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CONFORMATION OF ACYCLIC MOLECULES In this tutorial, you will learn how and why acyclic molecules adopt the three-dimensional shapes that they do. Before you begin, take the time to jot down the definitions for the following terms: strain, bond rotation, conformation, staggered conformation, eclipsed conformation, dihedral angle, torsional strain, gauche conformation, anti conformation. All of these terms can be found in the Illustrated Glossary of Organic Chemistry. You should also be prepared to use a model kit (you can find a tutorial on Dr. Hardinger’s website). Strain measures the amount of energy that a molecule obtains due to structural distortion; it is a measure of structural instability that is usually a result of electron repulsion. All molecules have an optimal three-dimensional structure that allows for the least possible strain on the system. While some molecules alleviate this strain by bond rotation, others cannot.

With your model kit, construct ethane (CH3CH3). It should look like this:

Why can bonds rotate?

When two atoms can be rotated around each other without breaking the bond between them, they have free bond rotation. In the case of two carbon atoms that are bound together by a single σ bond, no matter how you rotate the atoms, the electron density between them will never be disturbed. Double bonds require two adjacent p-orbitals to be parallel to each other in order to form a π bond. If the p-orbitals are no longer parallel, the electron density cannot be shared between the two atoms. This means that bond rotation is not possible; rotation would disrupt the p-orbital orientation and therefore break the bond. Even though the nature of the single bond allows for free bond rotation, there is still an energy price in doing so. Free bond rotation can be unfavorable for other reasons.

sp3 Carbon Hydrogen

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The three-dimensional arrangement of a molecule that comes about from rotation around any single bond is called a conformation. It is possible to rotate around the C-C bond of ethane to find different conformations. These structures will be related to each other as conformers or conformational isomers.

Ethane can adopt a staggered conformation (Figure 1). This allows the hydrogens on either carbon to be as far away from each other as possible. Ethane can also adopt an eclipsed conformation (Figure 2). In this conformation, the hydrogen atoms are as close as possible to each other. Like a solar eclipse, the hydrogens in the front eclipse those in the back.

A common term that is used when describing components of conformation is dihedral angle. A dihedral angle (θ) is defined as the angle between two intersecting planes (a plane must be made up of at least three points). To

Figure 2. Eclipsed conformation of ethane (A) from the side and (B) looking down the C-C bond.

A   B  

Eclipsed

Figure 1. Staggered conformation of ethane (A) from the side and (B) looking down the C-C bond.

A   B  

Staggered

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simplify, when discussing dihedral angles in the context of conformations, it refers to the angle between two bonds: in the eclipsed conformation, the angle between two C-H bonds on each carbon is 0°. The conformation that has the least strain will be the optimal arrangement. In these two conformations of ethane, one is able to better reduce the strain between atoms by reducing electron repulsion interactions of the hydrogens. The staggered conformation is the lowest energy conformation due to its ability to get all the atoms as far from each other as possible. The eclipsed conformation is ethane’s highest energy conformation. Ethane will rotate about the carbon-carbon bond between these two conformations. The energy difference is approximately 12.6 kJ/mol. This is a measure of ethane’s torsional strain, or strain which can be alleviated by rotating around a bond (in this case the carbon-carbon bond). Now, construct butane (CH3CH2CH2CH3). It should look something like this: Butane has three carbon-carbon bonds. Since bond rotation is permissible about all three, butane has more possible conformations than ethane. Let’s explore the C2-C3 bond. When rotating around this bond, let’s consider how we found the staggered and eclipsed conformations of ethane.

C2

C3

Figure 3. Two eclipsed conformations of butane. (Left) The two methyl groups are eclipsed to give a dihedral angle of 0° looking down C2-C3, and (Center) from the side. (Right) The methyl groups are eclipsed with hydrogens as seen when looking down C2-C3 and their dihedral angle becomes 120°.

C2 C3 C2 C3 C2

C3

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Butane has two eclipsed conformations (Figure 3). The groups which are involved in the ‘eclipse’ are different between the two conformations. If we consider the dihedral angles between the two C-CH3 bonds, it will be 0° and 120°, respectively for each conformation. As we rotate around C2-C3 between the two eclipsed conformations, we pass through butane’s gauche conformation, in which the two methyl groups have a dihedral angle of 60° (Figure 4). This is one of butane’s three staggered conformations.

Let’s take a moment to consider the relative strain of these conformers. Strain is related to the size of an atom or group. As electron cloud gets larger, so does the amount of repulsion its surroundings will experience. If we look at the two eclipsed conformations, we can see that they have different amounts of strain. In the eclipsed conformation in which the dihedral angle is 0°, the methyl groups are the closest they can be, forcing the electron clouds to crowd each other. When the dihedral angle is 120°, the methyl group is eclipsed with a hydrogen, which is much smaller than a methyl group (Figure 5). Therefore, it is reasonable to assume that the closer the methyls are to each other, the higher the strain and less stable (higher energy) that conformation will be. If we compare the gauche and eclipsed conformation in which the dihedral angle is 120°, we can see that the eclipsed conformation forces the methyl group to lie in the same plane as a hydrogen. Since the electron cloud of a methyl group is much larger than that of a single hydrogen, this conformer is lower in energy, but still relatively unstable, as compared to the other eclipsed conformer. In the gauche conformation, since all the groups are staggered, the molecule is better able to avoid unnecessary strain. From this we can conclude that the gauche conformation is lower in energy than both eclipsed conformers. Can we find a lower energy conformation?

