Functional groups are an atom or a group of atoms bonded together in a unique manner attached to the alkyl chain or ring or any other organic molecule. They confer special characteristics and properties to the molecule they are attached. They are also usually the site of chemical activity of the molecule and the site where the reagents attack or interact. Organic compounds are thus classified into different classes based on the functional group that is attached to it.
Nomenclature of compounds due to functional groups
In case of compounds with multiple functional groups, a principal functional group is chosen and the compound is named on that basis with the name ending with the suffix related to that functional group. The remaining functional groups called subordinate functional groups (also called substituents) are referred to by using their respective prefixes. In order to decide the principal functional group and the groups that will be the substituents, a standard priority list is created. Whichever group comes first in the priority list, it is given greater importance and would be considered the principal functional group. The others would be regarded as the substituents.
Functional group Priority list
-COOH (carboxylic acid), -SO3H (sulphonic acids), -COOR where R is an alkyl group (ester), -COCl (acyl halides), -CONH2 (amides), -CN(nitriles), -CHO (aldehydes), >C=O(ketones), -OH (alcohols), -NH2(amines), >C=C< (alkene), -CC- (alkyne, triple bonded)
Some functional groups are always substituents. Hence, they are named by their prefix like -R(alkyl), -C6H5(benzyl), halogens, -NO2 (nitro compounds), -OR where R is an alkyl group (alkoxy), etc.
The carbon chain to which the leading functional group is attached is chosen as the main chain. The longest chain is chosen as the basis. The chain is numbered in such a way that the carbon to which the functional group is attached gets the lowest possible number in the chain.
If multiple functional groups of the same type are present, the number is indicated by adding di, tri, etc. before the suffix. In such types of compounds, the full name of the parent alkane is written before the suffix.
Some commonly encountered functional groups are the following:
Bulky group ionizes to release H+ ions which means the molecule shows acidic character. The general formula of the carboxylic acid is R-COOH and it ionizes to R-COO–.
Carboxylic acid molecules are polar in nature due to the presence of two electronegative oxygen atoms.
They also participate in the hydrogen bonding due to the presence of the carbonyl group (C=O) and the hydroxyl group.
When placed in nonpolar solvents, these compounds form dimers due to hydrogen bonding. This increases melting and boiling points and changes other colligative properties.
A few examples of carboxylic acid compounds are formic acid (found in ants), acetic acid (found in vinegar), ascorbic acid (vitamin C), citric acid (lemon), etc.
Ester molecules are polar due to the presence of electronegative oxygen atoms; however, there is no hydrogen attached to the oxygen atoms; thus, ester molecules do not participate in hydrogen bonding. Thus they have low boiling points compared to carboxylic acids of similar mass. Ester compounds of lower mass are soluble in water. Esters are sweet-smelling compounds.
Some examples of esters are ethyl acetate (CH3COOC2H5), propyl methanoate (CH3COOC3H7), etc.
Due to the presence of the oxygen in the carbonyl group, aldehyde molecules are slightly polar. However, there is no hydrogen attached to the oxygen atom; thus, they cannot form hydrogen points among themselves. They participate in hydrogen bonding in water and thus lower aldehydes are slightly soluble in water. Due to the double bond, these compounds are highly reactive and undergo a lot of reactions(nucleophilic addition reactions).
Some examples of aldehydes include formaldehyde (HCHO), acetaldehyde (CH3CHO), benzaldehyde (C6H5CHO), etc.
Also contains carbonyl group but no hydrogen is attached to oxygen, so it does not form hydrogen bonds with itself, so their boiling points are lesser than carboxylic acids of similar mass. However, ketones are polar due to the presence of oxygen. They are slightly soluble in water as they can form hydrogen bonds with water. The carbonyl part is reactive and thus the molecule may undergo nucleophilic addition reactions, oxidation, reduction, halogenation reactions, etc.
Some common examples of ketones are propan-2-one (CH3COCH3), 1-phenylethanone (C6H5COCH3), benzophenone (C6H5COC6H5), etc.
Aldehydes are more reactive than the ketones, due to a greater partial positive charge on the carbonyl carbon, thus making it a better site for nucleophilic attack. Aldehydes also have less bulky groups around the carbonyl carbon and thus provide a less steric hindrance.
Alcohols are polar due to the presence of oxygen atoms. As the hydrogen atoms are attached to oxygen, there is the presence of intermolecular hydrogen bonding. Due to this, alcohols have comparatively higher boiling points. They are also soluble in water. Alcohols can release H+ ions from their hydroxyl groups due to the polarity of the O-H bond.
Some common examples of alcohols are methanol (CH3OH), ethanol (CH3CH2OH), phenol (C6H5OH), etc
The general formula for ethers can be R-O-R, R-O-Ar or Ar-O-Ar where R represents an alkyl group and the Ar represents an aryl group. Ether molecules cannot form hydrogen bonds with itself; thus, their boiling points are comparable to alkanes of similar mass. Ethers do form hydrogen bonds with water and are thus slightly soluble in water. The C-O bonds in the ether are polar.
Some common examples of ethers are ethoxyethane (C2H5OC2H5), methoxy ethane (CH3OC2H5), phenoxy benzene (C6H5OC6H5), etc.
Due to the presence of polar atoms of halogens, halides are polar molecules. They have comparatively higher boiling points. As branching in alkyl halide increases the boiling point of alkyl halide decreases. Alkyl halides are readily soluble in organic solvents but only slightly soluble in water (as they are heavier and the interactions between the halides and the water molecules are not strong enough).
Some examples of halides are Fluro-ethane, 2-chloropropane, chlorobenzene, etc.
Lower amines are soluble in water due to the formation of hydrogen bonds. Amines are less polar than alcohols. They have lower boiling points than alcohols. Lower aliphatic amines are gaseous at room temperature. Due to the lone pair of electrons on nitrogen in the amine group, amines are basic nature.
Some examples of amines are methylamine, benzylamine, etc.
Alkenes contain a carbon-carbon double bond. This bond is highly reactive and the alkenes can undergo addition reactions (addition of H2, H2O, halides, etc.). Alkenes are sp2 hybridized. They are insoluble in water. The general formula of alkenes is CnH2n.
Some common examples of alkenes are ethene (C2H4), propene (CH2CHCH3), Buta-1,3-diene, etc.
Alkynes contain a triple bond between 2 carbon atoms. They are insoluble in water and like alkenes undergo addition reactions. Alkynes are sp hybridized. The general formula of alkynes is CnH(2n-2).
Some examples of alkynes are ethyne, propyne, etc.