Alkanes

Heterolytic fission

The breaking of a covalent bond, forming a positive ion and a negative ion (one of the atoms takes both electrons in the bond)

Homolytic fission

The breaking of a covalent bond, with one of the bonding electrons going to each of the atoms, forming 2 radicals

Radical

An atom or group of atoms with an unpaired electron

Addition reaction

When 2 reactants join to form 1 product

Substitution reaction

When an atom or group of atoms is replaced by another atom or group of atoms

Elimination reaction

Involves the removal of one smaller molecule from a larger molecule, 1 reactant forms 2 products

Why are alkanes unreactive?

• C-C and C-H sigma-bonds are strong

• C-C bonds are non-polar

  • The electronegativity of carbon and hydrogen is so similar, that the C-H bond can be considered non-polar

What are the products of the complete combustion of an alkane?

Water and Carbon dioxide

What are the products of the incomplete combustion of an alkane?

• Carbon monoxide and Water

  • (or Carbon and water)

Reaction mechanism

The sequence of bond breaking and bond making steps that show the path taken by electrons during a reaction

What are the names of the 3 steps in a radical substitution reaction?

• Initiation

• Propagation

• Termination

What happens in initiation step in a radical substitution reaction?

The covalent bond in the bromine radical is broken by UV radiation in homolytic fission, forming 2 radicals

What happens in propagation step in a radical substitution reaction?

• A bromine radical reacts with the C-H bond in the methane, forming a methyl radical and HBr

• Then the methyl radical reacts with another bromine molecule, forming bromomethane and a new bromine radical

What happens in termination step in a radical substitution reaction?

2 radicals collide forming a new molecule

Give the reaction mechanism for the bromination of methane?

Overall equation

• CH₄ + Br₂ —UV-→ CH₃Br + HBr

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Initiation

• Br2 —UV-→ 2Br^.

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Propagation

• CH4 + Br^. —→ ^.CH3 + HBr

• ^.CH₃ + Br₂ —→ CH₃Br +Br^.

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  Termination

• Br^. + Br^. —→ 2Br₂

• Br^. + ^.CH₃ —→ CH₃Br

• ^.CH₃ + ^.CH₃ —→ CH₃CH₃

What are the limitations of radical substitutions?

• Further substitution

• Substitution into different parts of the chain

• Reaction not done to completion