physical properties of alkanes

Question

What are the physical properties of all alkanes. are they all odourless and do they all react with litmus paper.

Answer 1

Alkanes are not very reactive and have little biological activity; all alkanes are colorless and odorless non-polar compounds. The relative weak London dispersion forces of alkanes result in gaseous substances for short carbon chains, volatile liquids with densities around 0.7 g/mL for moderate carbon chains, and solids for long carbon chains. The differences in the physical states occurs because there is a direct relationship between the size and shape of molecules and the strength of the intermolecular forces (IMFs).

Because alkanes have relatively predictable physical properties and undergo relatively few chemical reactions other than combustion, they serve as a basis of comparison for the properties of many other organic compound families. Let’s consider their physical properties first.

1. Structures of Alkanes
All the carbon atoms present in an alkane are sp³ hybridized that is, every carbon atom forms four sigma bonds with carbon or hydrogen atoms. General configuration of alkane is C_nH_2n+2. They exhibit tetrahedral geometry with a bond angle of 109.47° between them.
2. Solubility of Alkanes

• Due to very little difference of electronegativity between carbon and hydrogen and covalent nature of C-C bond or C-H bond, alkanes are generally non-polar molecules.
• As we generally observe, polar molecules are soluble in polar solvents whereas non-polar molecules are soluble in non-polar solvents. Hence, alkanes are hydrophobic in nature that is, alkanes are insoluble in water.
• However, they are soluble in organic solvents as the energy required to overcome the existing Van Der Waals forces and generate new Van Der Waals forces is quite comparable.

2. Boiling Point of Alkanes

• As the intermolecular Van Der Waals forces increase with the increase of the molecular size or the surface area of the molecule we observe
• The boiling point of alkanes increases with increasing molecular weight.
• The straight-chain alkanes are observed to have a higher boiling point in comparison to their structural isomers.

3. Melting Point of Alkanes

• The melting point of alkanes follows the same trend as their boiling point that is, it increases with an increase in molecular weight.
• This is attributed to the fact that higher alkanes are solids and it’s difficult to overcome intermolecular forces of attraction between them.