Functional groups are the foundation of organic chemistry as they define the structure, reactivity, and properties of organic compounds. Here, we explain what functional groups are (by looking at – wait for it – fruit salad), and introduce the most common functional groups.
Did you know that functional groups were at the root of dispute and friendship of two chemistry legends? Let’s get into it!

What are Functional Groups in Organic Chemistry?

Functional groups in organic chemistry are specific groups of atoms that have characteristic properties (e.g., polarity). Functional groups determine the reactivity of molecules.

Instead of blindly memorizing a long list of functional groups, let us try to understand three key ideas first.

1. Many functional groups are related to each other. For instance, by putting a C=O double bond next to an amine, we get an amide. Others are variants of each other. For example, a sulfur atom instead of an oxygen turns an ether group (R-O-R) into a thioether (R-S-R). When learning these groups, try to find the similarities and differences between them.
There are different sub-families of functional groups. For instance, everything with a C=O double bond can be called a carbonyl. Aldehydes, ketones, amides, esters… are all relatives of the carbonyl family! Similarly, primary amines, secondary amines and tertiary amines are all amines!

2. Properties of functional groups are largely independent of the molecule’s broader environment. The hydroxyl group in ethanol behaves the same way as in octanol (even though the latter is much bigger!).

3. Connected to the above, functional groups have an inherent polarity and thus, nucleophilicity or electrophilicity (link). For example, aldehydes are electrophilic on carbon but nucleophilic on oxygen. This baseline polarity of each functional group actually already explains most of the organic chemistry reactions that you will encounter. Interconversion of functional groups can change this natural polarity. For instance, enol forms of carbonyls are not electrophilic at the central carbon anymore, and are nucleophilic at the alpha-carbon!

What are Functional Groups – SIMPLY EXPLAINED?

Functional groups are just like different fruits in a fruit salad. Let’s see how the points above match to this analogy.

1. Some fruits are related, like lemons and oranges as they are both citrus fruits. We mentioned carbonyls and ethers above. Another example are primary amines and tertiary amines – very similar but not identical!

2. Whether you catch a strawberry in one fruit salad or another one (these are different molecules in our analogy), they basically taste the same.

3. Lemons are inherently acidic. Well, carboxylic acids are also always acidic. (Sometimes more, sometimes less – due to things like inductive and mesomeric effects.) Just like fruits have characteristic taste, functional groups have characteristic properties.
This also nicely illustrates the keto-enol interconversion or even the advanced idea of “Umpolung“. Here, chemists invert the natural reactivity of the original functional group. The key example is conversion of aldehydes to dithianes which can be nucleophilic at the carbon (instead of electrophilic)!
Using our fruit analogy, by cooking and caramelizing lemons (Umpolung), we can make them more sweet instead of bitter.

History of Functional Groups in Organic Chemistry

Here’s some random background (if you know my content, you will realize I like this type of stuff):
As the backbone of organic chemistry, functional groups as a concept have their origin in the early 19th century. At that point, elemental analysis had allowed chemists to determine the molecular formula of most inorganic compounds. These lack carbon-hydrogen bonds and are thus not organic (shocker!). The widespread theory of vitalism suggested that only living organisms can produce organic substances.

Many students know that the German chemist Friedrich Wöhler overthrew this theory by synthesizing urea from inorganic ammonium cyanate in 1828. Less known is the controversy between Wöhler and Justus Liebig:
Both of them were doing experiments on inorganic salts that they believed had the molecular formula “AgCNO“. However, Liebig’s salt was a powerful explosive while Wöhler’s was not.

“The obvious conclusion was that one of the analyses must be wrong, and one of the chemists must be a poor analyst! Liebig, pushed by his aggressive character, rapidly accused Wöhler of erroneous results. But Liebig analyzed a sample of the silver cyanate supplied by Wöhler and verified that they were correct. At this point, Liebig openly admitted that he had made a mistake in his initial accusation. And curiously this was the starting point of a friendship and even a scientific collaboration between the two scientists.”

S. Esteban in J. Chem. Educ. 2008, 85, 9, 1201

The legendary Swedish chemist Berzelius (Wöhler’s professor) explained this controversy by proposing the concept of isomers. The realization that constitution of molecules can be different despite having the same atoms paved the way for discovery of all the functional groups we know today.

Common functional groups

Here are the most common functional groups. The bold ones are particularly important as they are invoked to explain some of the basic reactions in organic chemistry, such as nucleophilic substitutions (alkyl halides) or electrophilic additions.

Instead of belabouring basic information you can find everywhere already, I want to draw your attention to two themes:

1. Functional group relationships:
   What makes functional groups (in a certain row or across rows) similar?
   For instance, the first row are C-H hydrocarbon functional groups.
   Which functional groups are based on combinations of simpler functional groups?
   For example, an enone is an alkene that is connected to a ketone.
   Which functional groups are oxidized/ reduced versions of each other, and which have the same oxidation state?
   For instance, carboxylic acids are more oxidized versions of aldehydes and ketones, and have the same oxidation state as nitriles. This means nitriles can be converted to acids without reductants or oxidants!
   
2. Functional group polarity:
   What do the red and blue colored atoms correspond to? How does the polarity and reactivity differ across related functional groups?
   For example, why is the central carbonyl carbon blue in aldehydes and ketones, but not blue anymore in carboxylic acids?
   Why is the hydrogen of the O-H bond of the carboxylic acid blue but not blue for the C-H in the aldehyde?
   Should the carbons of alkenes and arenes be marked red or rather blue? What does this depend on?
   
3. Advanced: Functional group geometry: Can you rationalize all bond angles (e.g., alkynes vs. alkenes) and geometries of functional groups? Which parts of them are flat/ in one plane, and which ones are not?
   For example, why are in esters both the C=O and C-O-R bonds in the same plane? Why do esters prefer the Z-conformer where both C=O and C-O-R bond are facing the same side?

In other posts, we will deep dive into individual functional groups and families to explain properties and most common reactions in more detail. Functional groups clearly also form the basis of protecting groups and their reactivity.

Some final advice: Instead of learning a bunch of facts about 30 functional groups by heart in a brain dead manner, try to work first identify the commonalities and differences from a high level. The names might seem random at first but many of them will make sense once you take a look at what atoms or combinations of functional groups are present!