Organic Chemistry I: Hydrocarbons - SS2 Chemistry Past Questions and Answers - page 2

Hydrocarbons are organic compounds composed of hydrogen and carbon atoms. They can be classified into several categories based on their structure:

1.    Alkanes: Alkanes are saturated hydrocarbons that contain only single bonds between carbon atoms. They have the general formula CnH2n+2. Alkanes are relatively inert and have low reactivity. They are commonly used as fuels due to their high energy content. Examples of alkanes include methane (CH4), ethane (C2H6), and propane (C3H8).

2.    Alkenes: Alkenes are unsaturated hydrocarbons that contain at least one carbon-carbon double bond. They have the general formula CnH2n. Alkenes exhibit greater reactivity than alkanes due to the presence of the double bond. They are used in the production of polymers, such as polyethylene and polypropylene. Examples of alkenes include ethene (C2H4) and propene (C3H6).

3.    Alkynes: Alkynes are unsaturated hydrocarbons that contain at least one carbon-carbon triple bond. They have the general formula CnH2n-2. Alkynes are even more reactive than alkenes due to the presence of the triple bond. They find applications in the synthesis of organic compounds and as starting materials for the production of plastics and synthetic fibres. Examples of alkynes include ethyne (C2H2) and propyne (C3H4).

4.    Aromatic Hydrocarbons: Aromatic hydrocarbons are characterised by a ring structure with alternating double bonds, known as a benzene ring. They exhibit unique properties and are often used as solvents, fuel additives, or as precursors in the production of various chemicals. Benzene (C6H6) is a well-known aromatic hydrocarbon, along with toluene (C7H8) and xylene (C8H10).

Hydrocarbons can also be classified based on the carbon skeleton, which refers to the arrangement of carbon atoms in the molecule. The main types of hydrocarbons based on carbon skeleton are:

1.    Straight-Chain Alkanes: Straight-chain alkanes have a linear arrangement of carbon atoms. Examples include methane (CH4), ethane (C2H6), and octane (C8H18). Straight-chain alkanes have higher boiling points and melting points as the molecular weight increases. They exhibit regular packing in the solid state and show relatively low branching, leading to higher surface area and intermolecular interactions.

2.    Branched-Chain Alkanes: Branched-chain alkanes have carbon atoms arranged in a non-linear or branched manner. Examples include isobutane (C4H10), isopentane (C5H12), and neopentane (C5H12). Branched-chain alkanes have lower boiling points and melting points compared to their straight-chain counterparts. The branching reduces the surface area and intermolecular interactions, leading to weaker van der Waals forces between molecules.

3.    Cycloalkanes: Cycloalkanes are hydrocarbons with carbon atoms arranged in a closed ring or cyclic structure. Examples include cyclohexane (C6H12) and cyclopentane (C5H10). Cycloalkanes have higher boiling points compared to their straight-chain counterparts due to the formation of more compact structures. They exhibit ring strain due to bond angle distortions in small rings.

4.    Aromatic Hydrocarbons: Aromatic hydrocarbons, as mentioned earlier, contain a benzene ring or other aromatic ring structures. Examples include benzene (C6H6), toluene (C7H8), and naphthalene (C10H8). Aromatic hydrocarbons have distinct properties, such as high stability and resonance delocalization of pi electrons in the ring structure.