## 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 (-CH₂-). 3. Each series can be represented by a general formula. For example: * **Alkanes:** $C_nH_{2n+2}$ * **Alkenes:** $C_nH_{2n}$ * **Alcohols:** $C_nH_{2n+1}OH$ 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. > **Interesting Information:** Alkanes are considered a homologous series with no specific functional group. The general formulas provided are for open-chain (aliphatic) compounds. The table below lists important homologous series and their corresponding functional groups. | Homologous Series | General Formula | Functional Group | Example | | :--- | :--- | :--- | :--- | | **Alkanes** (R-H) | $C_nH_{2n+2}$ | No functional group | $\mathrm{CH_3-CH_3}$ (Ethane) | | **Alkenes** | $C_nH_{2n}$ | $>C=C<$ (Double bond) | $\mathrm{H_2C=CH_2}$ (Ethene) | | **Alkynes** | $C_nH_{2n-2}$ | $-C \equiv C-$ (Triple bond) | $\mathrm{H-C \equiv C-H}$ (Ethyne) | | **Halogenoalkanes** (R-X) | $C_nH_{2n+1}X$ | $-X$ (Halo group) | $\mathrm{CH_3CH_2Cl}$ (Chloroethane) | | **Alcohols** (R-OH) | $C_nH_{2n+1}OH$ | $-OH$ (Hydroxyl group) | $\mathrm{CH_3CH_2OH}$ (Ethanol) | | **Ethers** (R-O-R') | $C_nH_{2n+1}OC_mH_{2m+1}$ | $-O-$ (Ether linkage) | $\mathrm{CH_3-O-CH_3}$ (Methoxymethane) | | **Amines** (R-NH₂) | $C_nH_{2n+1}NH_2$ | $-NH_2$ (Amino group) | $\mathrm{CH_3NH_2}$ (Methylamine) | | **Nitriles** (R-CN) | $C_nH_{2n+1}CN$ | $-C \equiv N$ (Nitrile group) | $\mathrm{CH_3CN}$ (Ethanenitrile) | | **Aldehydes** (R-CHO) | $C_nH_{2n+1}CHO$ | $-CHO$ (Aldehyde group) | $\mathrm{CH_3CHO}$ (Ethanal) | | **Ketones** (R-CO-R') | $C_nH_{2n+1}COC_mH_{2m+1}$ | $>C=O$ (Carbonyl group) | $\mathrm{CH_3COCH_3}$ (Propanone) | | **Carboxylic Acids** (R-COOH)| $C_nH_{2n+1}COOH$ | $-COOH$ (Carboxyl group) | $\mathrm{CH_3CH_2COOH}$ (Propanoic acid) | | **Esters** (R-COO-R')| $C_nH_{2n+1}COOC_mH_{2m+1}$ | $-COO-$ (Ester group) | $\mathrm{CH_3COOCH_2CH_3}$ (Ethyl ethanoate) | | **Amides** (R-CONH₂) | $C_nH_{2n+1}CONH_2$ | $-CONH_2$ (Amide group) | $\mathrm{CH_3CONH_2}$ (Ethanamide) | | **Acid Halides** (R-COX) | $C_nH_{2n+1}COX$ | $-COX$ (Acid halide group)| $\mathrm{CH_3COCl}$ (Ethanoyl chloride) | | **Arenes** | $C_nH_{2n-6m}$ (m=rings) | Phenyl group ($\mathrm{C_6H_5-}$) | $\mathrm{C_6H_6}$ (Benzene) | ### Concept Assessment Exercise 15.2 Write the general formulae and functional groups of the following homologous series: 1. **Alcohols:** General Formula: $C_nH_{2n+1}OH$, Functional Group: $-OH$ (Hydroxyl) 2. **Aldehydes:** General Formula: $C_nH_{2n+1}CHO$, Functional Group: $-CHO$ (Aldehyde) 3. **Ketones:** General Formula: $R-CO-R'$, Functional Group: $>C=O$ (Carbonyl) 4. **Ethers:** General Formula: $R-O-R'$, Functional Group: $-O-$ (Ether linkage) 5. **Carboxylic acids:** General Formula: $C_nH_{2n+1}COOH$, Functional Group: $-COOH$ (Carboxyl) 6. **Esters:** General Formula: $R-COO-R'$, Functional Group: $-COO-$ (Ester) 7. **Amines:** General Formula: $C_nH_{2n+1}NH_2$, Functional Group: $-NH_2$ (Amino) 8. **Nitriles:** General Formula: $C_nH_{2n+1}CN$, 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. | Bond | Bond Energy ($kJ/mol$) | Bond | Bond Energy ($kJ/mol$) | | :--- | :--- | :--- | :--- | | C-C | 346 | Si-Si | 222 | | N-N | 167 | P-P | 201 | | O-O | 142 | S-S | 226 | 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. This class includes alkanes and cycloalkanes. The term "saturated" signifies that each carbon atom is bonded to the maximum possible number of atoms (four), having reached its full combining capacity. *Examples:* * Ethane: $\mathrm{CH_3-CH_3}$ * Butane: $\mathrm{CH_3CH_2CH_2CH_3}$ * Cyclobutane: A four-carbon ring * Cyclopentane: A five-carbon ring ### 15.3.4 Alkyl Group (R-) An **alkyl group** is formed by removing one hydrogen atom from an alkane molecule. They are named by replacing the -ane suffix of the parent alkane with -yl. * **Primary (1°), Secondary (2°), and