L4 - Alkanes

objectives

• Explanation of the trends in boiling points of members of a homologous series.
• The general formulas (eg CnH2n+2) of alkanes should be known.
• Alkanes have low reactivity
• Writing equations for the complete and incomplete combustion of hydrocarbons.

The Alkanes

• Alkanes are the simplest hydrocarbons.
• They are made of carbon and hydrogen only.
• They are joined by single covalent bonds.
• C makes 4 bonds and H makes 1 bond

Example 1 - Methane

Naming Hydrocarbons

General formula: C_nH_2n+2

This means for every n carbons there are (2n+2) hydrogens

These hydrocarbons are said to be saturated.

(They have single bonds between carbon atoms).

Shapes of Alkane Molecules

Physical Properties of Alkanes

Boiling point

• increases as they get more carbon atoms in their formula
• more atoms = greater induced dipole-dipole interactions
• greater intermolecular force = more energy to separate the molecules
• greater energy required = higher boiling point

CH₄ (-161°C) C₂H₆ (-88°C) C₃H₈ (-42°C) C₄H₁₀ (-0.5°C)

• difference gets less - mass increases by a smaller percentage

Straight chains molecules have greater interaction than branched

“The greater the branching, the lower the boiling point”

Melting point

• general increase with molecular mass
• the trend is not as regular as that for boiling point.

Solubility

• alkanes are non-polar so are immiscible with water
• they are soluble in most organic solvents.

Viscosity

• As the intermolecular london forces increase between the larger molecules the liquid alkanes become more viscous - this means they are more thick, sticky substances
• This is because the intermolecular forces increase so each molecule is more attracted to one another

Chemical Properties of Alkanes

• Alkanes are unreactive. This is because the C-C bond and the C-H bond are very stable.
  • fairly unreactive; (old family name, paraffin, meant little reactivity)
  • have relatively strong, almost NON-POLAR, SINGLE covalent bonds
  • they have no real sites that will encourage substances to attack them

Combustion

• The main reaction they undergo is combustion.
• This means they make useful fuels - especially the lower members of the series
• Because they react with oxygen in an exothermic reaction
• the greater the number of carbon atoms, the more energy produced
• BUT the greater the amount of oxygen needed for complete combustion.
• When an alkane burns they produce carbon dioxide and water (CO₂ and H₂O)

activity 2 - properties of the alkanes

Combustion of Alkanes

Complete Combustion

Complete Combustion is characterised by:

• a Blue Flame (because the fuel burns more efficiently as there is an abundance of oxygen)
• The production of Carbon Dioxide and Water only
• As it is more efficient More energy if produced as heat per gram of fuel

Methane + oxygen → carbon dioxide + water

CH₄ + 2 O₂ → CO₂ + 2H₂O

Incomplete Combustion

Incomplete Combustion is characterised by:

• a Yellow Flame (because the fuel burns more efficiently as there is an abundance of oxygen)
• The production of Carbon OR Carbon Monoxide and Water only
• As it is less efficient less energy if produced as heat per gram of fuel

Methane + oxygen → carbon monoxide + water

Reaction of Alkanes with Halogens

Reagents:

• chlorine and methane

Conditions:

• UV light or sunlight

Equation(s):

1. CH₄(g) + Cl₂(g) ——> HCl(g) + CH₃Cl(g) chloromethane

Further Substitution:

1. CH₃Cl(g) + Cl₂(g) ——> HCl(g) + CH₂Cl₂(l) dichloromethane
2. CH₂Cl₂(l) + Cl₂(g) ——> HCl(g) + CHCl₃(l) trichloromethane
3. CHCl₃(l) + Cl₂(g) ——> HCl(g) + CCl₄(l) tetrachloromethane

Mixtures:

• free radicals are very reactive - they are trying to pair their electron with sufficient chlorine, every hydrogen will eventually be replaced.

Mechanism:

• Mechanisms portray what chemists think is going on in the reaction, whereas an equation tells you the ratio of products and reactants.
• Chlorination of methane proceeds via FREE RADICAL SUBSTITUTION because the methane is attacked by free radicals resulting in hydrogen atoms being substituted by chlorine atoms.
• The process is a chain reaction.
• In the propagation step, one radical is produced for each one used