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

2.5 Stereochemical nomenclature of ethane derivatives

2.5 Stereochemical nomenclature of ethane derivatives

　We have already learned several terms, such as gauche, anti, staggered or eclipsed, to name the conformers based on the Newman projection. These are, however, insufficient when a more complex molecule is involved. For instance, there are three staggered forms for 2,3-dimethylbutane as shown below. It is impossible to define these as gauche or anti as long as we use the relations of methyl groups as the standard.

　A systematic nomenclature was proposed which could be applicable to such complex cases. For a four-atom system A-C₁-C₂-B, a tortion angle q (23) is defined. Atoms C₁ and C₂ may be other types of atoms, but for simplicity we take these two as carbon atoms.

The definition of tortion angle is as follows.

• 1) We look the molecule from C₁-C₂ axis as shown in 23
• 2) Rotate A-C₁bond so that A overlaps B.
• 3) The tortion angle is defined as the necessary angle of rotation to overlap A with B.
• 4) The tortion angle is positive if the required rotation is clockwise, and negative if the rotation is anticlockwise.

　It is important to notice that the tortion angle has a sign while dihedral angle is not. We shall use tortion angle as far as possible in this textbook.

Ligands A and B are chosen from the ligands bonded to C₁and C₂, respectively

• 1) If all three ligands are different, the priority is determined by the sequence rule.
• 2) If two ligands are identical, the rest is chosen regardless of the priority based on the sequence rule.
• 3) If three ligands are identical, the one associated with the smallest tortion angle is chosen.

　In practice, the tortion angle of ethane derivatives is not necessarily a multiple of 60^o. Hence, the value of tortion angle itself is not included in the stereochemical names. A circle (tortion angle = 360^o) is divided into several sections as is shown in 24-26, and each section is given a name. By combining these names, a system of nomenclature of ethane derivatives is formed. The upper half of the circle is syn region, and the lower half is anti region (cf. 25). The p of 26 stands for periplanar region, and c for clinal region. By combining these regions, you can obtain 27. The essence of this nomenclature will be summarized in ▶S2.5.

▶Back to Q2.8 as necessary

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