Principal Functional group

• The principal functional group is used to define the class the compound belongs to e.g. an alcohol, ROH
• The principal functional group is the highest priority functional group. Functional group priority is discussed later.
• The principal functional group is usually given the lowest locant possible.
• The priority order applies to the functional groups.
• When a group is named as a substituent, the priority order does not apply.

• The longest continuous chain containing the principal functional group defines the root name. 
• Other groups attached to this chain are called substituents.
• If there are two chains of equal length, then the choice that gives the simplest substituents is chosen.

• The numbers that define the positions of the principal functional group and substituents are called locants.
• Compounds are numbered from one end of the longest continuous chain.
• The locants are assigned such that the principal functional group gets the lowest possible locant.
• If this results in a "tie" then the first point of difference rule is applied so that the first time a difference in numbering occurs, then the method that gives the lower number at this first difference is used.
• In the event that there is no first point of difference then alphabetisation is used.
• Note that a locant can only be omitted if they are a. "1" and b. would be unambiguous (see later).