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AS Chemistry > Module 4.1 - Basic concepts and hydrocarbons > Flashcards

Module 4.1 - Basic concepts and hydrocarbons Flashcards

1
Q

What is a structural formula?

A

Shows the arrangement of atoms carbon by carbon, with the attached hydrogens and functional groups.

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2
Q

What is a skeletal formula?

A

Shows the bonds of the carbon skeleton only, with any functional groups. The hydrogen and carbon atoms aren’t shown.

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3
Q

Why is skeletal formula used?

A

Useful for large complicated structures, like cyclic hydrocarbons.

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4
Q

What is a displayed formula?

A

Shows how all the atoms are arranged, and all the bonds between them.

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5
Q

What is a homologous series?

A

A bunch of organic compounds that have the same functional group and general formula.

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6
Q

What is the general formula for alkanes?

A

CnH2n+2

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7
Q

Describe an aliphatic hydrocarbon skeleton?

A

In which the C atoms are joined together in either straight (unbranched) chains or branched chains.

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8
Q

Describe an aromatic hydrocarbon skeleton?

A

In one which there is at least one benzene ring in the structure.

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9
Q

Describe an alicyclic hydrocarbon skeleton?

A

In which the C atoms are joined together in a ring structure but are NOT aromatic.

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10
Q

What does a compound being saturated mean?

A

Only contain C-C single bonds - like alkanes.

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11
Q

What does a compound being unsaturated mean?

A

Can have C=C double bonds, triple bonds or aromatic groups.

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12
Q

What is an alkyl group and what is it’s general formula?

A

An alkyl group is a fragment of a molecule and it has the general formula CnH2n+2.

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13
Q

What is the prefix/stem for a compound with 1 carbon?

A

Meth-

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14
Q

Name the 4 stems for 1-4 number of carbons in a compound?

A

Meth-, eth-, prop- and but-

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15
Q

What is an isomer?

A

Two molecules are isomers of one another if they have the same molecular formula but the atoms are arranged differently.

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16
Q

What makes a chain isomer?

A

The carbon skeleton can be arranged differently - for example as a straight chain or branched. These isomers have similar chemical properties - but their physical properties like boiling point, will be different because of the change in shape of the molecule.

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17
Q

What makes a positional isomer?

A

The skeleton and the functional group could be the same, only with the functional group attached to a different carbon atom. These also have different physical properties, and the chemical properties could be different.

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18
Q

What makes a functional group isomer?

A

The same atoms can be arranged into different functional groups. These have very different physical and chemical properties.

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19
Q

What shape are alkane molecules?

A

In an alkane molecule, each C atom has 4 pairs of bonding electrons around it. They all repel each other equally. So, the molecule forms a tetrahedral shape around each carbon. Each bond angle is 109.5 degrees.

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20
Q

What happens if you burn (oxidise) alkanes with oxygen?

A

It is a combustion reaction so you get carbon dioxide and water.

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21
Q

What form must alkanes be to combust?

A

Combustion reactions take place between gases, so liquid alkanes have to be vaporised first. Smaller alkanes turn into gases more easily (volatile) so they’ll burn more easily.

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22
Q

Why do alkanes make excellent fuels?

A

Larger alkanes release heaps more energy per mole because they have more bonds to react. And, because they release so much energy when they burn.

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23
Q

What happens if you burn alkanes when there isn’t oxygen?

A

Incomplete combustion will take place and carbon monoxide and water produced.

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24
Q

How is carbon monoxide poisonous?

A

Carbon monoxide is better at binding to haemoglobin than oxygen is, so it binds to the haemoglobin in your bloodstream before the oxygen can. This means that less oxygen can be carried around your body, leading to oxygen deprivation. At very high concentrations, carbon monoxide can be fatal.

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25
Q

What is bond fission?

A

Breaking a covalent bond.

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26
Q

What is heterolytic fission?

A

In heterolytic fission the bond breaks unevenly with one of the bonded atoms receiving both electrons from the bonded pair. Two different substances are formed - a positively charged cation and a negatively charged anion.

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27
Q

What is homolytic fission?

