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Conformationational Isomerism - What are Conformers, Key Terms in Conformational Analysis, Types of Conformers, Sawhorse Projection, Newman’s Projection, Conformers of Ethane, Conformers of Butane, Practice Problems and FAQs

Conformationational Isomerism - What are Conformers, Key Terms in Conformational Analysis, Types of Conformers, Sawhorse Projection, Newman’s Projection, Conformers of Ethane, Conformers of Butane, Practice Problems and FAQs

Do you Know that Dhoni was a goalkeeper of his school football team before becoming India's World Cup Winning Captain ? Yes he started his career with football as a goalkeeper.

But what is the role of goalkeeper in football game?

The goalkeeper's main role is to stop the opposing team from putting the ball over the goal-line of the goal. This is accomplished by having the goalkeeper move into the trajectory of the ball to either catch it or direct it further from the vicinity of the goal line.

Now , the main point is that to play a 90 minute of football game , each player of both teams needs to be physically fit. So to do so they all do lot of exercise in their gym session such as

Push-ups , squats , crunches etc.

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While doing these exercises you may have noticed that shifting body position can be done in a variety of ways. Alternately, we could assert that there are countless possible spatial configurations. Similarly in chemistry, by rotating the single bonds, a molecule can take on a number of shapes while maintaining its localised atom arrangement (every saturated carbon atom is tetrahedral).

Let us know more about this rotation and the results.

Table of content

  • What are Conformers?
  • Key Terms in Conformational Analysis
  • Sawhorse projection
  • Newman’s projection
  • Conformers of ethane
  • Conformers of butane
  • Difference between conformation and configuration
  • Practice problems
  • Frequently asked questions-FAQ

What are Conformers?

Two carbon atoms in an organic compound may have a single covalent sigma bond only or in addition pi-bonds. The respective carbon atoms can easily rotate around the single sigma bond, while the presence of pi-bonds prevents it from doing so. The rotation of carbon atoms will take the attached atoms or groups along with them, resulting in different orientations with respect to others. These different positions of atoms or groups constitute different spatial positions for the same molecules. But each spatial position will experience different forces of interactions and hence shall possess different energy. Of course, the spatial orientation with least energy will be the most stable configuration for the molecule.

All the possible spatial arrangements of atoms due to the free rotation around C-C single bonds are called conformers or rotamers.

Characteristics of conformers:

  1. What will be the number of conformers? Rotation around 360° could result in an innumerable number of isomers. But we shall consider rotation around an incremental 60° and the resulting conformers or rotamers
  2. The conformations around these rotation angles are referred as eclipsed, gauche, staggered and anti-conformational forms
  3. Is the rotation total free? No. It to needs energy for rotation. But it is so small that the room temperature is sufficient to provide enough energy for rotation
  4. The different conformations are going to have different interactional energies called “torsional energy” which will restrict free rotation and the preferred conformation over others.
  5. Can we separate the conformers?

No. Isomers have fixed configurations and possess a larger difference in energy between them. Conformers on the other hand differ by a mere 1 to 10kJ of energy. Hence isomers can be separated, while it is very difficult to separate conformers from one another.

  1. Nevertheless, the stable conformers are useful in explaining the reaction mechanism

Key Terms in Conformational Analysis

There are a few terms related to conformational analysis. These are explained below.

  1. Free Rotation

A sigma covalent bond which undergoes free rotation at room temperature such as C-C, C-N, C-O, O-O, N-N, etc.

  1. Conformers / Rotamers or Conformations

Due to free rotation along a sigma covalent bond, molecules can have an endless variety of spatial orientations which are referred to as conformers.

