Alkanes

Sigma Bonds

A bond formed by the lengthways overlap of orbitals directly between two bonding atoms

What type of bonds are sigma bonds?

• C-C and C-H bonds in alkanes

• They are the strongest type of covalent bond and will have a high bond enthalpy

• Due to the high bond enthalpy, alkanes are unreactive

Shape of Alkanes and Bond Angle

• Shape - Tetrahedral

• Bond Angle - 109.5

Factors affecting boiling point of alkanes: Carbon Chain Length

• Boiling point increase as carbon chain length increases as there are more electrons which means there are stronger London Forces between the molecules so more energy required

Factors affecting boiling point of alkanes: Branching

• As Branching increases boiling point decreases because there is less surface area contact so weaker London force between the molecules so little energy to overcome

Combustion of alkanes

Alkane + Oxygen → Carbon dioxide + water

More exothermic so it release more energy

Incomplete Combustion

• Alkane + Oxygen → Carbon Monoxide + Water

• Alkane + Oxygen → Carbon + Water

Using Combustion Equations

Free Radical Substitution

Alkane + Halogen → Haloalkane + Hydrogen Halide

CH₄ + X₂ → CH₃X = HX

Free Radical Substitution Mechanism

• Conditions - UV Light

• Reagents - Halogen Molecule

Three Stages of the Mechanism

• Initiation - Creates 2 Halogen Radicals

• Propagation: 1st Propagation - Alkane reacts with the Halogen radical, 2nd Propagation - Alkyl radical reacts with halogen molecule

• Termination - Radicals combine to make neutral molecules

What is UV light used for?

Energy required to break the X-X bond

Issues with free radical substitution: Low Yield

• Further substitution can occur forming a mixture of products

• Substitution can occur in different positions creating position isomers

Why is free radical substitution mechanism is likely to produce a mixture of organic products

Substitution can replace an H atom