Ch. 2 - Ethane an Butane


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

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


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 = 0o, 120o and 240o


staggered form 4: conformation of ethane and analogs with φ = 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 φ = 60o.

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


go to 2.3 Conformational Analysis of Butane




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

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
-ΔG [kJ/mol] 1.71 2.72 3.43 3.97 5.85 7.52 11.3 17.1 23.0

go to 2.4 Relative abundance of conformers.


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


tortion angle φ name  Symbol
0±30o synperiplanar sp
+30o+90o +synclinal +sc
+90o+150o +anticlinal +ac
+150o~+180o +antiperiplanar (+)ap
-30o~-90o -synclinal -sc
-90o~-150o -anticlinal -ac
ー150o~-180o -antiperiplanar (-)ap


go to 2.5 Stereochemical nomenclature of ethane derivatives