to remember:

• Does not provide structural information.
• Useful for determining potential isomers.
• Helps in calculating molecular weight and other properties.

Atom Connectivity

Atom connectivity in chemistry defines how atoms are connected to each other through chemical bonds in a molecule. In constitutional isomers, the connectivity differs, meaning that the way atoms are linked changes, resulting in different compounds with the same molecular formula.
For instance, in the case of cis-1,2-dibromocyclopentane, altering which carbon atoms the bromines attach to creates a new isomer. This process keeps the cyclopentane ring intact but rearranges the substituents, offering variety in its chemical identity. Understand these points:

• Determines structural changes in isomers.
• Essential for distinguishing different compounds with the same molecular formula.
• Not to be confused with changes in stereochemistry, which involves spatial arrangements rather than connectivity.