Conformation, Sawhorse and Newman Projections

Exactly assume that you are fitting a puzzle in the correct position according to the piece you have in your hand. You are turning, trying to adjust it to the best place with different angles; till it fits on each corner would be an obvious condition of molecules under different circumstances. For example, the use of the term conformation in organic chemistry is to help people have an understanding of how molecules can take on shapes and geometry by merely rotating single bonds that do not cleave any bonds but facilitate the molecule's search through a vast conformational space of shapes and orientations. This is a characteristic of how many biological processes are able to be floppy; one classic example is the very simple enzyme-substrate interaction principle, in which it is mainly the correct molecular fit that escalates the success of the reaction. The physical and chemical properties of a molecule are actually dependent upon its conformations. Chemists require different projection methods in the successful investigation of these conformations. Sawhorse and Newman projections are a few such devices that are useful in giving a 2D representation of a molecule's 3D structure. The projections make the chemist realize relationships between the spaces that exist between various atoms, and one can predict how these relationships determine the behavior of the molecule in concern. A sawhorse representation gives an angle view of a molecule, allowing better visualization of dihedral angles existing between different substituents on carbons. A Newman projection can easily help us compare the staggered and eclipsed conformations by looking down the axis of a carbon-carbon bond.

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This Story also Contains

1. Salient Features
2. Types of Different Conformations and Aspects
3. Sawhorse Projections
4. Summary

Salient Features

Conformations

Conformation about a molecule may be a spatial arrangement of its atoms which will be brought about by rotation about a single bond. It does not depend on connectivity, as in the case of the definition of an isomer; they only depend on the different orientations of the same molecule. The molecule has many arrangements that can be acquired defining its chemical and physical properties. Listen, for example, the conformation of the drug molecule at the receptor site determines everything concerned about the efficacy of that drug at the receptor site.

Conformations

Alkanes contain carbon-carbon sigma (σ) bonds. Electron distribution of the sigma molecular orbital is symmetrical around the internuclear axis of the C–C bond which is not disturbed due to rotation about its axis. This permits free rotation about C–C single bond. This rotation results in different spatial arrangements of atoms in space which can change into one another. Such spatial arrangements of atoms that can be converted into one another by rotation around a C-C single bond are called conformations or conformers or rotamers. Alkanes can thus have an infinite number of conformations by rotation around C-C single bonds. However, it may be remembered that rotation around a C-C single bond is not completely free. It is hindered by a small energy barrier of 1-20 kJ mol^–1 due to weak repulsive interaction between the adjacent bonds. Such a type of repulsive interaction is called torsional strain.

Sawhorse projections
In this projection, the molecule is viewed along the molecular axis. It is then projected on paper by drawing the central C–C bond as a somewhat longer straight line. Upper end of the line is slightly tilted towards the right or left-hand side. The front carbon is shown at the lower end of the line, whereas the rear carbon is shown at the upper end. Each carbon has three lines attached to it corresponding to three hydrogen atoms. The lines are inclined at an angle of 120° to each other. Sawhorse projections of eclipsed and staggered conformations of ethane are shown in the figure.

Newman projections

In this projection, the molecule is viewed at the C–C bond head-on. The carbon atom nearer to the eye is represented by a point. Three hydrogen atoms attached to the front carbon atom are shown by three lines drawn at an angle of 120° to each other. The rear carbon atom is represented by a circle and the three hydrogen atoms are shown attached to it by the shorter lines drawn at an angle of 120° to each other. Newman’s projections for ethane are shown in the figure.

