Structure of Aliphatic Carbons (Leaving Cert Chemistry): Revision Notes

Structure of Aliphatic Carbons

What are Aliphatic Hydrocarbons

Aliphatic hydrocarbons are organic compounds composed of hydrogen and carbon atoms arranged in straight or branched chains. They can be classified into three main types based on the types of bonds between carbon atoms: alkanes, alkenes, and alkynes.

Alkanes ($C-C$ Single Bonds)

• General Formula: $C_nH_{2n+2}$
• Bonding: Alkanes contain only single bonds between carbon atoms, making them saturated hydrocarbons.
• Homologous Series: Alkanes form a homologous series where each successive member differs by a $CH_2$ unit.

Systematic Names and Structures of Alkanes (C1–C5):

1. Methane ($CH₄$)
2. Ethane ($C₂H₆$)
3. Propane ($C₃H₈$)
4. Butane ($C₄H₁₀$)
5. Pentane ($C₅H₁₂$)

Structural Isomers:

Alkanes can have structural isomers, where the arrangement of carbon atoms differs.

For example:

• Butane has two isomers: n-butane and isobutane (branched).

Larger Alkanes:

• Hexane ($C₆H₁₄$): Linear structure.
• Heptane ($C₇H₁₆$): Linear structure.
• Octane ($C₈H₁₈$): Linear structure.
• Cyclohexane ($C₆H₁₂$): A cyclic alkane.
• 2,2,4-Trimethylpentane ($C₈H₁₈$): Known as iso-octane, an important component in fuels.

Alkenes ($C=C$ Double Bonds)

• General Formula: $C_nH_{2n}$
• Bonding: Alkenes contain at least one carbon-carbon double bond, making them unsaturated hydrocarbons.
• Homologous Series: Each alkene differs by a $CH_2$ unit.

Systematic Names and Structures of Alkenes (C1–C4):

1. Ethene ($C₂H₄$)
2. Propene ($C₃H₆$)
3. Butene ($C₄H₈$)

Structural Isomers:

Butene has two structural isomers: 1-butene and 2-butene.

Alkynes ($C≡C$ Triple Bonds)

• General Formula: $C_nH_{2n-2}$
• Bonding: Alkynes contain at least one carbon-carbon triple bond, making them highly unsaturated.
• Ethyne ($C₂H₂$): Also known as acetylene, it is the simplest alkyne.

Physical Properties of Aliphatic Hydrocarbons

State

Alkanes, alkenes, and alkynes are gases, liquids, or solids depending on the number of carbon atoms.

• Methane, ethene, and ethyne are gases at room temperature.
• Larger alkanes (e.g., pentane, and hexane) are liquids.

Solubility

Aliphatic hydrocarbons are non-polar, so they are insoluble in water but soluble in non-polar solvents (like hexane).

• For example, methane, ethene, and ethyne do not dissolve in water but are soluble in organic solvents like benzene.

Demonstrating Solubility Properties

To demonstrate solubility:

Methane, ethene, and ethyne are bubbled through water and a non-polar solvent like hexane.

• Result: These gases dissolve in the non-polar solvent but not in water, illustrating their non-polar nature.