Alkanes and halogenoalkanes

Question 1

What is an alkane?

Answer 1

Saturated hydrocarbons, where each carbon forms four bonds

Question 2

Describe the process of fractional distillation:

Answer 2

1. Crude oil vaporised at 350ºC
2. Vaporised crude oil goes into the bottom of the fractionating column and rises up through the trays
3. Fractionating column is hotter at the bottom and cooler at the top - temperature gradient
4. Because of the different boiling points each fraction condenses at a different temperature, with short chain hydrocarbons condensing at the top (where its cooler) and long chain hydrocarbons condensing at the bottom (where its hotter)

Question 3

What is cracking?

Answer 3

Breaking long chain alkanes into smaller hydrocarbons, involves breaking C-C bonds

Question 4

How does thermal cracking work?

Answer 4

Takes place at a high temp (up to 1000ºC), and up to 70atm pressure. Produces lots of alkenes which can be used to form polymers

Question 5

How does catalytic cracking work?

Answer 5

Uses a zeolite catalyst, at a slight pressure and around 500ºC. Mostly produces aromatic hydrocarbons and the alkenes needed to produce motor fuels. Catalytic cracking is cheaper and quicker.

Question 6

What is complete combustion?

Answer 6

Burning alkanes (and other hydrocarbons) with plenty of oxygen, producing carbon dioxide and water

Question 7

What is incomplete combustion?

Answer 7

Burning hydrocarbons when there is not enough oxygen, producing particulate carbon (soot) and carbon monoxide

Question 8

What are the three main pollutants in vehicle exhausts and how are they removed?

Answer 8

Nitrogen oxides, unburnt hydrocarbons and carbon monoxide.
Removed by catalytic converters:

C3H8 + 5O2 –> 3CO2 + 4H2O
2NO + 2CO –> N2 + 2CO2
2NO–> N2 + O2

Question 9

What is the issue with sulfur dioxide?

Answer 9

Reacts with moisture in the atmosphere to form acid rain (also caused by nitrogen dioxide), which destroys trees and vegetation, corrodes statues and kills fish

Question 10

How is sulfur dioxide removed?

Answer 10

Powdered calcium oxide is mixed with water to make an alkaline slurry. Flue gases mix with the slurry and sulfur dioxide reacts with the calcium compound to form a harmless salt
CaO + SO2 –> CaSO3

Question 11

What is the process of a free radical substitution reaction to form chloromethane?

Answer 11

Initiation:
Cl2 –> 2Cl* under ultraviolet light

Propagation:
Cl* + CH4 –> *CH3 + HCl
CH3 + Cl2 –> CH3Cl + Cl

Termination:
CH3 + Cl –> CH3Cl
2 CH3 —> C2H6
2 Cl –> Cl2

Question 12

What is a chlorofluorocarbon?

Answer 12

A halogenoalkane molecule where all hydrogens have been replaced by chlorine and fluorine atoms

Question 13

What is the purpose of the ozone layer?

Answer 13

Absorbs lots of UV radiation from the sun, preventing sunburn and skin cancer

Question 14

How do chlorofluorocarbons affect the ozone layer?

Answer 14

CCl3F –> CCl2F + Cl (under UV)
Cl* + O3 –> O2 + ClO*
ClO* + O3 –> 2O2 + Cl*

Question 15

What have replaced CFCs?

Answer 15

HCFCs (hydrochlorofluorocarbons) and HFCs (hydrofluorocarbons)

Question 16

What is a halogenoalkane?

Answer 16

An alkane with at least one halogen atom in place of a hydrogen atom

Question 17

Why are most carbon-halogen bonds polar?

Answer 17

Halogens are generally much more electronegative than carbon

Question 18

What species attacks halogenoalkanes?

Answer 18

Nucleophile. Attracted to the ∂+ carbon as it doesn’t have enough electrons

Question 19

How does a nucleophilic substitution reaction occur?

Answer 19

Lone pair of e- on nucleophile attacks the ∂+ carbon, creating new bond between the nucleophile and carbon. Bond breaks between carbon and halogen as carbon can only form 4 bonds

Question 20

What are the conditions for a nucleophilic substitution reaction?

Answer 20

Warm, aqueous

Question 21

What will be formed in a nucleophilic substitution reaction between a halogenoalkane and:
>OH-
>CN-
>NH3

Answer 21

OH- : an alcohol
CN- : a nitrile
NH3 : an amine

Question 22

What is the trend in reactivity of the halogenoalkanes?

Answer 22

The C-F bond is the strongest, so undergoes nucleophilic substitution reactions more slowly than other halogenoalkanes.
C-F<C-Cl<C-Br<C-I

Question 23

What is the conditions and reactants in an elimination reaction?

Answer 23

Hot alcoholic
Between a halogenoalkane and hydroxide ions to form an alkene

Question 24

Describe an elimination reaction:

Answer 24

OH- acts as a base and takes a H+ from the carbon adjacent to the one attached to the halogen
Spare pair of electrons on the adjacent carbon forms a double bond with the carbon attached to the halogen
Carbon-halogen bond breaks