Organic chem: Alkanes

Define structural isomers

Compounds with the same molecular formula but differen structural formula

Know how to draw the structural isomers of a compound

(pg 5 of notes book)

Bonding in alkanes

Alkanes are joined together by single covalent bonds called sigma bonds (σ - bonds). These happen when 2 orbitals on adjacent nuclei overlap and share a pair of electrons

Shape of molecules

VSEPR theory is used to predict the shape of molecules. Electron pairs in bonds repel each other so the shape of the molecule will maximise space between atoms

Shape and bond angle of alkanes

Tetrahedral shape

Bond angle 109.5 degrees

Draw methane using VSEPR theory

Boiling points of alkanes

They are determined by intermolecular forces called London forces.

London forces

London forces form between all molecules as they only need electrons to form

Formation of London forces

• In a molecule electrons are constantly moving this created instantaneous dipoles (partial charge)

• The instantaneous dipoles create induced dipoles in neighbouring molecules. These 2 dipoles can now attract each other

• The induced dipoles can induce further dipoles which attract one another

Factors affecting strength of London forces

Number of electrons

Surface area of contact

Effect of no. of electrons on boiling point

Less electrons leads to weaker London forces so less energy is needed to overcome them so they have lower boiling point

Effect of branching on boiling point

More branching decreases the surface area of contact resulting in weaker London forces that require less energy to overcome so lower boiling point

Alkanes are considered to be unreactive due to

• Having strong C-C and C-H σ-bonds

• The bonds are not polar

Equation for complete combustion of alkanes

alkane + oxygen -> carbon dioxide + water

Equation for incomplete combustion of alkanes

alkane + oxygen --> carbon monoxide (CO) + water

Incomplete combustion produces

Carbon monoxide or carbon particulates

General formula for balancing complete combustion

Define a radical

A species with an unpaired electron

Purpose of UV light in the reaction of halogens + alkanes

Provides energy to break the bonds during homolytic fission

What are the 3 steps during radical substitution

Initiation

Propagation

Termination

Revise steps of radical substitution

(examples in neat notes pg 8)

Limitations of radical substitution

• Further substitution reactions

• Substitution at different positions of the carbon chain creates a mixture of final products