Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

A-Level Chemistry (Organic) > 3.2 Alkanes > Flashcards

3.2 Alkanes Flashcards

You may prefer our related Brainscape-certified flashcards:
Periodic Table flashcards Chemistry 101 flashcards AP® Chemistry flashcards Organic Chemistry 101 flashcards
1
Q

What are Alkanes?

A

Compounds of carbon and hydrogen with only single bonds between them

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

The Process of Fractional Distillation of Crude Oil:

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Why are shorter chain Hydrocarbons in higher demand?

A

They are more useful as fuels

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What happens during Cracking?

A
  • In cracking these large, less useful hydrocarbon molecules found in crude oil are broken down into smaller, more useful molecules
  • The large hydrocarbon molecules are fed into a steel chamber and heated to a high temperature and then passed over an aluminium oxide (Al2O3) catalyst
  • The chamber does not contain any oxygen to prevent combustion of the hydrocarbon to water and carbon dioxide
  • When a large hydrocarbon is cracked, a smaller alkane and one (or more) alkene molecules are formed

E.g. octane and ethene from decane

Long hydrocarbons are cracked by heating them and using aluminium oxide catalyst into a smaller alkane and an alkene

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What are the two types of Cracking?

A
  • Thermal Cracking
  • Catalytic Cracking
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What are the conditions required for Thermal Cracking?

A
  • requires high temperatures (up to 1000 C)
  • and high pressure (up to 70 atmospheres) and produces alkanes and a lot of alkenes

Remember that cracking is an endothermic reaction.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What are the conditions required for Catalytic Cracking?

A
  • uses a lower temperature (around 450 C)
  • and slight pressure in the presence of a catalyst such as a zeolite or aluminium oxide to produce mainly aromatic hydrocarbons

Remember that cracking is an endothermic reaction.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the Word Equation for Complete Combustion?

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What conditions are necessary for Complete Combustion to occur?

A

Excess (plenty of) oxygen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What are the products of Incomplete combustion?

A
  • Carbon Monoxide
  • Carbon (Soot)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Why is Carbon Monoxide Harmful to Humans?

A
  • CO is a toxic, colourless and odourless gas which can cause dizziness, loss of consciousness and eventually death
  • The CO binds well to haemoglobin which therefore cannot bind oxygen and carbon dioxide
  • Oxygen is transported to organs
  • Carbon dioxide is removed as waste material from organs
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

How are Oxides of Nitrogen Formed?

A

In a car engine, high temperatures and pressures are reached causing the oxidation of nitrogen to take place:

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What are the Harmful effects of Oxides of Nitrogen?

A
  • In air, the nitrogen oxides can react with these VOCs (volatile organic compounds) to form peroxyacetyl nitrate (PAN) which is the main pollutant found in photochemical smog
  • PAN is also harmful to the lungs, eyes and plant-life
  • Nitrogen oxides can also dissolve and react in water with oxygen to form nitric acid which is a cause of acid rain
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What are the Harmful effects of Acid Rain?

A

Acid rain can cause corrosion of buildings/statues, endangers plant and aquatic life (as lakes and rivers become too acidic) as well as directly damaging human health

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is the purpose of Catalytic Converters?

A

To reduce the amount of pollutants released in car exhaust fumes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What metal is a catalytic converter made from?

A

Platinum

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

The reactions that take place in the catalytic converter include:

  • Oxidation of CO to CO2:
  • Reduction of NO to N2:
  • Oxidation of unburnt hydrocarbons:
A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Pollutants, their Effect & Removal Table:

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

How do we reduce sulfur dioxide emissions?

A
  • The main way to reduce sulfur dioxide emissions is to treat the waste gases from coal fired power stations
  • The waste gases are passed into a scrubbing chamber which sprays a wet slurry of calcium oxide and calcium carbonate into the gases
  • This process is also known as sulfur scrubbing or flue gas desulfurisation
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

How can Calcium oxide be used to reduce Sulfur Dioxide Emissions?

