Question

Tell whether the following pairs of compounds are identical, constitutional isomers, stereoisomers, or unrelated. (a) cis-1,3-Dibromocyclohexane and trans-1,4-dibromocyclohexane (b) 2,3 -Dimethylhexane and 2,3,3 -trimethylpentane

Short answer

Both pairs are constitutional isomers.

Step-by-step solution

Define Types of Isomers

Begin by understanding the different types of isomers: **Constitutional isomers** differ in the connectivity of atoms, **stereoisomers** have the same connectivity but differ in the spatial arrangement (cis or trans), **identical compounds** are the same in every aspect, and **unrelated compounds** differ entirely either in molecular formula or structural essence.

Analyze Pair (a)

For cis-1,3-Dibromocyclohexane and trans-1,4-dibromocyclohexane, examine the molecular structure. Both have the same molecular formula, so they are not unrelated. However, the 1,3-positioning vs. the 1,4-positioning affects connectivity, identifying them as **constitutional isomers** since the position and relationships of the bromine atoms differ fundamentally.

Analyze Pair (b)

For 2,3-Dimethylhexane and 2,3,3-trimethylpentane, compare the structures. Count carbon atoms; both have C8H18 so they're related structurally. However, connectivity differs since the second compound has an extra methyl group (a 3rd carbon substituent changing the skeleton), indicating these are **constitutional isomers** due to different branching even with the same molecular formula.

Key concepts

Constitutional Isomers

Constitutional isomers, also known as structural isomers, are crucial to understanding variations in molecular structure. They are compounds with the same molecular formula but differ in the connectivity of their atoms.
Unlike stereoisomers, constitutional isomers can have entirely different physical and chemical properties. This is because the way in which atoms are connected influences how molecules interact with other substances.
Consider the example of ethanol ( CH_3CH_2OH ) and dimethyl ether ( CH_3OCH_3 ). While both have the formula C_2H_6O , their differing structures result in them being entirely different compounds with vastly different properties.

• Example: Different branching patterns in hydrocarbons
• Role in different reactions due to varied connectivity

Grasping this concept allows for accurate identification of isomers that exhibit unique chemical behavior and physical characteristics.

Stereoisomers

Stereosiomers have the same structural formula and sequence of bonded atoms, but differ in the three-dimensional orientation of their atoms in space. These variations can result in different physical properties and chemical reactivities.
There are two primary types of stereoisomers: enantiomers and diastereomers. Enantiomers are non-superimposable mirror images of each other, much like left and right hands, while diastereomers are not.
Understanding stereoisomers is essential in fields such as pharmacology, as different spatial arrangements can result in different biological activity.

• Enantiomers have identical properties except in chiral environments
• Diastereomers can have significantly different properties

This concept helps chemists and researchers design molecules with desired structural characteristics and functions.

Molecular Structure Analysis

Molecular structure analysis is a powerful tool used to comprehend the shape and form of chemical compounds, ultimately aiding in determining their isomeric relationships. Techniques such as VSEPR theory and advanced spectrometric methods like NMR (Nuclear Magnetic Resonance) and X-ray Crystallography provide insights into molecular geometry and connectivity.
By analyzing a molecule's shape, scientists can predict its reactivity and interactions with other molecules, especially critical for identifying isomers. Correctly interpreting molecular structures enables the differentiation between various isomers: constitutional isomers, stereoisomers, and even beyond.

• VSEPR theory aids in predicting molecular geometry
• Spectrometric methods assist in understanding atom connectivity

Leveraging these analytical methods is essential for developing materials and compounds for industries ranging from pharmacology to materials science.

Cis-Trans Isomerism

Cis-trans isomerism (also known as geometric isomerism) is a subset of stereoisomerism. It occurs in molecules that have restricted rotation around a bond. This restriction is often due to double bonds or ring structures, like in alkenes or cycloalkanes.
"Cis" refers to the configuration where substituent groups are on the same side, while "trans" means they are on opposite sides. These differences crucially affect the properties of compounds, such as their boiling point, melting point, and biological activities.
The classic example is 2-butene, where the cis form has a higher boiling point than its trans isomer due to the presence of stronger intermolecular forces.

• Relevant in differentiating compounds with similar formulae but different geometry
• Key factor in determining compound stability and reactivity

Understanding cis-trans isomerism broadens comprehension of compound behaviors and how structural nuances influence chemical interactions.