Functional Groups

Functional groups definition

Alkenes, alcohols, and carboxylic acids - these are all types of organic molecules. In particular, they are examples of homologous series (also known as families). Molecules within a homologous series react in a very similar way to each other, and this is because they all contain the same particular collection of atoms - their functional group. Members of a homologous series may have different lengths or arrangements of their carbon chain but each one of them features the same functional group at some point within their structure. This means that they take part in the same general reactions.

Functional groups are useful because they allow us to predict the characteristics of an unknown molecule just by looking at its structure. If we identify its functional group, we can compare it to other molecules in the same homologous series or family and make confident guesses as to how it reacts.

Functional groups table

Let's find out about various different functional groups you'll encounter in organic chemistry with the help of a handy functional groups table. This table names the functional group associated with specific organic families, along with the functional group formula and an example molecule.

Examples of functional groups

In the table above, we provided you with the names of molecules that are great examples of functional groups. Want to see these molecules in real life? Here they are.

Alkanes, alkenes and alkynes

Fig. 1: Examples of alkanes, alkenes and alkyenes, with their functional groups highlighted. These molecules are all hydrocarbons.StudySmarter Originals

This first picture gives you examples of the functional groups found in alkanes, alkenes, and alkynes. These three different families of organic molecules are all hydrocarbons, meaning that they contain just carbon and hydrogen atoms. They are characterised by their type of carbon-carbon bond. Alkanes have just C-C single bonds, whilst alkenes have C=C double bonds and alkynes have C≡C triple bonds.

The C-C single bond, C=C double bond, and C≡C triple bond are all non-polar functional groups. This is because the only two elements involved in the group (carbon and hydrogen) both have very similar electronegativity values. Check out Polarity and Electronegativity for more.

Alcohols and halogenoalkanes

Fig. 2: Examples of the functional groups found in alcohols and halogenoalkanes: the hydroxyl group and a halogen atom respectively.StudySmarter Originals

Alcohols contain the hydroxyl functional group. This consists of a hydrogen atom bonded to an oxygen atom, which is then in turn bonded to the rest of the molecule. We represent this group using -OH.

The functional group in the halogenoalkane family is even simpler - it is just a halogen atom. We typically represent a halogen atom with X, giving halogenoalkanes the functional group -X.

Both the hydroxyl group and halogen atoms are polar functional groups. Their polarity opens up the door to multiple different types of reactions, such as Nucleophilic Substitution Reactions. The hydroxyl group can also form hydrogen bonds and is easily converted into other functional groups, which you'll discover in Reaction of Alcohols.

Aldehydes and ketones

Fig. 3: Examples of the functional group found in aldehydes and ketones: the carbonyl group.StudySmarter Originals

Members of the aldehyde and ketone families all have variants of the same functional group: the carbonyl group, C=O. The carbonyl group itself consists of an oxygen atom joined to a carbon atom with a double bond. In the aldehyde functional group, the C=O is found at the end of a carbon chain. This means that the carbon atom is joined to one R group and one hydrogen atom. In the ketone functional group, the C=O is found in the middle of a carbon chain. This means that the carbon atom is joined to two R groups. Notice how the placement of the C=O carbonyl group is the only difference between the aldehyde propanal (above, left) and the ketone propanone (above, right). However, it gives aldehydes and ketones different characteristic reactions. Find out more in Reactions of Aldehydes and Ketones.

Carboxylic acids, acyl chlorides and esters

Fig. 4: Examples of the functional groups found in carboxylic acids, acyl chlorides, and esters: the carboxyl group, acyl chloride group, and ester link respectively.StudySmarter Originals

Carboxylic acids contain the carboxyl functional group, -COOH. This group's name gives you some clues as to its structure. It is actually made up of two functional groups that we looked at earlier: the carbonyl group (C=O, found in aldehydes and ketones) and the hydroxyl group (-OH, found in alcohols). The combination of the two groups means that carboxylic acids react as typical acids by donating a proton in solution.

