Functional Groups Priority Order: Which One’s Come First?

In the present-day world, there are more than twenty million known organic compounds, and it is impossible to memorize the chemical reactions for each one. Fortunately, molecules with similar functional groups tend to undergo similar reactions.

Functional groups link to the rest of the molecules via covalent bonds. They may be neutral or charged, and their naming is usually done by adding suffixes like -ol, -ene, -ane, -oic acid, and so on. The most common problem arises when there is more than one functional group attached to a single compound. For this, we have functional group priority orders.

The priority order is important in the nomenclature of organic compounds, as the higher priority group is the principal functional group, which gives its suffix to the name of the molecule and is typically numbered such that it has the lowest number.

Functional Groups Priority Order

Related content

• Organic Compounds: Introduction, Applications, Examples
• Organic vs. Inorganic Compounds: The Main Differences
• Types of Organic Reactions and their Examples
• Carboxylic acids vs. Esters

 

How do you name a molecule containing more than one functional group?

You have to follow the list of priority orders shown above (it’s not a complete list but it includes the most commonly encountered functional groups).

• Highest Priority Groups: Carboxylic Acids, Sulfonic Acids, Esters, Acid Halides, Amides.
• The next priority is for; Nitriles, Aldehydes, Ketones, Alcohols, Thiols, and Amines.
• Finally, for Alkenes and Alkynes: the priority depends on the purpose:

For the purposes of the name, “-ene” comes before “-yne” alphabetically. So when an alkene and an alkyne functional groups are present in the same organic molecule, the ending will always be “yne”. In other words, if there is a tie between an alkene and an alkyne for determining the lowest locant, the alkene takes priority.

Note that aromatic systems (arenes) such as a benzene ring should also be thought of as a functional group, but they don’t fit into the priority order list shown above.

Examples of Priority nomenclature

Some molecules with more than one functional group have been shown. For naming them, we use functional group priority orders.

The following moelcule has carboxyl, hydroxyl and amide functional groups. As per priority order, it shall be named 2-hydroxyacetamide.

Similarly, a molecule containing carboxylic, and amine functional groups will be named as;

In another example, the priority order among carboxylic, hydroxyl, and halogens is clearly shown.

 

Key Takeaway(s)

Concepts Berg

What is a functional group?

A functional group is a group of atoms that has its own characteristic properties and is responsible for the compound’s unique chemical properties and reactions in organic chemistry.

How do you find the highest priority group?

The functional group which has a higher priority will be the one that lends its suffix to the name of the compound.

How do you order a functional group?

The highest precedence group takes the suffix, with all others taking the prefix form. However, double and triple bonds only take suffix form (-ene and -yne) and are used with other suffixes.

Which functional group has the highest priority?

The carboxylic acid group has the highest priority and therefore makes up the name of the base compound as “oic acid”.

Reference Links

• Table of functional group priority (www.masterorganicchemistry.com)
• Functional Group (www.chem.ucalgary.com)