15.3 Functional Group

A functional group is an atom or a group of atoms attached to a carbon chain or ring that determines the characteristic chemical properties of an organic compound. Organic compounds consist of two main parts: a reactive functional group and a relatively unreactive carbon network.

• Example:
  • The hydroxyl group ($-OH$) is the functional group for alcohols.
  • The carboxyl group ($-COOH$) is the functional group for carboxylic acids.

Chemical reactions primarily occur at the functional group. Therefore, the chemical properties of an organic compound are dictated by its functional group. Double and triple bonds, which determine the properties of alkenes and alkynes, are also considered functional groups.

15.3.1 Homologous Series

Due to the vast number of organic compounds, they are classified into groups to simplify their study. A homologous series is a group of organic compounds that share the same functional group but differ in the length of their carbon chain.

A general formula can be used to determine the molecular formula of any member in a series. This typically applies to monofunctional compounds.

Key Features of a Homologous Series:

1. All members share the same functional group.
2. Each successive member differs from the next by a methylene group ($-C{H}_2-$).
3. Each series can be represented by a general formula. For example:
   • Alkanes: $C_n{H}_{2n+2}$
   • Alkenes: $C_n{H}_{2n}$
   • Alcohols: $C_n{H}_{2n+1}OH$
4. All members exhibit similar chemical properties due to the same functional group.
5. There is a gradual change in physical properties (like boiling point and density) as the carbon chain length increases.
6. Members can be prepared by similar methods.

The table below lists important homologous series and their corresponding functional groups.

Concept Assessment Exercise 15.2

Write the general formulae and functional groups of the following homologous series:

1. Alcohols: General Formula: $C_n{H}_{2n+1}OH$, Functional Group: $-OH$ (Hydroxyl)
2. Aldehydes: General Formula: $C_n{H}_{2n+1}CHO$, Functional Group: $-CHO$ (Aldehyde)
3. Ketones: General Formula: $R-CO-R^{\prime }$, Functional Group: $>C=O$ (Carbonyl)
4. Ethers: General Formula: $R-O-R^{\prime }$, Functional Group: $-O-$ (Ether linkage)
5. Carboxylic acids: General Formula: $C_n{H}_{2n+1}COOH$, Functional Group: $-COOH$ (Carboxyl)
6. Esters: General Formula: $R-COO-R^{\prime }$, Functional Group: $-COO-$ (Ester)
7. Amines: General Formula: $C_n{H}_{2n+1}N{H}_2$, Functional Group: $-N{H}_2$ (Amino)
8. Nitriles: General Formula: $C_n{H}_{2n+1}CN$, Functional Group: $-C\equiv N$ (Nitrile)

15.3.2 Hydrocarbons

Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms. Carbon atoms form the molecular backbone, while hydrogen atoms are attached to it.

Alkanes are the simplest hydrocarbons, containing only single carbon-carbon bonds. They are relatively unreactive for two main reasons:

1. The C-C and C-H bonds are strong and require significant energy to break.
2. These bonds are non-polar, making them resistant to attack by polar reagents like acids, bases, or oxidizing agents.

The high bond energy of the C-C bond contributes to the stability of carbon chains compared to other elements.

Hydrocarbons are broadly classified into saturated and unsaturated hydrocarbons.

15.3.3 Saturated Hydrocarbons

Saturated hydrocarbons are molecules that contain only single covalent bonds between carbon atoms. The term "saturated" signifies that the carbon atoms are bonded to the maximum possible number of hydrogen atoms — they are "saturated" with hydrogen. This class includes:

• Alkanes — open-chain saturated hydrocarbons with general formula $C_n{H}_{2n+2}$. Example: Ethane ($C{H}_{3}C{H}_3$).
• Cycloalkanes — ring-shaped saturated hydrocarbons with general formula $C_n{H}_{2n}$. Example: Cyclohexane ($C_6{H}_{12}$).

Saturated hydrocarbons are relatively unreactive because all C-C and C-H bonds are strong single bonds with no $\pi$ electrons available for electrophilic attack.

15.3.4 Unsaturated Hydrocarbons

Unsaturated hydrocarbons are molecules that contain one or more double or triple covalent bonds between carbon atoms. The carbon atoms are not bonded to the maximum number of hydrogen atoms — they are "unsaturated". This class includes:

• Alkenes — contain at least one carbon-carbon double bond ($C=C$). General formula: $C_n{H}_{2n}$. Example: Ethene ($H_{2}C=C{H}_2$).
• Alkynes — contain at least one carbon-carbon triple bond ($C\equiv C$). General formula: $C_n{H}_{2n-2}$. Example: Ethyne ($HC\equiv CH$).

Unsaturated hydrocarbons are more reactive than saturated hydrocarbons because the $\pi$ bond in the double or triple bond is weaker than the $\sigma$ bond and is readily available for electrophilic addition reactions.