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

1.6 Nomenclature of stereochemistry

1.6 Nomenclature of stereochemistry

The nomenclature of compounds should have an important role because one or more molecules are present for single molecular formula. First of all, a name must specify only one compound (though two or more name is possible for one compound). Secondly, the structure of a compound should be deduced from its name unambiguously. After the age of Lavoisier, chemists made much effort to devise reasonable and universal and simple rules for nomenclature of compounds. As a result, even beginners can easily understand and use the nomenclatures especially for considerably simple compounds. For example, C₄H<sub">9OH 2~5 is named as follows.

The first nomenclature (A) is traditional or trivial one which have been used since 19 th century, while the second nomenclature (B) is more regular or systematic one. Once you understand its general rules, the latter may be more useful.

Structural isomers, which have the same molecular formula but differ in the order in which the atoms are arranged in space, also require a nomenclature system by which one name will correspond to only one compound. It should also be possible to reconstitute the structure from the name. Although the nomenclature based on stereochemistry of a molecule ­ stereochemical nomenclature -- is much newer than nomenclature for compounds, they shared their ways to develop. This means that stereochemical nomenclature for certain group of compounds (e.g. alkenes or oxime etc.) was established first, and subsequently systematic nomenclature which could be applied to every group of compounds was devised.

Although the systematic nomenclature will become the standard, the former system still plays some role. Indeed, both nomenclatures are actually used, and necessity of past literatures written with old-fashioned nomenclature still exists. Therefore, we will learn both nomenclatures in this textbook.

Sequence rule

The systematic nomenclature is based on the sequence rule. The sequence rule decides the priority sequence among ligands. The stereochemistry of a compound is described by the order ranked according to the sequence rule and several additional rules about each compounds.

Now, let us take a look at the compound 19 and rank the ligands that are bonded to the atom X in order of priority.

1. Generally, the greater the atomic number, the higher the priority.
2. In the case of isotopes, the mass number is used to determine their relative priority.
3. First of all, the priority depends on the atoms that directly attached to the atom X. 
4. In the case of a tie (i.e. A, A), the ligands that are attached to each of the atoms (i.e. B, C, D, and B, C, E, respectively) must be considered.
5. Even if there are still tie atoms remaining, the procedure must be repeated till the priority is determined.
6. If an atom is doubly or triply bonded to another atom, the priority system treats it as if it were bonded to two or three of those atoms.

Thus, in A-B double bond, the atom A is considered to be bonded to two of atom B, the atom B is to be bonded to two of atom A, and the sequence rules (1)~(5) are to be applied.

These atoms (A) or (B) are called replica atoms.

▶go to S1.4 Sequence rules

▶(Back to Q1.25 as necessary)

The priority order of common ligands is listed in ▶S1.5.. It may be worth warning that the order can be easily changed by substitution. For example, methyl group is ranked lower than ethyl group in the list, however fluoromethyl CH₂F-has higher priority than ethyl.

▶go to S1.5 Priority of representative ligands (lower to higher)