Alkanes

Sigma Bonds

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

Which bonds in alkanes are sigma bonds

All C-C, C-H, and C=C bonds

Why are alkanes unreactive?

Due to their high bond enthalpy

What property of alkanes explains their low activity?

Low polarity of σ bonds

Shape and Bond Angle of alkanes

• Shape - Tetrahedral

• Bond Angle - 109.5°

Why does the boiling point increase when carbon chain length increases?

Bigger molecule so there are more electrons so stronger London forces between the molecule

Why does boiling point decrease when branching increases?

There is less surface area contact so weaker London forces between the molecules

Complete combustion of alkanes

Alkane + Oxygen → Carbon Dioxide + Water

Incomplete combustion of alkanes

• Alkane + Oxygen → Carbon Monoxide + Water

• Alkane + Oxygen → Carbon + Water

Issues with Carbon Monoxide

Bonds to the haemoglobin in the blood and blocks cell respiration which can lead to reduced blood oxygen concentration which can result in unconsciousness

Homolytic Fission

Each bonding pair receives one electron from the covalent bond, forming two radicals

Radical

A species with an unpaired electron

Free Radical Substitution

A covalent bond is broken and each of the bonded atoms receives one electron to form two radicals

• Halogen + Alkane → Haloalkane + Hydrogen Halide

Purpose of Free Radical Substitution

Synthesise haloalkanes as they are good solvents and starting materials

Free Radical

Reactive species due to the presence of an unpaired electron

Free Radical Substitution Mechanism

• Condition - UV Light

• Reagents - Halogen Molecule

Stages of Free Radical Substitution

• Stage 1 - Initiation

• Stage 2 - 1st and 2nd Propagation

• Stage 3 - Termination

Limitations of free radical substitutions

• Further substitution can occur

• Substitutions can occur in different positions