2.3 Conformational Analysis of Butane
2.3 Conformational Analysis of Butane
Although propane has much the same tortion angle-energy curve with ethane, a considerably different result will be obtained when the conformational analysis for butane is attempted in relation to the rotation of central C-C bond. When the potential energy of the molecule is plotted against the tortion angle made by two C-CH3 bonds, there are obtained two conformers with maximum energy(A and C of Fig. 2.4) and two conformers with minimum energy(B and D of Fig. 2.4).
Fig. 2.4 The tortion angle-potential energy curve of butane.
The Newman projection corresponding to conformers A-D are shown below(5-8). In the case of ethane, staggered and eclipsed conformations can describe the feature of its conformational analysis; in the case of butane, however, two conformations are not enough to describe the feature of the curve. Of the eclipsed form, 5, the one with φ = 0o is called cis form, and of the staggered form, 6, the one with φ = 60o is called gauche form and the other, 8, with φ = 180o is referred to trans or anti form.
▶go to S2.3 Gauche and Anti Forms
Because of the proximity of two methyl groups, 5 is associated with larger energy than 7, and 6 is associated with larger energy than 8. Cases where this treatment is not applicable because of more complicated structures are involved will be discussed later.
One can write many Newman projection for one particular molecule. For an ethane derivative ABCC-CDEF, projection can be done either from ABC side(16) or from DEF side(15). In addition, there are many rotational isomers(rotamers) in relation to the rotation about C-C bond. Much the same is true for perspective drawings(11)-(14).