  1. Conformational Isomers
  • Conformational isomers are those conformations that are the most stable and have the lowest potential energy.
  • These can never be isolated because they are not real isomers.
  • They are defined as conformations that are most stable and have the lowest potential energy as a result of free rotation from 0° to 360°.
  • Degenerate isomers or equienergic isomers are those conformers that share the same energy.
  • Non-degenerate isomers are conformational isomers with varying energies.
  • According to the rotational angle, the conformational isomers are located at the potential energy minima in the potential energy diagram.
  1. Conformational Energy
  • The rotational energy barrier is known as conformational energy. It is the potential energy difference between conformations at potential energy minima and maxima.
  • We can spin roughly as many single bonds as we like to go from one conformation to another. Breaking any bonds is the one thing we are unable to accomplish.
  • Conformational energy is the required amount of energy to overcome this rotational barrier to convert one conformer into another.
  1. Dihedral Angle (DHA) / Torsion Angle / Angle of Rotation (ф)
  • The angle between a C-H bond at the closer carbon and a C-H bond at the farther carbon is what we refer to as the dihedral angle, which is also known as the torsion angle
  • It can also be defined as the angle formed by two intersecting planes with two atoms in common.
  • ф = 0° in the eclipsed configuration, ф = 60° in the staggered conformation, ф = 180° in the anti configuration.

  1. Torsional Strain
  • It is described as the electrical repulsion of the electrons in two neighbouring eclipsed bonds' bond pairs.
  • When the molecule is in an eclipsed conformation, it is active at torsional angles of 0°, 120°, and 240°.
  • In the staggered conformation, it is seen as being almost zero. (DHA. = 60º, 180º, 300º)
  • Torsional strain is a chemical compound's resistance to bond twisting.
  • It has to do with the bond angle with respect to bond rotation. The two big groups are furthest apart in the staggered conformation (anti), which reduces torsional strain.
  1. Van der Waals Strain
  • It is the repulsion of the group to the bonds that are adjacent to it. The van der Waal strain is intermediate in gauche conformations and is maximal in the eclipsed conformation and least in the anti-conformation.
  • When two substituents in a molecule approach one another at a distance that is less than the total of their Van der Waals radii, this is referred to as Van der Waals repulsion in chemistry.
  • Van der Waals repulsion, commonly known as Van der Waals strain, is connected to steric hindrance.
  • Since the sum of van der Waals radii is less than the distance between two -H atoms, it is almost zero for -H atoms.

Sawhorse Projection

In sawhorse projection, the wanted C-C molecular axis is viewed from sideways with each carbon atom on either end of the axis are attached to their respective atoms or groups at 120° angle. Sawhorse Projections can also be drawn in such a way that the groups on the front carbon are staggered (60 degrees apart) or eclipsed (directly overlapping) with the groups on the back carbon.

For example let us consider the two saw horse projection for ethane molecule

The upper end of the line is rotated towards the left or right-hand side along the carbon-carbon single bond. The front carbon present is attached with three hydrogen atoms similarly back carbon contains three hydrogen atoms respectively. Lines are inclined at an angle of 120 degrees to each other. We can rotate the carbon atom along the single bond at an angle of 120 degrees to obtain other sawhorse structures.

Newman’s Projection

In Newman’s projection the molecule is placed as a front and back model, with the C_C molecular axis directly in front, so that the first carbon and its attachment can be seen clearly. The second carbon and the bond connecting it to the first carbon will be invisible. Of course, the attachment to the back carbon can be seen in conformers other than eclipsed position. In the case of the back carbon. Newman projections are an easy visualization of conformers of rotation..

The three lines in the shape of a Y in a Newman projection represent the three bonds of the first carbon that are attached to three different valencies and the fourth valency of the front carbon cannot be seen as it is present behind the carbon atom connecting the back carbon atom. The back carbon is represented by a circle, and the three lines coming out of the circle represent the three bonds that come off of that carbon. It is important to note that the fourth bond for each of these carbons is the carbon-carbon bond which cannot be shown in the Newman projection. A Newman projection can aid in the analysis of rotation around a specific carbon-carbon bond.

Same group will be attached as shown in the image given below.

When we view the molecule as shown, the front carbon is represented by the dot with its three valencies attached by solid lines and the fourth valency is present behind therefore it can’t be seen in Newman's projection. The 2nd carbon connected with the 1st carbon is represented by the larger circle with its three valencies attached to it as shown in the image.

Conformations of Ethane

Ethane(C2H6) contains a carbon-carbon single bond and each carbon atom is also attached to three H atoms. For understanding the concept let's take an example with Newman's projection model. Keep one carbon atom stationary and rotate other carbon atoms in a carbon-carbon bond. This rotation results in many spatial arrangements of an H atom attached to a carbon atom and another hydrogen attached to another carbon. This process is called conformational isomers.

