Chapter 12 Alkanes

Question 1

Alkanes

Answer 1

• Main components of natural gas and crude oil
• Used as fuels

Question 2

Bonding in alkanes

Answer 2

• Each C atom is joined to 4 other atoms by single covalent bonds which are sigma bonds

Question 3

Sigma bond

Answer 3

• The result of the overlap of 2 orbitals, 1 from each bonding atom
• Each overlapping orbital contains 1 electron -> therefore the sigma bond has 2 electrons that are shared between the bonding atoms
• Positioned on a line directly between bonding atoms

Question 4

Shape of alkanes

Answer 4

• Tetrahedral arrangement around each C atom
• The sigma bonds act as an axes around which the atoms can rotate freely, so these shapes are not rigid

Question 5

General formula of alkanes

Answer 5

CnH2n+2

Question 6

Crude oil

Answer 6

• Contains hundred of different alkanes

Question 7

How is crude oil seperated

Answer 7

Fractional distillation - separates by boiling point of the alkanes

Question 8

Boiling point of alkanes

Answer 8

Increases as the chain length increases due to increase in intermolecular forces

Question 9

Effect of chain length on boiling point

Answer 9

• As chain length increases, the molecules have a larger S.A. so more surface contact between molecules so more london forces

Question 10

Effect of branching on boiling point

Answer 10

Branched isomers have lower boiling points due to fewer surface points of contact between molecules.

Question 11

Reactivity of alkanes

Answer 11

Fairly unreactive because:
* C-C and C-H sigma bonds are strong
* C-C bonds are non-polar
* C and H have a very similar electronegativity so C-H can be considered non-polar

Question 12

Carbon monoxide

Answer 12

• Colourless, odourless, highly toxic gas
• Combines irreversibly with haemoglobin in red blood cells to form carboxyhaemoglobin

Question 13

Alkane + halogen

Answer 13

halogoalkane + hydrogenhalide

Question 14

Mechanism for bromination of alkanes

Answer 14

1. Initiation
2. Propagation
3. Termination

Question 15

Bromination of alkanes: Initiation

Answer 15

1. The covalent bond of the bromine molecule is broken by homolytic fission, by UV radiation
2. Radicals are formed

Question 16

Bromination of alkanes: Propagation

Answer 16

Step 1: A bromine radical reacts with a C-H bond in the methane -> forms a methyl radical and 1 molecule of hydrogen bromide

Step 2: Each methyl radical reacts with another bromine -> forms bromomethane and a bromine radical

• These 2 steps continue to cycle in a chain reaction
• Theoretically should stop when all reactants are used up, but normally stops due to radicals colliding

Question 17

Bromination of alkanes: Termination

Answer 17

When 2 radicals collide, forming a molecule with all electrons paired.

Question 18

Limitations of radical substitution in organic synthesis

Answer 18

Synthesis of just 1 organic compound

Question 19

What can follow bromination of alkanes

Answer 19

Multiple substitution
* A bromine radical can collide with a bromomethane molecule, substituting a further H atom -> forms dibromomethane

Question 20

Substitution at different positions in a carbon chain

Answer 20

• When C chain is long, substitution can occur at different positions
• Mono-substituted isomers