Acyl chlorides and esters are both types of acid derivatives. Acid derivatives are close cousins of carboxylic acids, in which the hydroxyl group (-OH) has been replaced by another electronegative group. In acyl chlorides, the replacement group is a chlorine atom, which gives acyl chlorides the functional group -COCl (known as the acyl chloride group). In esters, the replacement group is an oxygen atom bonded to an additional R group. This gives esters the functional group -COO- (known as the ester link).

Amines and amides

Fig. 5: Examples of the functional groups found in amines and amides: the amine group and the amide group respectively.StudySmarter Originals

It's time to add an extra element to the mix: nitrogen. Our first example of a nitrogen-containing organic compound is an amine, which has a functional group with the same name. The amine group is formed from a nitrogen atom with a lone pair of electrons bonded to at least one organic R group. The presence of nitrogen's lone pair of electrons means that amines react as bases. Amine groups with one or more hydrogen atom can also form hydrogen bonds.

Amides contain nitrogen atoms too. They are characterised by the amide functional group, which in turn is made up of an amine group (-NH₂) directly bonded to a carbonyl group (C=O). Like amines, they too form hydrogen bonds.

Arenes, halogenoarenes and phenol

Fig. 6: Examples of the functional groups found in arenes, halogenoarenes and phenols. All three families contain an aromatic arene ring. Halogenoarenes are made up of a phenol group bonded to a halogen atom whilst phenols contain a phenyl group bonded to the hydroxyl group.StudySmarter Originals

The next three examples of organic families all share an aromatic benzene ring in common. Benzene rings are also known as the arene functional group and can be represented with the formula C₆H₆. However, it is much clearer to draw the arene group out, using a hexagon with a circle inside.

Unlike the other hydrocarbons we've considered before (such as alkanes, alkenes, and alkynes), benzene rings contain a stable circle of delocalised pi electrons. This ring is strong and hard to break, which means that arenes and their derivatives tend to take part in substitution reactions instead of addition reactions. You'll learn more about this in Benzene Electrophilic Substitution.

Arene derivatives include halogenoarenes and phenols. In arene derivatives, one or more of the arene ring's hydrogen atoms is swapped for a different group. We call an arene ring that is missing a hydrogen atom the phenyl group. For example, halogenoarenes exchange hydrogen atoms in the arene ring for halogen atoms, meaning that their functional group combines the phenyl group and a halogen atom. Phenols exchange hydrogen atoms for the hydroxyl group (-OH) found in alcohols, meaning that their functional group combines the phenyl group and the hydroxyl group.

Functional group practice problems and examples

Now that you have learned about different types of functional groups, it's time for you to put your knowledge to the test. Have a go at identifying the functional groups in various molecules with the help of our functional group practice questions.

Identify the functional groups in the following molecule.

Fig. 7: Can you spot the functional groups?StudySmarter Originals

We can see that this molecule has three functional groups:

1. A chlorine atom (-Cl) on the left.
2. A C=C double bond in the middle.
3. A carboxyl group (-COOH) on the right.

We've highlighted them in the diagram below.

Fig. 8: The functional groups in our unknown molecule, labelled.StudySmarter Originals

Here's another practice question. This time, it uses skeletal formula.

Identify the functional groups in the following organic molecule.

Fig. 9: Can you spot the functional groups?StudySmarter Originals

This molecule is a little trickier. We need to pay special attention to the C=O bonds in order to figure out which functional group they belong to. Like in the first example, this molecule has three functional groups in total.

1. The C=O bond on the left is joined to an amine group (-NH₂). The two combine to form an amide group (-CONH₂).
2. The C=O bond in the middle of the molecule is joined to two R groups. It is therefore a ketone carbonyl group (-CO-).
3. There is also a hydroxyl group (-OH) on the right.

Once again, we've labeled the functional groups in the diagram down below.

Fig. 10: The functional groups in our second unknown molecule, labeled.StudySmarter Originals

Here's a different sort of question.