When one carbon is rotated along with the valency attached to it keeping the valency of other carbon atoms constant we obtain the different conformational isomers. When the angle between the front carbon atom and the back carbon atom is changed at a multiple of 60 degrees( until it is rotated to 360 degrees) two standard forms called eclipsed and staggered conformations will be obtained. Staggered and eclipsed forms are interposed between each other. The conformations in which hydrogen atoms attach to two carbon makes an angle of 0 degrees (i.e dihedral angle corresponds to 0 degrees) is called Eclipsed. The eclipsed conformation of ethane is less stable than staggered.

Staggered or anti - The conformation in which hydrogen atoms are at an angle of 180 degrees is called staggered or anti forms. The stable conformation of ethane is staggered conformation.

Skew or gauche - Intermediate conformation between two standard conformation is called Skew or gauche conformers.

When the bond is rotated along the single bond to form different conformers dihedral angle which is represented by () changes but the bond angle between carbon and the attached hydrogen remains the same.

The relative energies of conformers can be plotted as shown.

Eclipsed conformation can have two types of strains-

  1. Torsional: This is the repulsive force which is operated between the molecule due to bond-pair bond-pair repulsion between the adjacent atom attached to two different atoms. It operates in case of eclipsed conformation.
  2. Van der Waal strain: This strain is observed due to the steric hindrance of the bulky group and in the stable conformation bulky group tries to orient themselves to a maximum distance so that there is no steric hindrance or steric crowding in the conformation.

The most stable form of the ethane conformation is known as staggered conformation because both the torsional strain and Van der Waal strain will be absent. Whereas, in the case of the most unstable form of the ethane conformation, the eclipsed form there is a torsional strain which is present due to bond pair bond pair repulsion as dihedral angle corresponds to 0 degree. But the Van der Waal’s strain will not be observed because the size of the hydrogen atom is small and the difference in the energy of most unstable conformation and most stable conformation corresponds to the value corresponds to 12.5 Kj mol -1. This energy is also called energy barrier which is generally available at room temperature for rotation of the molecule.

Conformers of butane molecule

Consider the butane molecule along the (along C2-C3) bond. Two hydrogens and one methyl group will be attached to the front carbon(shown in solid lines) and the 4th valency cannot be seen as it is behind the 1st carbon atom. In the case of the back carbon ( shown by a circle), the same groups will be attached as shown in the image given below.

When the methyl group attached to the back carbon is just behind the methyl group of front carbon such that the dihedral angle corresponds to 0 degrees it is said to be in eclipsed conformation. When the group is rotated at an interval of 60 degrees the six different conformers are obtained. When the dihedral angle corresponds to 60 degrees the obtained form is known as gauche form, at 120 degrees it is known as partially eclipsed form, at an angle of 180 degrees it is said to be in staggered form. On further rotation at an angle of 240 degrees, 300 degrees and 360 degrees the form obtained is known as partially eclipsed, gauche and eclipsed conformation.

The most stable form of the butane conformation is known as staggered conformation because both the torsional strain and Van der Waal strain is absent. Whereas, in case of the most unstable form of the butane conformation is known as an eclipsed form in which there is a torsional strain which is present due to bond pair repulsion as dihedral angle corresponds to 0 degree as well as van der Waal strain because of two methyl group are present behind one other.

The gauche form is comparatively more stable than the partially eclipsed form and eclipsed form because in gauche form only a slight amount of Van der Waal strain is present but there is no torsional strain. Whereas in the case of partially eclipsed form torsional strain is present but there is no Van der Waal strain. The difference in the energy of most unstable conformation and most stable conformation corresponds to the value corresponding to 19 Kj mol -1 this energy is also called energy barrier which is generally available at room temperature for rotation of the molecule.

The order of stability of different conformers of butane are

Staggered > gauche > partially eclipsed > eclipsed

respectively.

Difference between conformation and configuration

S. No.

Properties

Conformation

Configuration

1

Definition

The distinct arrangement of atoms in any molecule that can easily interconvert is known as conformation.

The distinct arrangement of atoms in any molecule that cannot easily interconvert is referred to as its configuration.

