プログラム学習・有機化学 Organic chemistry Programmed Study

Ch. 2 - Ethane an Butane

Aims

In this chapter we shall learn the rotation about a single bond of ethane and the effect of rotation on the shape and energy of ethane molecule. The study on the relation between the rotation of single bonds and molecular structure and molecular energy is called conformational analysis. With the aid of Newman projection formula, conformational analysis of propane, butane and polysubstituted ethanes will be attempted. Naming of conformers will also be attempted by expanding the sequence rule.

Goal of this of this chapter 

After you learn chapter 2 completely and successfully, you can do the following things.

1. To draw Newman projection of ethane in any conformation.
2. To draw a dihedral angle-energy diagram to describe the energy change of ethane by rotation of the single bond.
3. To distinguish eclipsed and staggered conformations.
4. To distinguish two staggered conformers, anti form and gauche form, in the conformational analysis of butane.
5. To understand the nomenclature of conformers based on the sequence rule.

New terms and concepts

If you have learned about half of the following terms, go directly to the Question.

Else, please click the terms which you have not yet learned, and review them.

• Perspective representation　（透視画法）

• Dash-and-wedge representation　（くさび画法）
• 
Newman projection

• conformational analysis

• tortion angle-energy diagram 

• conformer (conformational isomer)

• eclipsed form
• staggered form

• anti form

• gauche form
• 
tortion angle
• 
periplanar

• clinal

Now, you may read through Summary to ensure the point, or go to Questions to test yourself.

Summary

S2.1 Conformational Analysis

conformational analysis: a study of the change of molecular structure (accompanied by the change of energy and properties) caused by a rotation of single bond.

the tortion angle-potential energy curve: a plot of potential energy(abscissa) versus twist angle (horizontal). indispensable for conformational analysis of simple molecule.

S2.2 Eclipsed and Staggered Forms.

eclipsed form 3: conformation of ethane and analogs with φ = 0^o, 120^o and 240^o

staggered form 4: conformation of ethane and analogs with φ = 60^o, 180^o and 300^o 

go to 2.2 Conformational analysis of ethane  

S2.3 Gauche and Anti Forms.

gauche form: conformation of 1,2-disubstituted ethanes with φ = 60^o.

anti(trans) form: conformation of 1,2-disubstituted ethanes with φ = 180^o. 

go to 2.3 Conformational Analysis of Butane

S2.4

The relation between the value of ΔG and composition of conformers A and B are tabulated as below. Table ΔG and relative amount. K(298K) amount of the more stable isomer (%) ΔG (kJ/mol)

ΔG = -2.303 RT log K

go to 2.4 Relative abundance of conformers.

S2.5 The stereochemical nomenclature based on the rotation about a single bond

go to 2.5 Stereochemical nomenclature of ethane derivatives