Chemistry

How are bis, tris, and tetrakis prefixes treated in alphabetical ordering?

Step-by-step chemistry solution: How are bis, tris, and tetrakis prefixes treated in alphabetical ordering?

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1. What the question is asking

A student wants to know how to alphabetise substituent names that contain the complex‑multiplicative prefixes bis‑, tris‑, tetrakis‑, ….
When we list substituents in a systematic IUPAC name we put them in alphabetical order ignoring certain prefixes.
The question is:

Do we alphabetise a substituent that begins with “bis(…)”, “tris(…)”, “tetrakis(…)”, etc. under the letter *b, t, …, or do we ignore the prefix and alphabetise it according to the first letter of the group that appears inside the parentheses?*

The answer must be based on the current IUPAC “Blue Book” (Nomenclature of Organic Chemistry, 2013).

2. Step‑by‑step reasoning

2.1 Identify the type of prefix

  • simple multiplicative prefixes – di‑, tri‑, tetra‑, penta‑ … used directly before a simple alkyl or substituent name (e.g. dimethyl).
  • complex multiplicative prefixes – bis‑, tris‑, tetrakis‑, etc. used when the repeated group is written in parentheses because the group itself contains locants, unsaturation, hetero‑atoms, etc. (e.g. bis(2‑chloroethyl)).

Both kinds are multiplicative prefixes in IUPAC terminology.

2.2 The rule that governs alphabetical ordering

IUPAC “Blue Book”, Section P‑15.2.1 (Alphabetical ordering of substituents) states:

When a list of substituents is ordered alphabetically, ignore:
a) any locants (numerical prefixes) attached to the substituent,
b) any simple multiplicative prefixes (di‑, tri‑, tetra‑, …), and
c) any complex multiplicative prefixes (bis‑, tris‑, tetrakis‑, …).

The rule continues:

The alphabetical order is then determined from the first letter of the substituent name that remains after the ignored prefixes have been removed. Hyphens and commas are also ignored.

Thus, bis‑, tris‑, tetrakis‑, etc. are ignored exactly the same way as di‑, tri‑, tetra‑.

2.3 Apply the rule to a concrete example

Consider the two substituents

  1. bis(2‑chloroethyl)

  2. dimethyl

  3. Remove the ignored prefix bis‑ → we are left with the group (2‑chloroethyl).
    Ignoring the locant “2‑” and the hyphen, the first substantive letter is c (from chloroethyl).

  4. Remove the ignored prefix di‑ → we are left with methyl, whose first letter is m.

Alphabetical order: c (bis‑…) comes before m (dimethyl).

A second example:

  • tris(tert‑butyl) → ignore tris‑, ignore the hyphen, the first substantive letter is t (from tert‑butyl).
  • tert‑butyl (no multiplicative prefix) → also starts with t.
    Since the first letters are the same, the next letters are compared (“e” vs “e”), and the two names would be ordered according to the full spelling of the remaining part of the name (i.e., “tert‑butyl” before “tert‑butyl” – they are identical, so any tie‑breaking rule such as the number of repetitions would be used).

2.4 Summary of the procedure

Step What to do
1. Write each substituent exactly as it appears in the systematic name.
2. Delete any leading multiplicative prefix – di‑, tri‑, tetra‑, bis‑, tris‑, tetrakis‑, … .
3. Delete any locants (numbers) and the hyphens that separate them from the rest of the name.
4. Compare the remaining strings alphabetically (case‑insensitive).
5. If two strings are identical up to the point where they differ only by the deleted prefix, the order is decided by the numerical multiplicity (e.g., bis(…) follows mono(…)), but this situation is rarely encountered in ordinary organic‑compound names.

3. Final answer

In alphabetical ordering of substituent names, the prefixes bis‑, tris‑, tetrakis‑ (and the related complex multiplicative prefixes) are **ignored.**
The ordering is determined by the first letter of the substituent that follows the parentheses, after also ignoring any locants and hyphens.

Consequently, a substituent such as bis(2‑chloroethyl) is alphabetised under “c” (for chloroethyl), not under “b”. The same applies to tris(…), tetrakis(…), etc.

4. Common mistakes

Mistake Why it is wrong Correct approach
Treating bis‑, tris‑, tetrakis‑ as ordinary lexical prefixes and alphabetising under “b”, “t”, etc. The IUPAC rules explicitly state that these are complex multiplicative prefixes and must be ignored for ordering. Remove the prefix before comparing letters.
Ignoring the rule for simple prefixes (di‑, tri‑, tetra‑) but remembering it for complex ones. Both simple and complex multiplicative prefixes are ignored in the same way. Apply the same “ignore” rule to all multiplicative prefixes.
Including the hyphen or the locant number in the comparison (e.g., ordering “2‑chloroethyl” before “chloroethyl”). Hyphens and locants are also ignored when determining alphabetical order. Strip numbers and hyphens before the comparison.
Using the whole phrase “bis(2‑chloroethyl)” as a single word in the list. This treats the prefix as part of the sortable text, violating the rule. Delete bis‑ and the locant “2‑” → compare “chloroethyl”.
Assuming that the alphabetical order depends on the order of the parentheses. The parentheses are merely a way of writing complex substituents; they have no effect on sorting. Look only at the name inside the parentheses after the prefix is removed.

By following the steps outlined above, the alphabetical list of substituents will always be consistent with the current IUPAC recommendations.

Original question: How are bis, tris, and tetrakis prefixes treated in alphabetical ordering? on Chemistry Stack Exchange, licensed CC BY-SA.

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