11.2 Hydrocarbons

• Hydrocarbons are compounds composed only of carbon and hydrogen atoms. Examples include Methane (CH4), Ethene (C2H4), Cyclohexane (C6H12), and Benzene (C6H6). These compounds contain no other elements except carbon and hydrogen.

• Hydrocarbons are categorized into two main types:

1. Aliphatic Hydrocarbons

2. Aromatic Hydrocarbons

11.2.1 Aliphatic Hydrocarbons

• The term Aliphatic means “fatty” or “straight,” and this type of hydrocarbon is mainly derived from animal fat.

• Aliphatic Hydrocarbons are further divided into two subcategories:

1. Open Chain Hydrocarbons

2. Closed Chain Hydrocarbons

(i) Open Chain Hydrocarbons

• These hydrocarbons are named “Open Chain” because the two terminal carbon atoms of their carbon chain are free and not connected to any other part of the molecule. Examples include:

• Butane (CH3-CH2-CH2-CH3)

• Ethene (CH2=CH2)

• Open Chain Hydrocarbons are classified into two types:

1. Saturated Open Chain Hydrocarbons

2. Unsaturated Open Chain Hydrocarbons

(a) Saturated Open Chain Hydrocarbons

• These contain only single bonds between the carbon atoms (C-C). They are called saturated because each carbon atom is bonded to as many hydrogen atoms as possible, without any double or triple bonds.

• Example: Propane (CH3-CH2-CH3)

• Example: Pentane (C5H12)

(b) Unsaturated Open Chain Hydrocarbons

• These contain one or more double bonds (C=C) or triple bonds (C≡C) between carbon atoms. Unsaturated hydrocarbons include:

• Alkenes (contain at least one C=C bond)

• Alkynes (contain at least one C≡C bond)

• Examples of Unsaturated Open Chain Hydrocarbons:

• Alkene (Double Bond): Propene (CH2=CH-CH3)

• Alkyne (Triple Bond): Ethyne (C2H2), Propyne (C3H4)

(ii) Closed Chain Hydrocarbons

• In these hydrocarbons, the terminal carbons of the chain are connected, forming a closed loop or circle. The loops can be of varying sizes depending on the number of atoms involved. These compounds may also be:

1. Saturated Closed Chain Hydrocarbons

2. Unsaturated Closed Chain Hydrocarbons

• Closed Chain Hydrocarbons are also known as Alicyclic Hydrocarbons.

• Examples:

• Saturated Closed Chain: Cyclopropane (C3H6), Cyclobutane (C4H8)

• Unsaturated Closed Chain: Cyclopropene (C3H4), Cyclobutene (C4H6)

11.2.2 Aromatic Hydrocarbons

• The term Aromatic comes from the Greek word “Aroma,” meaning fragrance or scent. These compounds were originally named because they were sweet-smelling.

• Aromatic Hydrocarbons are typically cyclic compounds with alternating single and double bonds between carbon atoms. These compounds are often composed of 5, 6, or 7 carbon atoms in a co-planar structure.

• Examples:

• Benzene (C6H6): A 6-membered aromatic ring with alternating single and double bonds.

• Naphthalene (C10H8): Composed of two fused benzene rings.

Homologous Series

• Homologous Series refers to a group of related compounds that share similar physical and chemical properties because they have the same active radicals. Members of a homologous series follow a common chemical formula.

• Each series is represented by a general formula, where each compound differs by a methylene group (-CH2).

• Examples of Homologous Series:

Homologous Series General Formula Names and Formulas of Some Members

Alkane CnH2n+2 Methane (CH4), Ethane (C2H6), Propane (C3H8)

Alkene CnH2n Ethene (C2H4), Propene (C3H6)

Alkyne CnH2n-2 Ethyne (C2H2), Propyne (C3H4)

Alcohol CnH2n+1OH Methanol (CH3OH), Ethanol (C2H5OH)

Aldehyde CnH2n+1CHO Ethanal (C2H5CHO), Propanal (C3H7CHO)

Carboxylic Acid CnH2n+1COOH Ethanoic Acid (CH3COOH), Propanoic Acid (C3H7COOH)

• Each compound in a homologous series has similar chemical behavior but varies in terms of boiling point, melting point, and other physical properties due to the increasing size of the molecules.