What are Functional Groups?

In organic chemistry, we encounter a number of special substituent groups which are attached to the hydrocarbon backbone. These groups impart certain characteristics to the molecule of which it is a part of and thus, become the highlight of that particular molecule.

Classes of Functional Groups    

There are a number of functional groups in organic chemistry and functional groups can be defined as a group of atoms placed in a molecule. They possess their own characteristic properties, regardless of the other atoms of the same molecule. Common examples are carboxylic acids, alcohols, ketones, amines, etc.

The major functional groups which we encounter most commonly at lower levels are-

• Alkanes.  
• Alkenes.
• Alkynes.
• Phenyl group (a benzene ring).
• Amines.
• Alcohols.
• Ethers.
• Alkyl halides where X is a halogen.
• Thiols.
• Aldehydes.
• Ketones.
• Carboxylic acids.
• Esters.
• Amides.
• Nitro groups.

What is the need for Functional Groups?

Compounds belonging to the same functional group will undergo same set of reactions and behave similarly.  Hence, studying a set of compounds with similar behaviour is made easy by the concept of functional groups.

Nomenclature of compounds is made convenient by grouping them into various functional groups and in this way, it is more systematic and less complicated.  

Characteristic Properties of different Functional Groups

The carbon directly attached to the functional group in an organic molecule is referred to as the alpha carbon; the carbon next to an alpha carbon is a beta carbon, the one next to that is gamma carbon and so on.

Saturated alkyl chains are found to be less reactive but unsaturated and substituted alkyl chains which have the presence of functional groups are more reactive.

Some common functional groups which are to be studied in detail are-

• Alcohols  
  • Alcohols have the presence of –OH group. This means that intermolecular hydrogen bonding is facilitated from oxygen of one molecule with hydrogen of another. As a result of this, alcohols usually possess higher boiling points.
  • Owing to the electronegativity difference between carbon and oxygen, an alcohol molecule is always polar and will undergo reactions accordingly.
  • Alcohols undergo nucleophilic substitution reactions, wherein the –OH group is replaced or substituted by another atom/group. They undergo reaction with carboxylic acids yielding esters. They also undergo oxidation reactions to yield aldehydes or ketones or carboxylic acids.
  • Based on the number of atoms attached to the alpha carbon atom (the carbon bearing -OH group), alcohols can be primary, secondary and tertiary.
  • Alcohols are of significance in pharmaceutical and medical industry where they are used as an antiseptic and antimicrobial. They are also exploited in the beverage industry and consumed as liquor (ethanol). Another important use is that it makes for an excellent solvent and so it is used as a reagent and solvent for both polar and non-polar substances.

• Ethers 
  • An ether group contains an oxygen atom bonded to two alkyl or aryl groups or a mixture of both. Even this molecule is polar due to the electronegativity difference between carbon and oxygen.
  • In ethers, oxygen is sandwiched between two bulky groups and hence it remains unavailable for hydrogen bonding and therefore, ethers have a lower boiling point than alcohols.
  • These are less reactive than alcohols but more reactive than alkanes.
  • These are miscible with water and are commonly used solvents.

• Aldehydes and ketones  
  • Since they contain a carbonyl group, these are known as carbonyl compounds.
  • They undergo keto-enol tautomerism which tells us that they are quite reactive in nature.
  • They undergo oxidation to form carboxylic acids. They get reduced to form alcohols. They also undergo a number of nucleophilic addition reactions which are called condensation reactions. One such very important condensation reaction is called ‘aldol condensation’.
  • Ketones and aldehydes do not participate in intermolecular hydrogen bonding. But however, in aqueous solutions, they form hydrogen bonds with water molecules.

• Carboxylic reactions
  • Carboxylic acids contain an acid group which is a combination of both carbonyl and hydroxyl groups.
  • These molecules participate in intermolecular as well as intramolecular hydrogen bonding which implies they have fairly high boiling points.
  • These molecules are proton donors and are Bronsted-Lowry acids. But these are weak acids.
  • Because they display intermolecular hydrogen bonding, they generally exist as dimers.
  • When treated with a base, they form a carboxylate salt. They react with alcohols to yield esters. In addition, they undergo specific reactions to convert to amines, aldehydes and ketones respectively.

• Esters
  • The product of a condensation reaction between an acid and an alcohol is an ester.
  • Esters have a characteristic fragrant odor as they are sweet-smelling’.
  • Esters are less polar than alcohols nut more polar than ethers.

• Amines 
  • Amines have a nitrogen atom bonded to three alkyl or aryl or hydrogen atoms or a combination of these.
  • Based on the number of carbons attached to the nitrogen atom, we can have primary, secondary and tertiary amines. Although a quaternary amine is possible (quaternary ammonium salt may be formed), it is very unstable due to the positive charge on the nitrogen atom.
  • The lone pair of electrons on the nitrogen atom renders the molecule basic and imparts reactivity.
  • Amines exhibit hydrogen bonding (which makes it soluble in water and increases its boiling point).
  • Amines undergo reactions with carboxylic acids and form amides which are biologically very important molecules.
  • Several neuro-transmitters and amino acids have presence of amino acids.

• Thiol
  • In organic chemistry, a functional group similar to alcohol is thiol.
  • Thiol an organosulfur compound in which carbon directly directly bonded with sulfhydryl group (-SH).
  • Thiol is also known as sulfhydryl group.

Context and Applications

This topic is a prerequisite for the completion of certain undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry and Chemical Engineering courses.

Practice Questions

Q1. Out of alcohols, amines, acids and the other functional groups discussed above, which of them exhibits highest extent of hydrogen bonding?

Answer- Alcohols followed by acids and lastly amines.

Q2.What functional groups are capable of hydrogen bonding?

Answer-Groups containing electronegative atoms like nitrogen, oxygen and fluorine.

Q3. What causes reactivity differences in organic molecules possessing different functional groups?

Answer- The carbon-carbon and carbon-hydrogen bonds are quite strong between them and the electrons of the covalent bonds of hydrogen and carbon are more or less equally distributed. Between carbon and heteroatoms (like oxygen, nitrogen, halogen, Sulphur, etc.), there will be development of partial charges between them and leads to polarity resulting in weakening of the bond. Polar bonds make the molecules reactive.