Figure 4. Gauche conformation of ethane (A) looking down the C2-C3 bond and (B) from the side.

A   B  

C2

C3 C2

C3

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If we orient the methyl groups so that they are as far away as possible, with a dihedral angle of 180°, we have found butane’s anti conformation (Figure 6). This is another staggered conformer of butane. Since all the groups in the molecule are positioned with as much space between them as possible, this is the optimal, lowest energy conformation of butane.

We can also plot the relative energies of these conformers as a function of their dihedral angles (Figure 7). This helps to illustrate that butane will exist predominantly in its anti and gauche conformations because these have much higher stability due to the reduced torsional strain. It is also important to see that rotating from gauche to anti requires the molecule to pass through a less stable conformer which does require some energy.

Figure 6. Butane’s anti conformation looking down C2-C3 (A) and side view (B).

A   B  

Figure 5. Graphic representation of electron clouds in eclipsed conformations if butane. (A) Methyl groups are eclipsing each other. (B) The methyl group is eclipsed with a hydrogen.

A   B  

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To summarize:

1. Acyclic molecules can freely rotate around single bonds. 2. These molecules can arrange their groups by rotating around these single

bonds. 3. A molecule will arrange its groups to adopt the most stable, least energy

conformation. 4. This optimal conformation is characterized by reduced torsional strain, or

alleviated electron repulsion between groups. 5. Free bond rotation requires energy.

CH3

HHCH3

H H

H

H3C

H

CH3

HHH

H3C

H

H

CH3H

CH3

HHH

H CH3

CH3

HHCH3

H H

H

H3C

H

CH3

HHH

H3C

H

H

CH3H

CH3

HHH

H CH3

Ener

gy

Dihedral Angle 0° 60° 120° 180°

Anti

Eclipsed

Eclipsed

Gauche

Figure 7. Plot of energy vs dihedral angle of butane’s conformations.

CH3

HHCH3

H H

H

H3C

H

CH3

HHH

H3C

H

H

CH3H

CH3

HHH

H CH3

CH3

HHCH3

H H

H

H3C

H

CH3

HHH

H3C

H

H

CH3H

CH3

HHH

H CH3

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Exercise. (Answers provided following the references section) Make a model of 1-chloro-3-methylbutane. For this exercise, let’s consider looking down C1-C2.

A. Torsional strain in 1-chloro-3-methylbutane will come about from the relative sizes of the hydrogens, chloride and the propyl group. Rank these from largest to smallest.

B. What is the highest energy conformation that 1-chloro-3-methylbutane can adopt? Why? What is the dihedral angle between the chloride and propyl group?

C. What is the lowest energy conformation that 1-chloro-3-methylbutane can

adopt? Why? What is the dihedral angle between the chloride and propyl group?

D. What is the conformation when the dihedral angle is 60°? What is the

conformation when the dihedral angle is 120°?

E. Sketch a plot of the dihedral angles used in A-C vs the (relative) energies of each conformation.

F. Consider the conformation when the dihedral angle is 300°. What is the

name of this conformation? Where does it fall in your energy plot? References 1. Brown, W.H.; Foote, C.S.; Iverson, B.L.; Anslyn, E.V. Organic Chemistry, 5 ed;

Brooks/Cole: Belmont, CA, 2009. 2. Hardinger, S.A. Chemistry 14C Lecture Supplement, 8 ed; Hayden-McNeil:

Plymouth, MI, 2015.

C1 C2

C3

C4

Methyl

Chlorine

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3. Hardinger, S.A. Chemistry 14C Thinkbook, 12 ed; Hayden-McNeil: Plymouth, MI, 2015.

4. Hardinger, S.A. Dr. Hardinger’s Organic Chemistry Page-UCLA. Illustrated Glossary. http://www.chem.ucla.edu/harding/index.html (accessed March 31, 2015).

Exercise Answers A. Propyl > Chloride > Hydrogen B. The highest energy conformation is eclipsed, with the chloride and propyl group at 0°. This is the least stable conformation due to the electron repulsion between the two large groups.

C. The lowest energy conformation is the anti conformation, with the chloride and propyl group at 180°. This is the most stable conformation due to the alleviated torsional strain between the two groups.

Eclipsed 0°

C1

C2

C1

C2

Anti 180°

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D. When the dihedral angle is 60°, the molecule has adopted a gauche conformation. When the dihedral angle is 120°, the molecule has adopted an eclipsed conformation.

E.

Gauche 60°

Eclipsed 120°

Cl

H CH(CH3)2Cl

HH

H

H

CH(CH3)2

Cl

HHH

(H3C)2HC

H

Cl

HH

CH(CH3)2

H HCl

HH

Ener

gy

Dihedral Angle 0° 60° 120° 180°

Anti

Eclipsed

Eclipsed

Gauche

Cl

H CH(CH3)2Cl

HH

H

H

CH(CH3)2

Cl

HHH

(H3C)2HC

H

Cl

HH

CH(CH3)2

H HCl

HH

Cl

H CH(CH3)2Cl

HH

H

H

CH(CH3)2

Cl

HHH

(H3C)2HC

H

Cl

HH

CH(CH3)2

H HCl

HH

Cl

H CH(CH3)2Cl

HH

H

H

CH(CH3)2

Cl

HHH

(H3C)2HC

H

Cl

HH

CH(CH3)2

H HCl

HH

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F. The conformation when the dihedral is 300° is another gauche conformation. It is the reflection of the conformation adopted when the dihedral angle is 60°. These conformations will be of equal energy.