Tertiary (3°) Carbons and Hydrogens:** * A **primary carbon** is attached to one (or zero) other carbon atom. * A **secondary carbon** is attached to two other carbon atoms. * A **tertiary carbon** is attached to three other carbon atoms. * Hydrogens are classified based on the type of carbon they are attached to. * **Examples of Alkyl Groups from Butane Isomers:** 1. **From n-butane ($\mathrm{CH_3-CH_2-CH_2-CH_3}$)**: * Removing a primary hydrogen gives the **n-butyl** group: $\mathrm{CH_3-CH_2-CH_2-CH_2-}$ * Removing a secondary hydrogen gives the **secondary-butyl (sec-butyl)** group: $\mathrm{CH_3-CH_2-CH(-)-CH_3}$ 2. **From isobutane ($\mathrm{(CH_3)_2CH-CH_3}$)**: * Removing a primary hydrogen gives the **isobutyl** group: $\mathrm{(CH_3)_2CH-CH_2-}$ * Removing a tertiary hydrogen gives the **tertiary-butyl (tert-butyl)** group: $\mathrm{(CH_3)_3C-}$ > **Key Information:** The prefix *"iso"* refers to a structure with a methyl group substituent on the second-to-last carbon of the parent chain. ### 15.3.5 Unsaturated Hydrocarbons **Unsaturated hydrocarbons** are molecules containing one or more carbon-carbon double or triple bonds. The term "unsaturated" implies that more atoms can be added to the carbon skeleton across these multiple bonds. This class includes alkenes and alkynes. * **Alkenes** (double bonds): Each double-bonded carbon is attached to three atoms and can bond to one more. * **Alkynes** (triple bonds): Each triple-bonded carbon is attached to two atoms and can bond to two more. Because of this, unsaturated hydrocarbons readily undergo **addition reactions**. *Examples:* * Propene: $\mathrm{CH_3-CH=CH_2}$ * Propyne: $\mathrm{CH_3-C \equiv CH}$ * Cyclobutene: A four-carbon ring with one double bond. ### 15.3.6 Terminology Associated with Naming Organic Compounds Understanding the following terms is essential for naming organic compounds according to IUPAC rules. 1. **Prefix:** Indicates a substituent or branch attached to the main carbon chain (e.g., methyl-, chloro-). 2. **Root Name (or Stem):** Indicates the number of carbon atoms in the longest continuous chain (e.g., meth-, eth-, prop-). 3. **Suffix of Root Name:** An ending added to the root name to indicate the type of carbon-carbon bonds in the main chain (-an- for single, -en- for double, -yn- for triple). 4. **Suffix:** The final part of the name that indicates the principal functional group or family of the compound (e.g., -ol, -al, -oic acid). **Frequently Used Root Names:** | No. of Carbons | Root Name | No. of Carbons | Root Name | | :--- | :--- | :--- | :--- | | 1 | meth- | 6 | hex- | | 2 | eth- | 7 | hept- | | 3 | prop- | 8 | oct- | | 4 | but- | 9 | non- | | 5 | pent- | 10 | dec- | **Prefixes and Suffixes for Common Classes:** | Class/Family | Prefix/Suffix | Example | | :--- | :--- | :--- | | **Alkanes** | Suffix: -ane | $\mathrm{CH_3CH_2CH_2CH_3}$ (Butane) | | **Alkenes** | Suffix: -ene | $\mathrm{CH_3CH=CHCH_3}$ (But-2-ene) | | **Alkynes** | Suffix: -yne | $\mathrm{CH_3CH_2C \equiv CH}$ (But-1-yne) | | **Halogenoalkanes** | Prefix: halo- | $\mathrm{CH_3CH(Cl)CH_3}$ (2-Chloropropane) | | **Alcohols** | Suffix: -ol | $\mathrm{CH_3CH(OH)CH_2CH_2CH_3}$ (Pentan-2-ol) | | **Ethers** | Prefix: alkoxy- | $\mathrm{CH_3OCH_2CH_3}$ (Methoxyethane) | | **Aldehydes** | Suffix: -al | $\mathrm{CH_3CH_2CHO}$ (Propanal) | | **Ketones** | Suffix: -one | $\mathrm{CH_3(CO)CH_2CH_2CH_3}$ (Pentan-2-one) | | **Amines** | Suffix: -amine | $\mathrm{CH_3CH_2NH_2}$ (Ethylamine) | | **Nitriles** | Suffix: -nitrile | $\mathrm{CH_3CH_2CN}$ (Propanenitrile) | | **Carboxylic Acids** | Suffix: -oic acid | $\mathrm{CH_3CH_2COOH}$ (Propanoic acid) | | **Acid Halides** | Suffix: -oyl halide | $\mathrm{CH_3COCl}$ (Ethanoyl chloride) | | **Amides** | Suffix: -amide | $\mathrm{CH_3CONH_2}$ (Ethanamide) | | **Arenes** | Suffix: -benzene, Prefix: phenyl- | $\mathrm{C_6H_5CH_3}$ (Methylbenzene) |