Physical Properties of Alkanes

Physical Properties of Alkanes

Alkanes, also known as paraffins, derive their name from the Latin words "parum" (meaning "little") and "affinis" (meaning "affinity"), which together suggest that alkanes have "little or no affinity" for interacting with other substances. This highlights two key characteristics of alkanes:

1. They do not interact strongly with each other.
2. They are relatively unreactive compared to other organic compounds.

Because alkanes are non-polar, they experience only weak London dispersion forces, meaning they don't form strong bonds with themselves or with other substances. This lack of affinity is evident in their low reactivity and minimal tendency to participate in chemical reactions.

In terms of their physical properties, this minimal interaction results in predictable trends, particularly in their boiling points, water solubility, and density, which are discussed in more detail below.

1. Boiling Points of Alkanes

The boiling point of an alkane depends mainly on the size (molecular weight) and structure of the molecule.

• Molecular Size and Boiling Point:

  • As the number of carbon atoms in the alkane increases, its boiling point rises. This is because larger molecules have greater surface areas, allowing for stronger London dispersion forces (temporary dipoles due to electron movement) between molecules.
    
    
  • For example:
    • Methane (CH₄): Boiling point of -161.5°C.
    • Pentane (C₅H₁₂): Boiling point of 36.1°C.
    • Decane (C₁₀H₂₂): Boiling point of 174°C.
  • With each additional CH₂ group, the boiling point generally increases by about 20-30°C.

• Branching and Boiling Point:

  • Straight-chain alkanes have higher boiling points than their branched isomers. Branching reduces the surface area available for intermolecular contact, weakening the London dispersion forces.
  • For example:
    • n-Pentane (straight-chain) has a boiling point of 36.1°C, while isopentane (branched) has a boiling point of 27.7°C.
      
      

2. Water Solubility of Alkanes

Alkanes are non-polar molecules, which means they do not form strong interactions with polar solvents like water. Water molecules, which are highly polar and participate in hydrogen bonding, do not readily mix with alkanes.

• "Like dissolves like" principle: Since alkanes are non-polar and water is polar, alkanes are insoluble in water. Instead, they are soluble in non-polar solvents like hexane, ether, and other hydrocarbons.

• Hydrophobicity: Alkanes are hydrophobic ("water-fearing"). This means that in aqueous environments, alkanes tend to group together and separate from water, forming distinct layers (e.g., oil and water separation).

• Alkane Structure and Solubility:

  • The longer the alkane chain, the less likely it is to dissolve in water. Shorter alkanes (methane, ethane) are gases and don’t interact with water, while longer alkanes (pentane, hexane, etc.) are liquids that form separate phases when mixed with water.

3. Other Physical Properties

• Density:

  • Alkanes are less dense than water (density of water = 1 g/cm³). The density of alkanes ranges between 0.6 - 0.8 g/cm³ for liquid alkanes. This is why alkanes float on water when mixed.
  • The density increases slightly with chain length, but they still remain less dense than water.

• State at Room Temperature:

  • Small alkanes (C₁ to C₄): Gases at room temperature (e.g., methane, ethane, propane).
  • Medium alkanes (C₅ to C₁₅): Liquids at room temperature (e.g., pentane, hexane).
  • Larger alkanes (C₁₆ and above): Waxy solids or high-viscosity liquids.

4. Viscosity

• Longer alkanes have higher viscosity due to stronger London dispersion forces, meaning they flow more slowly than shorter alkanes. For example, hexadecane (C₁₆H₃₄) is a thick, waxy solid, while butane (C₄H₁₀) is a gas with low viscosity.

Summary of Physical Properties:

• Boiling point: Increases with molecular size; branched alkanes have lower boiling points than straight-chain alkanes.
• Water solubility: Alkanes are insoluble in water due to their non-polar nature.
• Density: Less dense than water and float when mixed with water.
• Viscosity: Increases with molecular size; larger alkanes are more viscous.