3.2 - Alkanes and Halogenoalkanes

Question 1

How is crude oil separated?

Answer 1

Crude oil is separated by fractional distillation

Question 2

What are the steps in separated crude oil?

Answer 2

1) The crude oil is vaporised at around 620K
2) The vaporised crude oil travels up the fractionating column
3) Due to different sized alkanes having different boiling points, they condense at different temperatures in the column
4) The smallest hydrocarbons do not condense, and the largest do not vaporise (they are a thick liquid)

Question 3

What is cracking?

Answer 3

Cracking is the process of breaking long-chain alkanes into smaller hydrocarbons (which can include alkenes)

Question 4

What is thermal cracking?

Answer 4

Thermal cracking takes place at a high temperature (1250K) and a high pressure (up to 70 atm). Thermal cracking produces a lot of alkenes

Question 5

What catalyst is used in catalytic cracking?

Answer 5

Catalytic cracking uses zeolite (hydrated aluminosilicate) as a catalyst

Question 6

What is catalytic cracking?

Answer 6

A catalyst, slight pressure and a medium temperature (800K) are used to mainly produce aromatic hydrocarbons and the alkanes needed to produce motor fuels

Question 7

What are the benefits of catalytic cracking?

Answer 7

Using a catalyst cuts costs and speeds up the rate of reaction

Question 8

What are nitrous oxides?

Answer 8

Nitrogen oxides are a series of toxic and poisonous molecules that are produced in car engines

Question 9

How is ozone formed in a car engine?

Answer 9

Unburnt hydrocarbons and nitrogen oxides react to form ozone, a major component in smog and can cause lung damage and aggravate asthma

Question 10

What are catalytic converters?

Answer 10

Catalytic converters (made of precious metals like platinum, palladium and rhodium) remove pollutants like nitrogen oxides, unburnt hydrocarbons and carbon monoxide

Question 11

How is sulphur dioxide removed from power stations?

Answer 11

It is reacted with calcium oxide or calcium carbonate to form calcium sulphate, a harmless salt

Question 12

How does burning fossil fuels contribute to global warming?

Answer 12

The carbon dioxide released is a greenhouse gas, and therefore contributes to the enhanced greenhouse effect

Question 13

What is formed when halogens react with alkanes in photochemical reactions?

Answer 13

They form a halogenoalkane. A hydrogen atom is substituted by a halogen; this is called free radical substitution

Question 14

How are photochemical reactions started?

Answer 14

They are started by UV (ultraviolet) light

Question 15

What is a free radical?

Answer 15

A free radical is a particle with an unpaired electron

Question 16

How are free radicals formed?

Answer 16

Free radicals are formed when a covalent bond splits equally, giving one electron to each species.

Question 17

How is a free radical shown?

Answer 17

A dot is put next to the atom

Question 18

What are the three stages of free radical substitution?

Answer 18

Initiation, propagation and termination

Question 19

What happens in the ‘Initiation stage?

Answer 19

Free radicals are produced under UV light e.g. Cl2 → 2Cl•

Question 20

What happens during the ‘Propagation’ stage?

Answer 20

Free radicals are used up and created in a chain reaction. The first stage is when the free radical attack a molecule, forming another free radical and a separate compound. The second stage is when the free radical previously created attacks the halogen diatomic atom, creating the first free radical and a halogenoalkane

Question 21

What are the two propagation stages in the formation of chloromethane?

Answer 21

First Stage: Cl• + CH4 → •CH3 + HCl

Second Stage: •CH3 + Cl2 → CH3Cl + Cl•

Question 22

What happens in the reaction with chlorine and methane when the chlorine is in excess?

Answer 22

The products will go from chloromethane, dichloromethane, trichloromethane and tetrachloromethane

Question 23

What happens during the ‘Termination’ stage?

Answer 23

Free radicals are used up; two free radicals join together. This ends the chain reaction

Question 24

What are the three possible termination stages for the synthesis of chloromethane?

Answer 24

1) •CH3 + Cl• → CH3Cl
2) •CH3 + •CH3 → C2H6
3) Cl• + Cl• → Cl2

Question 25

What are chlorofluorocarbons?

Answer 25

CFCs are halogenoalkane molecules where all of the hydrogen atoms have been replaced by chlorine or fluorine atoms

Question 26

What is a halogenoalkane?

Answer 26

A halogenoalkane is an alkane with at least one halogen atom in place of a hydrogen atom

Question 27

What are the polarities of halogenoalkanes?

Answer 27

Most carbon-halogen bonds are polar due to the fact that the halogen has a much higher electronegativity than carbon atoms. The slightly positive carbon means it can be attacked by a nucleophile

Question 28

What is a nucleophile?

Answer 28

A nucleophile is an electron-pair donor

Question 29

What happens in a nucleophilic substitution reaction?

Answer 29

A nucleophile attacks a polar molecule, kicks out a functional group and settles itself in its place. The lone pair of electrons on the nucleophile creates a new bond between the nucleophile and the carbon

Question 30

What is produced when a halogenoalkane reacts with hydroxides and what conditions are needed?

Answer 30

An alcohol is formed. Warm aqueous sodium hydroxide or potassium hydroxide is needed

Question 31

What is produced when a halogenoalkane reacts with ethanolic potassium cyanide?

Answer 31

A nitrile is formed - the solvent cannot be water as it could act as a competing nucleophile and some alcohol would be formed.

Question 32

What is produced when a halogenoalkane reacts with ethanolic ammonia?

Answer 32

An amine is formed by a nucleophilic substitution reaction - the solvent cannot be water as it could act as a competing nucleophile and some alcohol would be formed

Question 33

What is the trend in nucleophilic substitution reaction rates with different halogen polar bonds?

Answer 33

The stronger the bond, the slower the reaction takes place and therefore fluorine has the slowest reaction, followed by chlorine, bromine and iodine

Question 34

What is an elimination reaction?

Answer 34

A small group of atoms break away from the larger molecule and are not replaced

Question 35

What happens when you warm a halogenoalkane with hydroxide ions dissolved in ethanol (not water)?

Answer 35

An elimination reaction occurs and an alkene is formed. This is because the OH- ion acts as a base in an elimination reaction (attacking an H+ ion to form water) instead of acting like a nucleophile (attacking the delta positive carbon)