Properties of hydrocarbons

Forces between molecules
Different forces between hydrocarbons
Hydrocarbons in water

Forces between molecules

Intramolecular forces:

The forces holding molecules together within a molecule are called intermolecular forces.

Types of intramolecular forces of attraction

Ionic bond (Cations to anions)
Covalent bond:
- Nonpolar covalent bond (Nuclei to shared electrons)
- Polar covalent bond (Partially charged cation to partially charged anion)
Metallic bonding (Metal cations to delocalized electrons)

Intermolecular forces:

Intermolecular forces are forces that exist between molecules.

Types of intermolecular forces

Dipole-dipole interactions (Partially oppositely charged ions)
Hydrogen bonding (H atom and O, N or F atom)
London dispersion forces (Temporary or induced dipoles/van der Waals force)

Questions

1. Which is the dominating type of bond between two molecules of 1,2-dimethylcyclohexane?
2. Which is the dominating type of bond between two molecules of lactic acid?
3. Phosphorous trihydride or Phosphine can bond a proton and form the phosphonium ion, in the same way as ammonia can bond a proton a form the ammonium ion (see the reaction diagram). Which different kinds of bonds are there between the atoms in the phosphonium ion?

Phosphonium ion

feedback

Which is the dominating type of bond
1. between molecules of xylene?
2. between molecules of butanoic acid?
3. in solid CaO?
4. Is the compound with the molecular formula C2Cl4 a dipole? (Motivate your answer.)

feedback

Different forces between hydrocarbons

Always present are the London dispersion forces:

The bigger the molecule the larger surface area and the more induced dipoles can be formed.

Many physical properties are determined by the intermolecular forces.

Melt temperature T_m: energy required to break the dispersion forces that connect the molecules in solid phase.

Melt temperature T_bp: energy required to break the dispersion forces that connect the molecules in the liquid phase.

The dispersion forces in the solid phase are strong since the molecules are arranged like bricks.

dispersion forces in solid/liquid and gas forms

T_m varies with how well the molecules can be packed.

T_bp varies with molecular surface.

variance in boiling point due to heat/surface area

Hydrocarbons in water

Hydrocarbons are insoluble in water.

To dissolve strong intermolecular forces between water molecules need to be broken.

Removing bonds increase the systems energy – not favourable. Formation of bonds decreases the systems energy (favourable but the bonds formed between hydrocarbon and water molecules are very weak).

All molecules containing H(ydrogens) can function as an acid.

According to Brønsted’s definition an acid is a substance that can release protons (Hydrogen atoms without electrons).
A base is according to the same man a substance that can receive and bind protons.

Hydrocarbons are full of hydrogens. We can therefore discuss their acid and base properties. (The acid strength can however be 10³⁰ – 10⁴⁰ times weaker than e.g. HCl or H₂SO₄)