Sawhorse Projections

These projections are nothing but the representation of the three-dimensional conformation of the molecule into two dimensions. Key features of such a projection are: the view of a molecule is from an oblique manner, two neighboring carbons are shown and the spatial orientation of the groups about those two carbons are shown. The advantage of this method is that it gives a clear-cut view of the dihedral angles subtended by two substituents, and by that, it is of immense help in the process of the determination of the conformer stability, especially for the steric effects. Newman Projections The Newman projections are the second method to realize molecular conformations. This projection views along a carbon-carbon bond. In this view, we look down the viewer's line of sight at the substitutes which are attached to a pair of carbons. New man projections are particularly useful. They are in identifying and differentiating between properly staggered and eclipsed conformations of molecules. Generally more stable are the staggered conformations in which the substituents are now 60 apart, as greater separation of the atoms begins to become the point of minimum steric repulsion. In eclipsed conformations, the increase in higher energy is the result of an increase in steric strain, which derives from the overlapping of substituents.

Types of Different Conformations and Aspects

Different conformations mainly deal with staggered and eclipsed conformations. The staggered conformation is an arrangement in which the substituents of neighboring carbons are located as far from each other as possible with reduced energy, therefore maximizing stability. Two forms exist of a staggered conformation: gauche conformation and anti-conformation. The substituents in the gauche conformations are staggered 60° apart, while the substituents in the conformation are 180° apart. The radicals in the latter are the most twisted conformations because steric hindrance is the least there.

Sawhorse Projections

Dihedral angles can make the substituents differ to denote different conformations in the sawhorse projection. For example, in the staggered anti-conformation, the substituents need to be 180 degrees apart; in the gauche conformation, the substituents are 60 degrees apart. Chemists determine the most stable conformation of a molecule from sawhorse projections; this tells plenty about reactivity, stability, and interaction.

The use of Newman projections also allows a comparison of the staggered conformation with the eclipsed conformation. In a staggered conformation, the substituents of the front carbon are midway between the substituents of the rear carbon, hence reducing the effect of the repulsive forces, whereas, in an eclipsed conformation, there is the viewing of the substituents as well as its neighboring ones from the point of view of either substituent. This is held responsible for being associated with a higher energy of the conformation. This is exactly when the energy of several conformations can actually be observed by rotating Newman projections in the hope of observing which conformation is energy-favorable.

Relevance and Applications

Real-World Applications

Conformational analysis is a very important field, and the impacts that its consequences portray are huge for all fields, from pharmaceuticals to material science and biochemistry. For drug design, for instance, knowing the rightful preferred conformation of a given molecule will then be an easy task to predict motion toward the biological target. It is the adaptation of preferred conformational shapes for correct receptor fitting that makes beta-blockers more in their treatment of heart conditions in treatment.

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Some Solved Examples

Example 1
Question:
Which one of the conformed of butane is the most stable?

1)Anti

2) Gauche

3) Eclipse

4) Fully Eclipse

Solution:

In anti, the two $-\mathrm{CH}_3$ remain farthest from each other, due to which it has the least bond

strain and has the highest stability,

Hence the correct option is (1)

Example 2
Question:
Staggered and eclipsed conformers of ethane are:

1) Polymers

2) (correct) Rotamers

3) Enantiomers

4) Mirror images

Solution:
Conformational isomers are called rotamers as they can be converted into each other by rotation about a single bond.

Hence, the correct answer is option (2).

Example 3
Question:
The dihedral angle in the staggered form of the Newman projection of 1,1,1-Trichloroethane is ......... degree. (Round off to the nearest integer)

1) (correct) 60

2) 45

3) 20

4) 90

Solution:
The staggered form of 1,1,1-Trichloroethane is given as:

The dihedral angle is 60 degrees.

Hence, the answer is 60.

Summary

In general, the structure or, better put, quality of the arrangement of atoms in a molecule is deduced by the conformation. There are Tucker's and other more advanced conformational analyses. Stability/ reactivity/prediction forms the tool of conformation analysis, which is exceptionally important both in the drug and material science field. These concepts' complexity allows chemists much information about molecular behavior and ways to design new compounds and materials with valued properties. So, this review identified some important features, elaborated on diversified varieties of conformations and their practical application, and, finally, gave a detailed guideline to this organic chemistry feature.