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Isomerism in chemistry means that compounds with the same formula have different chemical structures and have different properties. The different arrangement of carbon and hydrogen atoms caused by the rotation of the C-C single sigma bond is called conformation or conformers.
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Keyterms: Isomerism, hydrogen, atoms, single sigma bond, C-C bond, alkanes, carbons, electrons, ethane, hydrogen
Read Also: Class 12 Saturated Hydrocarbons
Conformation
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Conformation can be defined as the shape produced by a molecule due to rotation around one or more single bonds. For example, in the case of alkanes, there is a distribution of electrons around the axis between the nuclei of the C-C bond. This further allows free rotation around the C-C single bond, resulting in a different spatial arrangement of the carbons. In addition, alkanes therefore also have an infinite number of conformations by rotating around single C-C bonds.
Also Read: Isomerism in Coordination Compounds
Conformational Isomers
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The concept of conformation can be understood with the help of the Ethane molecule.
- The ethane molecule (C2H6) contains a single chain of two carbon atoms (C-C) with three hydrogens attached to each carbon atom.
- If we see the ball and stick model of ethane, one carbon atom is rotated, keeping the other carbon atom stationary about the C-C axis. In that case, an infinite number of configurations or arrangements occurs between the hydrogen atoms attached to the rotated carbon atom and the hydrogen atoms of the fixed carbon atom.
- These are called spatial arrangements of atoms and can be converted into one another based on the rotation of the C-C single bond and are called conformers or conformations.
Also Read: Hydrogen Bonding
Types of Conformational Isomers or Conformers
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a. Eclipsed conformation
If during rotation, the three hydrogen atoms of each of the carbon atoms are at such an angle to each other, which makes them the closest to each other, they are called eclipsed conformation.
b. Staggered conformation
If the hydrogen atoms of each of the carbon atoms are at such an angle that they are the farthest from each other, then that conformation is known as staggered conformation.
c. Skew Conformation
All other arrangements of the hydrogen atoms of each carbon atom are known as skew conformation. There can be an infinite number of conformations as a result of complete free rotations. Apart from the two mentioned above, all of them are skew conformations.
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Representation of Eclipsed and Staggered Conformation
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a. Sawhorse projections
- In this projection, a long straight line denotes the bond between (C-C) carbon atoms.
- The lower end of the line represents the front carbon atom of the molecule whereas the upper end of the line represents the rear carbon atom.
- Taking the example of ethane, we know that each of the carbon atoms has three hydrogen atoms. Here they are denoted by three lines arising out of both the lower and far end of the line. They are at an angle of 120 degrees to each other.
The Sawhorse projection of both eclipsed and staggered conformation of ethane molecule is given below:
b. Newman projections
- In this projection, a dot denotes the carbon atom in the front part, whereas a circle around the dot represents the carbon atom at the rear end.
- The three hydrogen atoms of the front carbon atom arise from the dot, and the other three hydrogen atoms of the rear carbon atom appear from the circle's circumference.
- Again, the hydrogen atoms of individual carbon atoms are at 120 degrees to each other.
Here both the staggered and the eclipsed conformation of ethane is shown using the Newman projection.
Also Read: Bond Enthalpy
Things to Remember
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- The shape produced by a molecule due to rotation around one or more single bonds is called conformation.
- Conformational Isomers or Conformers are the different arrangements of the carbon atom.
- Conformational Isomers are divided into two types- Eclipsed conformation and Staggered Conformation.
- Eclipsed and Staggered Conformation can be represented with the help of Sawhorse projections and Newman projections.
- In Sawhorse projection, the ends of long straight lines represent the carbon atom and the small lines arising from the ends represent the bonded atoms.
- In Newman projections, a dot denotes the carbon atom in the front part, whereas a circle around the dot represents the carbon atom at the rear end. The lines arising from the dot and the circle represent the bonded atoms.
Sample Questions
Ques.1: Which conformation of ethane is more stable? (1 Mark)
Ans: Staggered conformation of ethane is a more stable conformation.
Ques.2: Which conformation of ethane has the lowest potential energy? (1 Mark)
Ans: Staggered conformation of ethane has the lowest potential energy.
Ques.3:Draw a Newman projection of the most stable conformation of propane and 2-methylpropane. (1 Mark)
Ans: Newman projection of the most stable conformation of propane and 2-methylpropane are-
Ques.4: Is rotational energy required more in the case of a double bond or single bond in C-C atoms? (1 Mark)
Ans: In the case of the double bond, more rotational energy is required as compared to the single bond in C-C atoms.
Ques.5: From the perspective of viewing down the C2-C3 bond, draw the Newman projection of the most stable conformation of 2,3-dimethylbutane. (1 Mark)
Ans: Newman projection of the most stable conformation of 2,3-dimethylbutane is
Ques.6: Define the term conformation. (1 Mark)
Ans: Conformation can be defined as the different arrangements of the same molecule formed by rotations about single bonds of the carbon atom.
Ques.7: Draw the Newman projection of the most stable conformation about C1−C2 bond of 1-Bromo-2-methylpropane? (2 Marks)
Ans: The Newmann's projection of the most stable conformation about C1−C2 bond of 1-bromo-2-methylpropane can be represented as-
The Br atom and the methyl groups are trans to each other and the steric interactions are minimized.
Ques.8: (a) What is the dihedral angle between two C-H bonds in the staggered conformation of ethane?
(b) At which temperature can conformational isomers form? (2 Marks)
Ans: (a)The dihedral angle between two C-H bonds in the staggered conformation of ethane is 60 degrees.
(b) At room temperature, the conformational isomers are formed.
Ques.9: What is the dihedral angle? (2 Marks)
Ans: The dihedral angle is also known as the torsional angle and is denoted by the symbol θ—the angle formed between two bonds arising from different atoms in a Newman projection.
For example, let's take the ethane molecule in its Newman projection form,
The angle formed between the hydrogen atom of the front carbon atom (represented by the dot) and the hydrogen atom of the rear carbon atom (represented by the circle) is called the dihedral angle.
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