Introduction to organic chemistry

Topics: Stereochemistry, Atom, Chemical bond Pages: 11 (1093 words) Published: September 18, 2014


At the end of this unit, the students should be able to:

1. Learn the Types of Structures
a. Sawhorse Structures
b. Newman Projections

2. Learn to draw the different Isomers
a. Sawhorse Structures
b. Newman Projections
Types of Structures
1. Two-dimensional structural formula

4. Three-dimensional structural formula

2. Condensed structural formula

5. Sawhorse structures

3. Bond-line structural formula

6. Newman Projection

Three-dimensional structural formula

Sawhorse Structures

In a sawhorse projection, the backbone carbons are represented by a diagonal line, and the terminal carbons are shown in groups.

In a staggered conformation the atoms and groups attached to each backbone carbon fit in the voids around the groups on the adjacent carbon.

In an eclipsed conformation the groups and atoms on adjacent carbons are in line with each other.

Steps in Drawing Sawhorse Structures
Draw the sawhorse projection of 1-iodo-1,2-dicholoro-2-fluoroethane in staggered conformation.

Step 1 Draw a line representing the carbon-carbon bond sloping upward and to the right.

Step 2 Then draw a bond at each carbon, one going exactly straight up and the other exactly straight down.

Step 3 Next, add the two remaining bonds at each carbon, arranged as shown to give the correct bond angles.

Step 4 Finally, add the symbols for the elements.


Draw a sawhorse projection of 1,2-ethanediol, HO-CH2-CH2-OH in both staggered and eclipsed conformations. staggered conformation
eclipsed conformation

Newman Projection

End-on representations for conformations are commonly drawn using a convention called a Newman projection. A Newman projection is a graphic that shows the three groups bonded to each carbon atom in a particular C – C bond, as well as the dihedral angle that separates them.

How to Draw a Newman Projection
Step 1 Look directly down the C – C bond (end-on), and draw a circle with a dot in the center to represent the carbons of the C – C bond.

Step 2 Draw in the bonds.

Step 3 Add the atoms on each bond.

Newman Projections for the Staggered and Eclipsed Conformations ethane


In a Newman projection it doesn’t matter which C you pick to be in the front or the back. Example: All of the Newman projections shown here represent the staggered conformation of propane.

The staggered and eclipsed conformations of ethane interconvert at room temperature, but each conformation is not equally stable. The staggered conformations are more stable (lower in energy) than the eclipsed conformations. Electron–electron repulsion between the bonds in the eclipsed conformation increases its energy compared to the staggered conformation, where the bonding electrons are farther apart.

There are also other factors that play into the Newman Projection. These include the kind of attachments on a molecule. Example: butane
If the carbon attachments are on opposite ends, 180 degrees part, this is considered an anti conformation.

If there is a chain of carbon as an attachment in both the front and back carbon, and the attachments are 60 degrees apart, this is considered a gauche conformation.

Why are there different Newman Projections for one molecule? Because the carbons are linked by single bonds, the bond is able to freely rotate, allowing for various conformations to exist.

Example: Draw the Newman projection of butane, C4H10.

The illustration of butane above is represented by the Newman Projections below by designating the two middle carbons, one as the “front” and one as the “behind” carbon, and connecting the two end methyl groups accordingly.

The stability of butane: anti  gauche  eclipsed  totally eclipsed

How were these Newman Projections of butane obtained?...
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