Chemical Literature (Chem 184/284)

Substance Indexing: The Challenge of Nomenclature

• In order to ensure that each substance has a unique possible name, and to group "like" compounds together, CA has devised their own system of nomenclature (not necessarily IUPAC) and scheme for arranging them in the Chemical Substance Index.
• Unfortunately, this system can be hideously complex. Here's a hideous example
• Dodecahedrane (C20H20) used to be listed as simply dodecahedrane.
• Then a systematic name was assigned:

5,2,1,6,3,4-[2,3]Butanylidenedipentaleno [2,1,6-cde:2',1',6'-gha]pentalene, hexadecahydro-
• Now it's treated as a member of the fullerene family:

[5]Fullerane-C20-Ih
• It is important to remember that the CAS nomenclature has changed over time, as in the case of dodecahedrane above. The most important change took place in 1972; nomenclature has been fairly stable since then. But if you are using the older literature, you may have to do some checking to be sure of the correct terminology.

Basic Rules of CAS Nomenclature

• CAS indexers select the "main" part of the compound to act as the heading parent.
• Substituents to the parent are listed after it. This is referred to as inverted order
• What constitutes a parent compound and how it would be named are not always obvious, even to a chemist.
• Examples
• Toluene is

Benzene, methyl-
• ortho-Xylene is

Benzene, 1,2-dimethyl-
• Benzyl alcohol is

Benzenemethanol
• When there are multiple substituents, they are listed in alphabetical order, including the prefixes.
• Carbon tetrachloride is

Methane, tetrachloro-
• CCl₂F₂ is

Methane, dichlorodifluoro-
• CCl₃F is

Methane, fluorotrichloro-
• Polymers are listed by the monomer(s) or repeating unit, with polymer or homopolymer appended.
• Teflon is

Ethene, tetrafluoro-, homopolymer

Alphabetization of Compounds

• Compounds are listed first by parent compound, with the parent compound itself first (with any qualifiers and categories), then by substituted forms in alphabetical order.
• Substituents are read from left to right, ignoring numbers and punctuation.
• Example: Benzene
• Benzene
• Benzene, analysis
• Benzene, uses and miscellaneous
• Benzene, compounds
• Benzene, polymers
• Benzene, azido-
• Benzene, chloro-
• Benzene, 1,2-dibutyl-

Special Cases: Salts

• Salts of organic acids, or inorganic oxyacids are named as derivatives of the parent acid.
• Potassium chloride is

Potassium chloride
• But: Potassium sulfate is

Sulfuric acid, potassium salt (2:1)

Helps for finding CAS Chemical Names

• In general, it can be very tricky to look at the structure of a complex compound and decide what the CA name will be.
• However, in many cases, you can use a variety of resources to help find the CA name.

Using the Index Guide for Chemical Names

• If the compound has a common or trade name, check the Index Guide.
• The Index Guide is especially good for drugs, natural products, dyes, etc.
• For other common chemicals, even if you can't find the specific chemical you want, you may be able to find a similar one and get a clue to follow.

Using the Registry Number Handbook for Chemical Names

• CAS publishes a "handbook" which lists Registry Numbers and gives the CAS systematic name for the substance.
• Remember that there are many sources you can use to find Registry Numbers which have good synonym indexes: Merck, HODOC, Aldrich, Kirk-Othmer
• The UCSB library's copy is shelved just after Chemical Abstracts itself.

Molecular Formula Index

• While most molecular formulas have a large number of possible compounds. it is far easier to look at a possible name and decide whether it matches your compound than to guess at a name.
• Note that the Molecular Formula Index just gives a list of abstract numbers, not a breakdown by subheadings.

Molecular Formula Index Organization

• Molecular formulas are listed in Hill order.
• If carbon is present, it comes first, followed by hydrogen, then all other elements in alphabetical order.
• If not, then all (including H) in alphabetical order.
• Note that the rules for salts apply to molecular formulas, too.
• Molecular Formula Examples
• Benzene is C₆H₆
• Teflon is (C₂F₄)x
• Ferrocene is C₁₀H₁₀Fe
• Hydrochloric acid is ClH
• Benzoic acid is C₇H₆O₂
• Sodium benzoate is C₇H₆O₂, sodium salt...NOT C₇H₆NaO₂

Using the Ring System Handbook for Chemical Names

• Most compounds with a polycyclic ring system use the name of the ring system as the parent compound.
• The Handbook lists ring systems in order of:
• Increasing number of rings
• Increasing number of atoms in the ring
• Increasing Hill order formula of the ring
• Step 1: Count number of rings, using the smallest rings in the structure which will take in all the atoms in the ring system - 5
• Step 2: Count the number of atoms in each ring - 5, 6, 6, 5, 5
• Step 3: Note the "molecular formula" of each ring - C₅, C₆, C₅O, C₄O, C₅
• Step 4: Arrange the formulas in order of increasing size and Hill order: C₄O, C₅, C₅, C₅O, C₆
• Step 5: Look up the ring systems that fit the formula and pick the correct one by inspection (not always easy).
• Resulting name: 5H-4a,11a-epoxy-7,9a-methano-1H-cyclopenta[b]heptalene
• The entry for a given ring system gives structure diagram with CAS locant numbers, name, Registry Number of the parent ring.
• Rings which are less unsaturated will (for complex rings) have the same parent name, but with, for example, "decahydro" added.

When print tools fail you...

• Even with all of the above, sometimes none of the tools will help you find the correct name.
• And if you don't find something, does it mean that you haven't guessed the right name, or that it hasn't been reported??
• Enter the power of computer searching - structure searching can give a definitive answer to most questions.