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In naming organic chemicals, the nomenclature must specify the location of all special features. This is commonly done using "locants" such as numbers, as in 2-bromobutane. There are some cases where the number is omitted. However, students are sometimes confused about when they can omit a number. This page summarizes the cases where the number, most often a 1, can be omitted, and discusses a common misconception about when it can be omitted.
The general rule is that the locant (number) can be omitted when there is no ambiguity; that is, when only one location is possible. It cannot be omitted by making any assumption that a certain location is more likely.
The examples below are presented without showing the structures. All are simple compounds, and you should be able to draw them. In fact, this page will probably work best if you draw the structures for yourself as you go, so you can think about whether or not a number is required.
The cases where the number can be omitted can be considered in three classes:
The aldehyde and carboxylic acid functional groups can occur only at the end of a chain. When one of these is the main functional group, its position is defined as "1". Thus butanal (aldehyde) and butanoic acid (carboxylic acid) have the indicated functional group at the 1-position.
Similarly, butanedial and butanedioic acid have one functional group at each end; there is no other possibility, and the numbers are omitted.
All positions on a ring are equivalent. If there is one substituent, that substituent defines the "1" position. Thus bromocyclobutane and bromobenzene do not need numbers. There is only one possible compound of each name, and the bromo position is defined as "1".
This applies to single ring compounds, where all positions on the ring are equivalent.
There are several chemicals, generally small ones, where the number for a substituent is commonly omitted, even though they do not quite fit one of the specific exceptions discussed above. However, they do fit the general criterion that the number is not needed, because there is no other choice. A list of some of those special cases follows. I encourage you to draw the structures, and be sure you agree that only one structure is possible in each case:
Sometimes people think that the 1 is a "default" number. That is, they think that the number 1 is implied if no number is given. Using this logic, they might give octanol or octene as (incorrect) names for 1-octanol or 1-octene. As you read the cases above where the number is omitted, it is important to realize that it is omitted because there is no other possibility, not because there is a default number. That is, one never "assumes" that a group is at "1".
A chemical name conveys information about the structure of the chemical; a name can be thought of as an "instruction manual" for how to draw the structure. When we first started discussing the nomenclature of organic compounds, we made the point that there are two types of wrong names. One type of incorrect name fails to provide the correct information. Another type of incorrect name may provide the information, but not follow the official (IUPAC) "rules" for doing so.
As an example, consider the simple chemical 1-chloropropane. If someone names this chloropropane, they have failed to communicate what compound they mean (since the chloro could be at either the 1-position or the 2-position). If someone names it 3-chloropropane, it is quite clear what they mean, even though they have violated the naming rules (give the substituent the lowest possible number).
The first type of error is much more important, as it means the primary goal of naming -- communication of the structure -- has failed.
How is this relevant to the discussion of omitting numbers? Well, you can do more harm by omitting a number that is needed than by giving one that is not needed. If it doubt, include the number. Omit the number only when it is perfectly clear what the meaning is.