Conformations of 1,2-Dihaloethane in Newman Projections: A Detailed Explanation with Potential Energy Diagram
1,2-Dihaloethane is an organic compound that contains two halogen atoms attached to adjacent carbon atoms in an ethane (C2H6) backbone. This molecule exhibits different conformations due to the rotational freedom about the single bonds between the two carbon atoms. The conformations can be visualized using Newman projections, a method that helps represent the 3D structure of molecules by showing the relative orientations of atoms in a staggered or eclipsed configuration when looking down the bond between two carbon atoms.
1. Newman Projection: Introduction
Newman projection is a way to illustrate the conformation of molecules, particularly alkanes and derivatives like 1,2-dihaloethane. By looking down the bond connecting two atoms (usually carbons), the perspective shows how atoms or groups attached to those carbons are oriented. In a Newman projection:
- The front carbon atom is represented as a dot, while the back carbon is represented by a circle.
- Bonds to the front carbon are drawn as lines radiating out from the dot, while bonds to the back carbon are shown as lines radiating from the circle.
- The bond between the two carbons can be either staggered (where substituents on adjacent carbons are as far apart as possible) or eclipsed (where substituents are aligned directly in front of each other, creating strain).
2. Conformations of 1,2-Dihaloethane
Let’s analyze the conformations of 1,2-dihaloethane, where the two halogen atoms are on adjacent carbon atoms. The different conformations of 1,2-dihaloethane can be grouped as staggered and eclipsed, and their potential energies will depend on the interactions between the substituents (including the halogen atoms, typically chlorine or bromine).
a) Staggered Conformation (Anti Conformation)
In the staggered conformation, the two halogen atoms are positioned opposite each other, with other substituents like hydrogen atoms in the most separated positions. This is the most stable conformation since it minimizes steric strain. The staggered conformation can be represented as follows:
- Newman Projection: When viewed from the front, the two halogen atoms are 180° apart, and the hydrogens are also staggered around the central carbon-carbon bond.
- Energy Level: This conformation has the lowest energy because the halogen atoms are as far apart as possible, minimizing repulsion between them. The bond torsional strain is minimal.
b) Eclipsed Conformation (Syn Conformation)
The eclipsed conformation occurs when the halogen atoms are aligned with each other across the C-C bond, and other substituents (like hydrogens) are also in direct alignment. This conformation has higher energy due to torsional strain and steric hindrance between the atoms or groups on adjacent carbons.
- Newman Projection: In the eclipsed conformation, the two halogen atoms are directly in front of each other, leading to a higher potential energy due to the repulsion between the bulky halogen atoms. The hydrogens are also eclipsed.
- Energy Level: This conformation is higher in energy due to the steric repulsion between the halogens and the hydrogen atoms. The torsional strain due to the eclipsing bonds also contributes to the increase in energy.
c) Gauche Conformation
The gauche conformation is an intermediate between the staggered and eclipsed forms, where the two halogen atoms are 60° apart. This conformation exhibits a balance between steric strain and torsional strain, resulting in a moderate level of energy.
- Newman Projection: The two halogen atoms are not exactly opposite one another but are positioned 60° apart. Other substituents are staggered as well, but there is still some steric repulsion between the halogens.
- Energy Level: The gauche conformation has a higher energy than the staggered (anti) conformation due to the proximity of the halogen atoms, but it is more stable than the fully eclipsed conformation.
d) Other Intermediate Conformations
In addition to the typical staggered (anti) and eclipsed (syn) conformations, there are also other intermediate conformations, which are variations of the gauche conformation with varying degrees of steric interactions. These include the gauche-anti conformation, where the two halogens are still somewhat close to each other but positioned in a staggered manner.
3. Potential Energy Diagram
The potential energy diagram of 1,2-dihaloethane as a function of torsional angle (rotation about the C-C bond) demonstrates how the molecule changes energy as it adopts different conformations. The diagram shows the following:
- Staggered Conformation (Anti Conformation): This is the lowest energy conformation, as explained earlier. The potential energy is at a minimum here, and the molecule is most stable.
- Gauche Conformation: The energy rises slightly from the anti conformation due to some steric strain between the halogen atoms, but the molecule is still relatively stable.
- Eclipsed Conformation (Syn Conformation): The highest point on the potential energy curve corresponds to the eclipsed conformation. This is due to the steric repulsion between the halogen atoms and between the hydrogens and halogens, making it the least stable conformation.
The potential energy curve for 1,2-dihaloethane typically looks like a periodic function with alternating minima and maxima. The minima correspond to the staggered conformations (anti and gauche), and the maxima correspond to the eclipsed conformations.
4. Influence of the Halogens
The nature of the halogen atoms (chlorine, bromine, etc.) significantly influences the steric interactions in 1,2-dihaloethane. Heavier halogens, such as bromine, result in more significant steric repulsion compared to lighter halogens like chlorine. This will affect the relative stability of different conformations and shift the energy barriers between them.
- For example, bromine atoms will create higher steric hindrance due to their larger size, which would increase the energy difference between staggered and eclipsed forms, making the eclipsed conformation even less favorable.
Conclusion
In summary, 1,2-dihaloethane exhibits multiple conformations based on the rotation around the C-C bond. The staggered (anti) conformation is the most stable due to minimized steric and torsional strain, while the eclipsed conformation is the least stable due to the repulsion between substituents. The potential energy diagram highlights the energetic preferences for each conformation and explains the underlying factors driving molecular stability. Understanding these conformations is crucial for predicting the behavior of halogenated compounds in various chemical reactions, particularly in organic synthesis and reaction mechanisms.
Subscribe on YouTube - NotesWorld
For PDF copy of Solved Assignment
Any University Assignment Solution
.webp)