2

Interconversion

Interconversion or converting one conformation into another (of the same molecule), is simple (takes place at room temperature).

Interconversion, or converting one configuration to another, is difficult.

3

Mechanism of Conversion

Rotations around carbon to carbon single bonds is used to interconvert conformation.

Breaking and forming new chemical bonds allows for configuration interconversion.

4

Flexibility

Conformations are highly adaptable.

Configurations are less adaptable

Practice problems

Q1. Which of the following strain is possible in the eclipsed form of ethane molecule?

  1. Torsional strain
  2. Van der Waal strain
  3. Only Van der Waal strain
  4. Both A and B

Answer: (A)

Solution: The most stable form of the ethane conformation is known as staggered conformation because there is no torsional strain or Van der Waal strain. Whereas, in case of the most unstable form of the ethane conformation is known as an eclipsed form in which there is a torsional strain which is present due to bond pair bond pair repulsion as dihedral angle corresponds to 0 degree, Van der Waal strain which is observed due to the bulkiness of the molecule but will not be observed in ethane molecule because the size of a hydrogen atom is small.

Q2. Which of the following forms may be obtained when the molecule is rotated along C2-C3 bonds present in the butane molecule?

  1. Staggered
  2. Gauche
  3. Eclipsed
  4. All of the above

Answer: (D)

Solution: All the three forms will be observed in the case of butane molecule when rotated at an angle multiple of 60 degrees. Eclipsed form corresponds to the most unstable form as both the methyl group is present behind one another in two different carbon followed by the gauche form in which two methyl group is at an angle of 60 degrees adjacent to each other. The staggered form is the most stable form as both the methyl group is at an angle of 180 degrees with respect to each other.

Q3. Due to _______________, gauche conformation is less stable than eclipsed conformation in n-butane.

  1. Angle strain
  2. Hydrogen bonding
  3. van der Waals repulsion
  4. Covalent bonding

Answer: C)

Solution: As van der Waals’ force of attraction is weak, gauche conformation is less stable than eclipsed conformation of n-butane.

Q4. Select the correct option for the difference in the energy for a most stable and most unstable form of conformer in ethane molecule when rotated along (C1-C2) bond.

  1. 12.5 Kj mol -1
  2. 10.2 Kj mol -1
  3. 9.5 Kj mol-1
  4. 17.7 Kj mol -1

Answer: (A)

Solution: The most stable form the ethane conformation is known as staggered conformation because there is no torsional strain or Van der Waal strain is absent. Whereas, in case of the most unstable form of the ethane conformation is known as an eclipsed form in which there is a torsional strain which is present due to bond pair- bond pair repulsion as dihedral angle corresponds to 0 degree but van der Waal strain will not be observed because the size of a hydrogen atom is small. Due to this, there is a difference in the energy of the most stable form and the most unstable form and the value corresponds to 12.5 Kj mol -1

Frequently asked question-FAQ

Q1. How do we determine the relative stability of the conformers?
Answer:
There are two types of strain present in the conformers Van der Waal strain and torsional strain. In general van der Waal strain which is present due to the steric crowding of the group makes the conformation unstable and has higher energy than the conformer which exhibits torsional strain exhibited due to the bond-pair bond pair repulsion of the group attached on the front and back carbon. The relative stability of conformers is determined with the help of the type of strain present in it. Experimentally it is determined by the energy difference required to overcome the barrier.

Q2. What is a rotation restricted system?
Answer:
A rotation restricted system is the type of orientation in which rotation of the bond is not possible along the atom attached to it. In the case of double and triple bonds pure orbital participate in the bonding and it cannot be rotated until we break the bond.

Q3. What is the dihedral angle?
Answer:
The dihedral angle is defined as the angle between two groups attached to two different carbon atoms in the Newman configuration. When the valencies along the carbon are rotated it leads to the change in the dihedral angle and therefore alters the stability of the conformer.

Q4. How to determine the stability of staggered and gauche conformation?
Answer:
In general the staggered conformation is more stable than the gauche form because in the case of staggered form both torsional strain as well as the Van der Waal strain is not present as the dihedral angle corresponds to 180 degrees. But where intramolecular H-bonding is observed in such cases the stability of the gauche form is exceptionally more than the staggered form.

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