Takeuchi's Stereochemistry

Chapter 2. Ethane

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.

Object of Chapter 2.

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@i@j
Dash-and-wedge representation@iщ@j
Newman projection
conformational analysis
tortion angle-energy diagram
conformer (conformational isomer)
eclipsed form
staggered form
anti form
gauche form

tortion angle

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

Go to index page (International Version)


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 = 0o, 120o and 240o

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

go to 2.2 Conformational analysis of ethane

S2.3 Gauche and Anti Forms.

gauche form; conformation of 1,2-disubstituted ethanes with = 60 o.

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

go to 2.3 Conformational Analysis of Butane


The relation between the value of G and composition of conformers A and B are tabulated as below. Table G and relative amount. K(298K) amount of the more stable isomer(%) G(kJ mol-1)

G = -2.303 RT log K

K (298K) 2 3 4 5 10 20 100 1000 10000
amount of the more stable isomer(%) 67 75 80 83 91 95 99 99.9 99.99
-DG[kJ/mol] 1.71 2.72 3.43 3.97 5.85 7.52 11.3 17.1 23.0

Temperature and Free Energy Difference vs. Molar Ratio (JAVA)

go to 2.4 Relative abundance of conformers.

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

tortion angle q name
0}30o synperiplanar@sp
{30o`{90o {synclinal@{sc @
{90o`{150o@ {anticlinal@{ac
{150o`{180o {antiperiplanar@({)ap
-30o`-90o -synclinal@-sc
-90o`-150o -anticlinal@-ac @
150o`-180o -antiperiplanar@(-)ap@

Nomenclature of Conformers (JAVA)

go to 2.5 Stereochemical nomenclature of ethane derivatives