12.1, 12.2, Alkanes & Radical Reactions

Question 1

What is a sigma bond?

Answer 1

The bond present in alkanes

Question 2

What is the alternative name for a single covalent bond?

Answer 2

A sigma bond

Question 3

When is a sigma bond formed?

Answer 3

When p-orbitals from alkanes directly overlap

Question 4

What decreases as the amount of branching in a molecule increases?

Answer 4

The boiling point.

Question 5

Why does boiling point decrease as the amount of branching increases?

Answer 5

-Because there are fewer surfacce points of contact between molecules iwthin the branched alkane, resulting in fewer London forces.
-Additionally, because of their shape, the branches of the molecules get in the way, and prevent the branched molecules getting as close together as straight-chain molecules, decreasing the intermolecular forces further.

Question 6

What happens as the chain length of a molecule increases?

Answer 6

The molecules have a larger surface area, so more surface contact is possible between molecules.

Question 7

Why do longer chain length molecules have higher boiling points?

Answer 7

Because they have a larger surface area, so more surface contact. Therefore, the London forces between the molecules will be greater, so more energy is required to overcome the forces.

Question 8

How is crude oil separated into separate molecules?

Answer 8

By fractional distillation, in a distillation tower

Question 9

What are alkanes?

Answer 9

A component of natural gas and crude oil, amongst the most stable organic compounds

Question 10

Are alkanes saturated or unsaturated?

Answer 10

Saturated

Question 11

Are alkanes reactive or unreactive?

Answer 11

Unreactive

Question 12

What is lack of reactivity in alkanes a result of?

Answer 12

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

Question 13

Why are alkanes used as fuel?

Answer 13

-Because their conbustion reactions give out heat
-They are readily available
-They are easy to transport
-They can burn in a plentiful supply of oxygen without releasing toxic products

Question 14

What is produced in incomplete combustion?

Answer 14

Carbon Monoxide and Water

Question 15

What is produced in complete combustion?

Answer 15

Carbon Dioxide and Water

Question 16

What causes incomplete combustion?

Answer 16

A limited oxygen supply

Question 17

What causes alkanes to react with halogens?

Answer 17

The presence of sunlight (because of the UV radiation)

Question 18

What is a radical?

Answer 18

A chemical species with one or more unpaired electrons, that are very reactive

Question 19

What are the three steps that radical substitution takes place in?

Answer 19

Initiation, propagation and termination

Question 20

What happens in INITIATION?

Answer 20

The conversion of atoms into radicals (eg in homolytic fission)

Question 21

What happens in PROPAGATION?

Answer 21

The reaction propagates through 2 propagation steps, causing a chain reaction (a radical reacts with a non-radical)

Question 22

When does propagation stop?

Answer 22

When two radicals collide

Question 23

When should propagation stop in theory?

Answer 23

When all the reactants have been used up.

Question 24

What is TERMINATION?

Answer 24

When two radicals collide and react, forming a molecule with all electrons paired.

Question 25

Are termination reactions exothermic or endothermic? Why?

Answer 25

Exothermic, because bonds are formed.

Question 26

What are the limitations of radical substitution in organic synthesis?

Answer 26

-Leads to the formation of multiple products. The greater the carbon chain, the greater the chance of unwanted products
-If the carbon chain is longer, we will get a mixture of isomers with substitution at different points in the carbon chain.