A
  • Calcium oxide and water reacts with sulfur dioxide to initially produce calcium sulfiite, which is then further oxidised to calcium sulfate or gypsum:
  • Calcium carbonate can also be used

The scrubber sprays a lime slurry over the waste gases to remove 90 - 95% of the sulfur dioxide

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What is a Free Radical?

A

Any species with a free, unpaired electron

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

How can Alkanes undergo Free Radical Substitution?

A

Alkanes can undergo free-radical substitution in which a hydrogen atom gets substituted by a halogen (chlorine/bromine)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Why do Alkanes need UV light for Free Radical Substitution?

A

Alkanes are very unreactive

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What are the Three steps of Free Radical Substitution?

A
  • Initiation
  • Propagation
  • Termination
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

Free Radical Substitution

What happens during the Initiation Step?

A
  • In the initiation step the Cl-Cl or Br-Br is broken by energy from the UV light
  • Each atom takes one electron from the covalent bond
  • This produces two radicals in a homolytic fission reaction
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

Free Radical Substitution

What happens during the Propagation Step? (P1)

A
  • This refers to the progression (growing) of the substitution reaction in a chain reaction
  • Free radicals are very reactive and will attack the unreactive alkanes
  • A C-H bond breaks homolytically
  • Remember: Homolytic fissions is where each atom gets one electron from the covalent bond
  • An alkyl free radical is produced
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

Free Radical Substitution

What happens during the Propagation Step? (P2)

A
  • … This can attack another chlorine / bromine molecule to form a halogenoalkane and regenerate the chlorine / bromine free radical
  • The regenerated chlorine / bromine free radical can then repeat the cycle
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

Free Radical Substitution

Why is Free Radical Substitution not suitable for preparing specific haolgenoalkanes?

A

A mixture of substitution products are formed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

Free Radical Substitution

What happens during the Termination Step?

A
  • This is when the chain reaction terminates (stops) due to two free radicals reacting together and forming a single unreactive molecule
  • Two radicals → one product

  • Multiple products are possible, dependent on the radicals involved
  • For example, in the single substitution of ethane with chlorine:
  • Exam Tip - *If you are asked to give an equation for the termination step of a free radical reaction / mechanism, you should not give the equation reforming the original halogen as this is often marked as “ignore” on mark schemes *
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

Halogenoalkane Bond Energy Table:

A
  • The halogenoalkanes have different rates of substitution reactions
  • Since substitution reactions involve breaking the carbon-halogen bond the bond energies can be used to explain their different reactivities

The table above shows that the C-I bond requires the least energy to break, and is therefore the weakest carbon-halogen bond
During substitution reactions the C-I bond will therefore heterolytically break as follows:
R3C-I + OH- → R3C-OH + I-
The C-F bond, on the other hand, requires the most energy to break and is, therefore, the strongest carbon-halogen bond
Fluoroalkanes will therefore be less likely to undergo substitution reactions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

What happens when we react halogenoalkanes with aqueous silver nitrate solution?

A

This will result in the formation of a precipitate

The rate of formation of these precipitates can also be used to determine the reactivity of the halogenoalkanes

32
Q

What is the trend in reactivity of halogenoalkanes?

A

C-F Has the strongest bond

33
Q

What are the most common halogenoalkanes?

A

chlorofluorocarbons (CFCs)

  • These compounds contain carbon atoms with chlorine and fluorine atoms attached to them
  • E.g. CCl3F and CCl2F2
34
Q

Why are CFCs Useful?

A

CFCs have many uses due to their chemical inertness as they are non-flammable and non-toxic

35
Q

Uses of CFCs?

A
  • They are used as refrigerators
  • Propellants for aerosols
  • As solvents for dry cleaning
36
Q

What are the effects of CFCs on the Ozone Layer?

A
  • CFCs absorb a lot of UV radiation in the upper atmosphere
  • The CFCs are then broken down by the UV light causing the formation of chlorine radicals
  • These radicals react with ozone and break down the ozone layer

Chlorine radicals and the ozone layer

37
Q

Why are hydrofluorocarbons (HFCs) often used as an alternative to CFCs?

