What are Functional Groups?

Organic Chemistry deals mostly with carbon and hydrogens, also called hydrocarbons, but those groups which replace hydrogen and bonds with carbon to give a characteristic nature, unique of their own, to the hydrocarbon they are attached to, are called functional groups. All the compounds belonging to a functional group undergo reactions in a similar pattern and are known to have similar physical and chemical properties.

Why do we need Functional Groups in Chemistry?   

In inorganic chemistry, we have a periodic table to understand different elements and have segregated them into groups and periods to understand their physical and chemical properties. But in organic chemistry there are millions of compounds formed with different substituent along with hydrogen and carbons to give various sets of reactions and properties of their own. It is in fact humanly impossible to learn the individual properties of every other organic compound and study them.

In order to ease down this herculean task, we need to classify compounds that behave similarly and undergo similar reaction patterns based on some ground rules. Therefore, the concept of functional groups was adopted to study the compounds, when we discovered that all the compounds belonging to a particular functional group tend to have similar properties. For example, alcohols are always known to have lower boiling points and esters are always known to have a sweet smell, these are unique character traits that the functional group imparts to their compounds that makes it easier to distinguish one class of compounds from another.

Nomenclature of Functional Groups in Organic Compounds            

Before going into classification of functional groups, it’s better to understand how they are named,. According to International Union of Pure and Applied Chemistry (IUPAC), the nomenclature of compounds containing functional groups must obey the following rules.

• Select the longest chain in the compound with the carbon atom containing the functional group.
• The numbering of parent chain is done in such a way that the carbon atom bearing functional group gets the lowest number.
• If a compound contains several functional groups, then the group which is considered as the principle functional group (the most preferred one) is used as suffix always and all other groups must be treated as prefix.
• The order of preference of principle functional group is as follows:

Sulphonic acids > carboxylic acids > acetic anhydrides > esters > acid chlorides > amides > nitriles > aldehydes > ketones > amines > ethers.

All the remaining groups such as halogens, nitro group, alkoxy, nitrosyl, etc. are treated as substituent groups.

Example:

Here carboxylic acid group is numbered as first carbon, hence the suffix “oic acid”, aldehyde bearing carbon as third, hence the prefix “formyl” and ketone at sixth position prefixed as “oxonine”.

Classes of Functional Group        

There can be different ways in which we can classify functional groups, based on the atoms they contain, based on their reactivity, based on the reactions they usually undergo, etc.  Here, for better understanding we can learn classification of functional group based on the different atoms they contain along with their respective nomenclature.

Functional groups with only carbon and hydrogen  

Groups with the most basic atoms of organic chemistry, carbon and hydrogens can be broadly classified into aliphatic, alicyclic and aromatic compounds. The other major classification of hydrocarbons is based on the number of bonds in them. The compounds that contain only single bonds between the carbon atoms are called saturated compounds (alkanes), while the carbon with double and triple bonds are called unsaturated compounds (alkenes and alkynes).

Aliphatic compounds have straight and branched chains by which carbons are attached to each other, whereas in alicyclic and aromatic compounds, the carbons can be attached in a cyclic manner. Alicyclic compounds are plain ring structures with carbon and hydrogens whereas aromatic compounds must be cyclic, conjugated, planar and should have electrons according to Huckel’s rule. The table below shows these groups and their nomenclature to be used.

Functional groups with halogen groups       

Functional groups with oxygen atom

Oxygen is the next most commonly found atom in organic compounds and it has set of functional groups that are very important as shown below in the table.

Other few important functional groups with different atoms

Example illustration of Uniqueness in Properties of Functional Groups

Exclusiveness towards their chemical behavior exists within a functional group. For example, let’s take two functional group such as alcohols and aldehydes. The famous Tollen’s test or silver mirror test can be only undergone by aldehydes and not alcohols as shown.

Alcohols do not produce silver mirror on addition of Tollen’s reagent, this is because there is absence of carbonyl carbon in alcohol, which needs to react to give silver mirror.

Common Mistakes   

Remember the priority order of functional groups while naming the compounds.    

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry.