Alkanes (F6examonly)

Question 1

What are alkanes?

Answer 1

• saturated hydrocarbons containing single C-H and C-C bonds
• these bonds are σ bonds

Question 2

What are σ bonds?

Answer 2

• formed by orbitals overlapping directly between bonding atoms
• there is free rotation around the sigma bond

Question 3

What is the shape around the bonds in alkanes? Why?

Answer 3

• tetrahedral
• the 4 areas of electron density around the carbon atom repel, resulting in the tetrahedral shape, as the they arrange themselves as far apart as possible to minimise repulsion
• bond angle = 109.5

Question 4

Describe how the boiling point of an alkane depends on its size and shape:

Answer 4

• the longer the carbon chain, the stronger the induced dipole-dipole interactions (london forces)
• this is because there are more points of surface contact between molecules, and more electrons to interact
• therefore a greater amount of energy is required to overcome these london forces, so the boiling point increases
• the more branched the carbon chain, the weaker the induced dipole-dipole interactions
• this is because there are less points of contact between the molecules
• therefore less energy is required to overcome these london forces, so the boiling point decreases

Question 5

Why are alkanes unreactive?

Answer 5

• the sigma bonds in alkanes have low polarity, because the difference between the electronegativity of carbon and hydrogen is very small
• this means that the sigma bonds are unreactive
• alkanes also have high bond enthalpies, as the C-H and C-C bonds require a lot of energy to break
• this makes alkanes unreactive

Question 6

Describe the complete combustion of alkanes as used in fuels:

Answer 6

• alkanes react with O2 to produce H2O and CO2
• the alkanes must be vaporised first, as the combustion reaction happens between gases
• larger alkanes release more energy because more bonds are broken
• so they make good fuels

Question 7

Describe the incomplete combustion of alkanes:

Answer 7

• when there is a limited supply of oxygen, alkanes react with O2 to produce CO and H2O
• this releases less energy

Question 8

What are the dangers of CO being produced?

Answer 8

• carbon monoxide is poisonous
• it binds to the haemoglobin in the blood stream
• this is irrerversible
• therefore there is less haemoglobin available to transport O2 around the body, and this leads to oxygen deprivation

Question 9

What is homolytic fission?

Answer 9

when a bond is broken, and each bonding atom recieves one electron from the bonded pair

Question 10

What are the limitations of synthesising organic products by free radical substitution?

Answer 10

• a mixture of products is formed
• this is due to further substitution:
• e.g. when trying to produce CH3Cl
• CH3Cl + Cl· → ·CH2Cl + HCl
• ·CH2Cl + Cl2 → CH2Cl2 + Cl·
• also due to reactions at different positions in the carbon chain. this produces a mixture of isomers