A

As these compounds don’t contain any chlorine atoms, they will not have adverse effects on the ozone layer

38
Q

Why are Halogenoalkanes much more reactive than Alkenes?

A

Due to the presence of the electronegative halogens

The carbon-halogen bond is polar causing the carbon to carry a partial positive and the halogen a partial negative charge

39
Q

What Two reactions do halogenoalkanes undergo?

A
  • Nucleophilic substitution reactions
  • Elimination reactions
40
Q

What are Nucleophilic substitution reactions?

A
  • A halogen is substituted for another atom or group of atoms
  • The products formed when halogenoalkanes undergo this type of reaction are alcohols, amines and nitriles
41
Q

What are Elimination reactions?

A
  • A hydrogen halide is eliminated during the reaction
  • The key product formed from this type of reaction is an alkene
42
Q

Fractional Distillation (Key Points)

A
  • Oil is pre-heated
  • then passed into column
  • The fractions condense at different heights
  • The temperature of column decreases upwards
  • The separation depends on boiling point
  • Boiling point depends on size of molecules
  • The larger the molecule the larger the van der waals forces
  • Similar molecules (size, bp, mass) condense together
  • Small molecules condense at the top at lower temperatures
  • and big molecules condense at the bottom at higher temperatures.
43
Q

What is Vacuum distillation?

A
  • Heavy residues from the fractionating column are distilled
    again under a vacuum.
  • Lowering the pressure over a liquid will lower its boiling point.

Vacuum distillation allows heavier fractions to be further separated without high temperatures which could break them down.

44
Q

Define Cracking

A

conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds

45
Q

Thermal Cracking Summary:

A
46
Q

Catalytic Cracking Summary:

A
47
Q

What are the environmental effects of Carbon (Soot) ?

A

Carbon (soot) can cause global dimming - reflection of the sun’s light

48
Q

How can SO2 be removed from the waste gases from furnaces?

A
  • flue gas desulfurisation
  • The gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction

The calcium sulfite which is formed can be used to make calcium sulfate for plasterboard.

49
Q

What do Catalytic Converters Involve?

A
  • Converters have a ceramic honeycomb coated with a thin layer of catalyst metals platinum, palladium, rhodium
  • to give a large surface area
50
Q

Describe The Mechanism of greenhouse effect:

A
  • UV wavelength radiation passes through the atmosphere to the Earth’s surface and heats up Earth’s surface.
  • The Earth radiates out infrared long wavelength radiation.
  • The C=O Bonds in CO2 absorb infrared radiation so the IR radiation does not escape from the atmosphere.
  • This energy is transferred to other molecules in the atmosphere by collisions so the atmosphere is warmed.
51
Q

What is a Primary halogenoalkane?

A

One carbon attached to the carbon atom adjoining the halogen

52
Q

What is a Secondary halogenoalkane?

A

Two carbons attached to the carbon atom adjoining the halogen

53
Q

What is a Tertiary halogenoalkane?

A

Three carbons attached to the carbon atom adjoining the halogen

54
Q

Nucleophilic substitution reactions (Summary)

A
55
Q

What is Hydrolysis?

A

The splitting of a molecule by a reaction with water

56
Q

Why have many of the uses (of halogenoalkanes) been stopped ?

A

This is due to the toxicity of halogenoalkanes and also their detrimental effect on the atmosphere.

57
Q

What is the benefit of the Ozone Layer?

A

It filters out much of the sun’s harmful UV radiation.

58
Q

How can we compare the rate of hydrolysis reactions?

A
  • Aqueous silver nitrate is added to a halogenoalkane.
  • The halide leaving group combines with a silver ion to form a silver halide precipitate.
  • The precipitate only forms when the halide ion has left the halogenoalkane and so the rate of formation of the precipitate can be used to compare the reactivity of the different halogenoalkanes.

