Hydrocarbons
Table of Contents
Hydrocarbons are organic compounds made of only carbon and hydrogen. Strong covalent bonds hold these atoms together. The carbon–carbon (C–C) and carbon–hydrogen (C–H) bonds form the basic framework of organic compounds. Carbon can build extended structures, resulting in several hydrocarbons found in nature. [1-4]
In addition to their structural role, hydrocarbons are important fuel sources and key building blocks in organic chemistry. For example, methane (CH4) from natural gas is used for cooking and power generation.
Physical Properties of Hydrocarbons [9]
- Physical state at room temperature depends mainly on molecular size and the resulting strength of intermolecular (London dispersion) forces. Low molar mass hydrocarbons are gases, intermediate ones are liquids, and high molar mass compounds are solids.
- Boiling and melting points generally increase with increasing molecular mass due to stronger intermolecular forces. Straight-chain isomers usually have higher boiling points than their branched counterparts of similar molar mass.
- Solubility in water is very low because hydrocarbons are nonpolar (hydrophobic), but it is high in nonpolar organic solvents such as benzene, hexane, and ether.
- Density is lower than that of water; consequently, liquids such as petrol and kerosene float on the water surface.
- Electrical conductivity is negligible because hydrocarbons do not contain free ions or mobile charged particles.
Classification [1-3,5-7]
Hydrocarbons are broadly classified into acyclic (open-chain) and cyclic (closed-chain) hydrocarbons based on whether the carbon atoms form chains or rings.
1. Acyclic Hydrocarbons
In acyclic hydrocarbons, carbon atoms are arranged in straight or branched chains. Depending on the nature of carbon–carbon bonding, these hydrocarbons are further classified into:
- Saturated hydrocarbons: Contain only C–C single or sigma (σ) bonds between carbon atoms.
- Unsaturated hydrocarbons: Contain one or more multiple bonds, such as double (C=C) or triple (C≡C) bonds, between carbon atoms. They involve pi (π) bonds.
| Type | Formula | Saturation | Bonding | Examples |
|---|---|---|---|---|
| Alkane | CnH2n+2 | Saturated | Only C–C single bonds | Methane (CH4), ethane (C2H6) |
| Alkene | CnH2n | Unsaturated | At least one C=C double bond | Ethene (C2H4), propene (C3H6) |
| Alkyne | CnH2n-2 | Unsaturated | At least one C≡C triple bond | Ethyne (C2H2), propyne (C3H4) |
2. Cyclic Hydrocarbons
In cyclic hydrocarbons, carbon atoms are joined to form one or more closed rings. Based on bonding patterns and electronic structure, they are classified into:
- Alicyclic hydrocarbons: Similar to acyclic hydrocarbons in chemical behavior but arranged in rings.
- Aromatic hydrocarbons: Exceptionally stable due to delocalized π-electrons.
| Type | Formula | Structure | Bonding | Examples |
|---|---|---|---|---|
| Cycloalkane | CnH2n | Alicyclic | At least one C-C single bond in the ring | Cyclobutane (C4H8), cyclohexane (C6H12) |
| Cycloalkene | CnH2n-2 | Alicyclic | At least one C=C double bond | Cyclohexene (C6H10) |
| Cycloalkyne | CnH2n-4 | Alicyclic | At least one C≡C triple bond; significant ring strain | Cyclooctyne (C8H12) |
| Benzenoid aromatic compound | No single formula | Aromatic | One or more benzene rings with delocalized electrons | Benzene (C6H6), naphthalene (C10H8) |
| Non-benzenoid aromatic compound | No single formula | Aromatic | No benzene ring but show cyclic π-electron delocalization | Azulene (C10H8) |
Applications [2,4,8]
- Fuels and energy: Methane, propane, petrol, and diesel are commonly used for cooking, heating homes, generating electricity, and powering transportation.
- Chemical manufacturing: Alcohols, acids, detergents, and solvents are produced from compounds obtained from petroleum and natural gas. These products are collectively known as petrochemicals.
- Plastics and synthetic materials: Ethene and propene are used to manufacture plastics such as polyethylene and polypropylene, which are widely used in packaging, bottles, and household items.
- Lubricants and waxes: Heavier hydrocarbons are used as lubricating oils to reduce friction in machines and engines and to manufacture paraffin wax for candles, shoe polish, and waterproof coatings.
- Building materials: Bitumen is widely used for road surfacing, roofing, and sealing applications.
- Pharmaceuticals: Phenol is used in the manufacture of aspirin, while aniline derivatives are used in the synthesis of paracetamol.
- Dyes and pigments: Aniline and naphthol are widely used in the manufacture of synthetic dyes and pigments for textiles, inks, and paints.
