Functional Groups

What do you notice about the words introduce and reduction? They'e anagrams of each other - they are both made from an identical set of letters. But these two words have very different pronunciations, intonation, and meanings, all thanks to the way their letters are arranged.

Chemical molecules are similar. Just because they are made from the same number and type of atoms, it doesn't mean that they are similar in any other way. One might readily react with water or acid, whilst the other might make a great fuel. A molecule's characteristics, properties, and typical reactions are all governed by its arrangement of atoms into specific functional groups.

• This article is about functional groups in organic chemistry.
• We'll define functional groups, then provide you with a functional group table.
• After that, we'll look at examples of functional groups.
• We'll end with a couple of functional group practice questions.

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.

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.

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.