Chapter 12: Alkanes

Question 1

Sigma bond

Answer 1

The strongest type of covalent bond, formed by the head-on overlapping between atomic orbitals. Each orbital contains one electron, so the sigma bond has two electrons that are shared between the bonding atoms.

Question 2

How many sigma bonds do each carbon atom in an alkane have?

Answer 2

Four

Question 3

Shape and bond angle around each carbon atom

Answer 3

Tetrahedral, 109.5

Question 4

Effect of chain length on boiling point

Answer 4

As chain length increases, there is more surface area of contact, meaning more London forces between molecules, and more energy is enquired to overcome these forces.

Question 5

Effect of branching on boiling point

Answer 5

As there is more branching, there is fewer surface points of contact between molecules, meaning weaker London forces, and less energy is required to overcome these forces.

Question 6

Why are alkanes not usually reactive?

Answer 6

• C—C and C—H sigma bonds are strong
• C—C bonds are non-polar
• The electronegativity of carbon and hydrogen is so similar that the C—H bond can be considered to be non-polar

Question 7

Why are alkanes used as fuels

Answer 7

Readily available, easy to transport and burn in a plentiful supply of oxygen without releasing toxic products.

Question 8

Products of complete combustion

Answer 8

Carbon dioxide and water

Question 9

Products of incomplete combustion

Answer 9

Carbon monoxide / soot (carbon) and water

Question 10

What condition is required for the reactions of alkanes with halogens

Answer 10

High energy ultraviolet (UV) radiation

Question 11

Initiation

Answer 11

Covalent bond in the halogen molecule is broken by homolytic fission, each halogen atom takes one electron, forming two highly reactive radicals.
Br-Br —> Br• + •Br

Question 12

Propagation

Answer 12

Step 1: One of the halogen radicals reacts with the C-H bond in the alkane, forming an alkyl radical, and a molecule of the hydrogen halide.
CH4 + Br• —> •CH3 + HBr

Step 2: Each methyl radical reacts with another halogen molecule, forming the haloalkane and new halogen radical.
•CH3 + Br2 —> CH3Br + Br•

Question 13

Termination

Answer 13

Two radicals collide, forming a molecule with all electrons paired.
Br• + •Br —> Br2
•CH3 + •CH3 —> C2H6
•CH3 + •Br —> CH3Br

Question 14

Limitations of radical substitution in organic synthesis

Answer 14

Further substitution
Substitution at different positions in the carbon chain