12. ALKANES

Question 1

Are Alkanes saturated or unsaturated hydrocarbons?

Answer 1

Saturated

Question 2

State and explain the shape and angle of alkanes

Answer 2

There are 4 bonding regions which repel each other as far apart as possible.
Resulting in a tetrahedral shape with 109.5˚

Question 3

What is the bonding like in alkanes?

Answer 3

Single C-C and C-H bonds as sigma-bonds

Question 4

State and explain the effect of chain length on boiling point of alkanes

Answer 4

Boiling point increases as chain length increases

Increased chain length -> greater surface area of contact -> stronger london forces -> more energy required to overcome the forces -> higher boiling point.

Question 5

State and explain the effect of branching on boiling point of alkanes

Answer 5

Boiling point decreases as branching increases

More branching -> less surface area of contact -> weaker london forces -> less energy required to overcome the forces -> lower boiling point

Question 6

Why do alkanes not react with most common reagents?

Question 7

State and explain the shape and bond angles of alkanes

Answer 7

There are 4 electron pairs around each carbon, which repel each other as far apart as possible.

This results in a Tetrahedral shape, with 109.5˚

Question 8

What bonds do alkanes contain?

Answer 8

Single C-C and C-H bonds as sigma bonds

Question 9

How does boiling point vary with chain length? State and explain

Answer 9

Boiling point increases as chain length increases.

Increased chain length —> Greater surface area of contact —> Stronger london forces —> More energy required to overcome the forces —> Higher boiling point

Question 10

How does boiling point vary with branching?

Answer 10

Boiling point decreases as branching increases.

More branching —> Less surface area of contact —> Weaker london forces —> Less energy required to overcome the forces —> Lower boiling point

Question 11

Why do alkanes not react with most common reagents?

Answer 11

• The C-C and C-H sigma bonds are strong
• The C-C bonds are non-polar
• Similar electronegativity of C and H so C-H bonds are non-polar

Question 12

Write the general equation for the complete combustion of an alkane

Answer 12

Alkane + O2 (g) —> CO2 (g) + H2O (l)

Question 13

Write the general equation for the incomplete combustion of an alkane

Answer 13

Alkane + O2 (g) —> CO (g) + H2O (l)

or

Alkane + O2 (g) —> C (s) + H2O (l)

Question 14

When do alkanes undergo incomplete combustion?

Answer 14

In a limited supply of oxygen

Question 15

What is the general equation for an Alkane reacting with a Halogen?

Answer 15

Alkane + Halogen —>(UV) Haloalkane

Question 16

What type of reaction is Alkane + Halogen?

Answer 16

Free radical substitution

Question 17

What are the three steps of Free radical substitution?

Answer 17

Step 1: Initiation
Step 2: Propagation
Step 3: Termination

Question 18

What happens in the Initiation step?

Answer 18

Covalent bond is broken by homolytic fission, forming 2 radicals
(eg. Br-Br —> Br• + •Br)

Question 19

What happens in the Propagation step?

Answer 19

Propagation step 1:
One of the radicals reacts with a C-H bond forming a methyl radical and a hydrogen halide molecule.
(eg. CH4 + Br• —> •CH3 + HBr)

Propagation step 2:
Methyl radical reacts with halogen molecule forming organic product and halogen radical.
(eg. •CH3 + Br2 —> CH3Br + Br•)

This cycle continues as a chain reaction

Question 20

What happens in the termination step?

Answer 20

Two radicals collide forming a molecule with all electrons paired
(eg. Br• + •Br —> Br2)

When 2 radicals collide and react both radicals are removed form the reaction mixture, stopping the reaction

Question 21

What are the limitations of radical substitution in organic synthesis?

Answer 21

• Further substitution can occur
• Substitution at different positions in a carbon chain