The quicker the precipitate is formed, the faster the substitution reaction and the more reactive the halogenoalkane.
The rate of these substitution reactions depends on the strength of the C-X bond . The weaker the bond, the easier it is to break and the faster the reaction.

59
Q

Nucleophilic substitution with aqueous hydroxide ions (Summary)

A
60
Q

What is a Nucleophile?

A

Electron pair donator e.g. :OH- , :NH3 , CN-

61
Q

How are Alcohols formed from Halogenoalkanes?

A
  • The nucleophile in this reaction is the hydroxide, OH- ion
  • An aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) with ethanol is used
  • This reaction is very slow at room temperature, so the reaction mixture is warmed
  • This is an example of a hydrolysis reaction and the product is an alcohol
  • The rate of this reaction depends on the type of halogen in the halogenoalkane
  • The stronger the C-X bond, the slower the rate of the reaction
  • In terms of bond enthalpy, C-F > C-Cl > C-Br > C-I
  • Fluoroalkanes do not react at all, but iodoalkanes have a very fast rate of reaction

The halogen is replaced by the nucleophile, OH-

62
Q

Why is OH- A better Nucleophile than Water (H2O)?

This reaction (Formation of alcohols from hydrogenoalkanes) could also be done with water as the nucleophile, but it is very slow

A
  • The hydroxide ion is a better nucleophile than water as it carries a full negative charge
  • In water, the oxygen atom only carries a partial charge
63
Q

How are Nitriles formed from Halogenoalkanes?

A
  • The nucleophile in this reaction is the cyanide, CN- ion
  • Ethanolic solution of potassium cyanide (KCN in ethanol) is heated under reflux with the halogenoalkane
  • The product is a nitrile
  • Eg. bromoethane reacts with ethanolic potassium cyanide when heated under reflux to form propanenitrile

The halogen is replaced by a cyanide group, CN -

  • The nucleophilic substitution of halogenoalkanes with KCN adds an extra carbon atom to the carbon chain
  • This reaction can therefore be used by chemists to make a compound with one more carbon atom than the best available organic starting material
64
Q

How do we name Nitriles?

A
65
Q

How are Primary Amines formed from Halogenoalkanes?

A
  • The nucleophile in this reaction is the ammonia, NH3 molecule
  • An ethanolic solution of excess ammonia (NH3 in ethanol) is heated under pressure with the halogenoalkane
  • The product is a primary amine
  • Eg. bromoethane reacts with excess ethanolic ammonia when heated under pressure to form ethylamine

The halogen is replaced by an amine group, NH2

66
Q

Nucleophilic Substitution with OH- (Diagram)

A
67
Q

Nucleophilic Substitution with NH3 (Diagram)

A
68
Q

Nucleophilic Substitution with CN- (Diagram)

A
69
Q

What happens during an elimination reaction?

Halogenoalkanes

A
  • In an elimination reaction, an organic molecule loses a small molecule
  • In the case of halogenoalkanes this small molecule is a hydrogen halide (eg. HCl)
70
Q

What is the process of an elimination reaction?

Halogenoalkanes

A
  • The halogenoalkanes are heated with ethanolic sodium hydroxide causing the C-X bond to break
  • Forming an X- ion and leaving an alkene as an organic product

For example, Hydrogen bromide is eliminated to form ethene

71
Q

What products can form from elimination reactions depending on the conditions?

A
  • NaOH (hot, in ethanol): an elimination reaction occurs to form an alkene
  • NaOH (warm, aqueous): a nucleophilic substitution reaction occurs, and an alcohol is formed
72
Q

Elimination with OH- (Diagram)

A

Hydrogen bromide is eliminated to form ethene

73
Q

What type of reaction does an Aqueous solvent induce?

A

Substitution

74
Q

What type of reaction does an Alcoholic solvent induce?

A

Elimination

74
Q

How does the structure of the halogenoalkane also have an effect on the
degree to which substitution or elimination occurs?

A
  • Primary tends towards substitution
  • Tertiary tends towards elimination

A-Level Chemistry (Organic) (7 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions