Chapter 12 - Alkanes

Question 1

What is the bonding in alkanes?

Answer 1

σ bonds

Question 2

What is a sigma (σ) bond?

Answer 2

• Head overlap of two orbitals, one from each bonding atom
• Electron density in a straight line

Question 3

What is the shape of alkanes?

Answer 3

Tetrahedral

Question 4

What is the bond angle around carbon atoms in alkanes?

Answer 4

109.5º

Question 5

Explain what happens to boiling point of alkanes when chain length increases?

Answer 5

• More surface points of contact
• Greater London forces
• Higher boiling point

Question 6

What happens to boiling point of alkanes when branching increases?

Answer 6

• Fewer surface points of contact
• Fewer London forces
• Lower boiling point

Question 7

Explain lack of reactivity of alkanes?

Answer 7

• C-C and C-H have high enthalpy
• C-C and C-H bonds non polar

Question 8

Word equation for complete combustion of alkanes?

Answer 8

Alkane + oxygen -> carbon dioxide + water

Question 9

What is produced when alkane undergoes incomplete combustion?

Answer 9

Carbon monoxide or carbon
+ water

Question 10

What is the name of the mechanism in which alkanes react with halogens?

Answer 10

Radical substitution

Question 11

What type of fission breaks the covalent bond in the initiation stage?

Answer 11

Homolytic fission

Question 12

Bromination of methane

Give the equation for the initiation stage?

Answer 12

Br₂ -> Br• + •Br

Question 13

Bromination of methane

Give the equation for the first propagation step?

Answer 13

CH₄ + Br• -> •CH₃ + HBr

Question 14

Bromination of methane

Give the equation for the second propagation step?

Answer 14

•CH₃ + Br₂ -> CH₃Br + Br•

Question 15

What happens to the Br• radical formed in the propagation step?

Answer 15

Can react again with the alkane molecule so creates a chain reaction, however it is normally stopped by termination

Question 16

Bromination of methane

Give the equations for the three termination steps?

Answer 16

Br• + •Br -> Br₂
•CH₃ + •CH₃ -> C₂H₆
•CH₃ + •Br -> CH₃Br

Question 17

What are the limitations of radical substitution?

Answer 17

• Further substitution
• Substitution at different positions in carbon chain

Question 18

Bromination of methane

What would the be final product of further substitution for this reaction?

Answer 18

CBr₄