4.1.2 Alkanes

Question 1

Alkanes

Answer 1

Saturated hydrocarbons containing single C-C and C-H bonds as σ-bonds, with free rotation of the σ-bond

Question 2

σ-bond

Answer 2

Overlap of orbitals directly between the bonding atoms

Question 3

Shape

Answer 3

Each carbon atom in an alkane is surrounded by 4 electron pairs in 4 σ-bonds that arrange in a 3D tetrahedral shape due to the repulsion between the electron pairs (EPR theory). Each bond angle in the tetrahedral arrangement is ~109.5°

Question 4

Relationship between Carbon-chain length and boiling point

Answer 4

As the chain length increases, the surface area increases and there are more contact points between alkane chains, therefore there are greater London forces between molecules. This results in an increased strength of attraction between molecules so more (heat) energy is needed to overcome the forces, thus increasing the boiling point.

Question 5

Relationship between branching and boiling point

Answer 5

As the branching increases, there are fewer surface contact points between alkane chains, therefore there are fewer London forces between molecules. This results in a decreased strength of attraction between molecules so less (heat) energy is needed to overcome the forces, thus decreasing the boiling point.

Question 6

Reactivity

Answer 6

Alkanes have low reactivity due to

• strong σ-bonds between the C-C and the C-H
• C-C bonds are non-polar (same electronegativity)
• C-H bonds are considered non-polar (similar electronegativity)

Question 7

Complete combustion

Answer 7

Alkane + plentiful supply of O2 -> CO2 + H2O
Used as fuels
- readily available
- easy to transport
- do not release toxic products

Question 8

Incomplete combustion

Answer 8

Alkane + limited supply of O2 –> CO + H2O

• CO is a colourless, odourless, toxic gas
• Can also produce soot (C)

Question 9

Halogenation

Answer 9

Alkanes undergo a substitution reaction in the presence of UV radiation
UV radiation (from sunlight) provides initial energy needed for the reaction to take place

Question 10

Radical Substitution

Answer 10

Mechanism for the halogenation of alkanes involving homolytic fission in the presence of UV radiation, takes place in 3 stages:

• Initiation
• Propagation
• Termination

Question 11

Initiation

Answer 11

covalent bond in halogen molecule is broken by homolytic fission to form 2 highly reactive radicals
Energy for bond fission provided by UV radiation
Br—Br –> Br• + Br•

Question 12

Propagation

Answer 12

A chain reaction through two steps
1. A halogen radical reacts with a C—H bond in the alkane to form an alkane radical + a halogen halide compound
CH4 + Br• –> CH3• + HBr
2. Each alkane radical reacts with another halogen molecule to form a haloalkane compound + a new halogen radical
CH3• + Br2 –> CH3Br + Br•

Question 13

Termination

Answer 13

2 radicals collide and pair up their unpaired electrons to form a covalent bond
Stable molecules are formed and radicals are removed from the reaction mixture, stopping the reaction
There are a number of possible termination steps:
1. Br• + Br• –> Br2
2. CH3• + CH3• –> C2H6
3. CH3• + Br• –> CH3Br

Question 14

Further substitution

Answer 14

Another halogen radical can collide with a haloalkane molecule in propagation to substitute a further H atom to form di(haloalkane), etc.
Further substitution can continue until all atoms have been substituted resulting in a mixture of organic products formed
Limitation of radical substitution in synthesis of alkanes

Question 15

Reactions at different positions in a Carbon-chain

Answer 15

With longer alkane chains there will be a mixture of monosubstituted isomers by substitution at different positions along a carbon chain
Results in even more possibilities e.g. with further substitution can form 2 di(haloalkane) isomers and a larger mixture of organic products formed
Limitation of radical substitution in synthesis of alkanes