A

In homolytic fission, the bond breaks evenly and each bonding atom receives one electron from the bonded pair. 2 electrically uncharged ‘radicals’ are formed.

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28
Q

What are radicals?

A

Radicals are particles that have an unpaired electron. Because of the unpaired electron, radicals are very reactive.

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29
Q

What does a curly arrow show?

A

The movement of an electron pair.

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30
Q

Why do halogen and alkane reactions require UV light to start?

A

Halogens react with alkanes in photochemical reactions and these reactions are started by light.

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31
Q

What forms when halogens and alkanes react together?

A

Forms haloalkanes.

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32
Q

When a hydrogen atom is substituted (replaced) by a chlorine, bromine etc, what is this reaction called?

A

Free-radical substitution reaction.

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33
Q

What are the 3 stages of a free radical substitution reaction mechanism?

A

Initiation, Propagation and Termination.

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34
Q

What happens in the initiation stage of a free radical substitution reaction mechanism?

A

> Free radicals are produced.

1) Sunlight provides enough energy to break the (example - Cl-Cl) bond - this is photodissociation.
2) The bond splits equally and each atom gets to keep one electron (homolytic fission). The atom becomes a highly reactive free radical.

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35
Q

What happens in the propagation stage of a free radical substitution reaction mechanism?

A

> Free radicals are used up and created in a chain reaction.

1) A radical attacks another molecule.
2) A new radical is formed and attacks another molecule.
3) The new radicals can continue to attack other molecules until all the molecules are wiped out.

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36
Q

What happens in the termination stage of a free radical substitution reaction mechanism?

A

> Free radicals are mopped up.

1) If 2 radicals join together, they make a stable molecule.
2) There are heaps of possible termination reactions.

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37
Q

Describe the reaction mechanism of chlorine and methane that react with a bit of a bang to form chloromethane.
CH4 + Cl2 ->(UV) CH3CI + HCI

A 
Initiation - 
CI2 ->(UV) 2Cl*
Propagation -
Cl* + CH4 ->  CH3* + HCI
CH3* + Cl2 -> CH3Cl + Cl*
Termination - 
Cl* + CH3* -> CH3Cl
CH3* + CH3* -> C2H6
Cl* + Cl* -> Cl2
38
Q

What is a problem with free radical substitution?

A

> If you’re trying to make a particular product, you don’t only get the desired product you’re after but a mixture of products.
This means you have to separate the desired product from the other unwanted by-products.

39
Q

Give a problem this reaction?

CH4 + Cl2 ->(UV) CH3CI + HCI

A

If there’s too much chlorine, some of the hydrogen atoms on the chloromethane molecule will be swapped over for Cl atoms making dichloromethane. This could continue making trichloromethane and tetrachloromethane.

40
Q

How can you solve the problem of there being too much chlorine in this reaction?
CH4 + Cl2 ->(UV) CH3CI + HCI

A

To have an excess of methane. This means there is a greater chance of a chlorine radical colliding only with a methane molecule and not a chloromethane molecule.

41
Q

What is another problem with free radical substitution structurally?

A

Free radical substitution can take place at any point along the carbon chain. So a mixture of isomers can be formed. (For example - propane reacting with chlorine it will produce a mixture of 1-chloropropane and 2-chloropropane.

42
Q

Do alkenes have saturated or unsaturated hydrocarbons?

A

Unsaturated, so have at least one C=C bond.

43
Q

What is the general formula for alkenes?

A

CnH2n

44
Q

What is a double bond made from?

A

A sigma bond and a pi bond.

45
Q

What is a sigma bond?

A

> Is formed when 2 orbitals overlap, in a straight line.

>This gives the highest electron density between the 2 nuclei. This is a single covalent bond.

46
Q

Why does a sigma bond having a high electron density mean that they are the strongest type of covalent bond?

A

The high electron density between the nuclei means there is strong electrostatic attraction between the nuclei and the shared pair of electrons. This means that sigma bonds have a high bond enthalpy and are the strongest type of covalent bond.

47
Q

What is a pi bond?

A

> A pi bond is formed by the sideways overlap of 2 adjacent p orbitals.
It’s got two parts to it, one ‘above’ and one ‘below’, this is because the p orbitals that overlap are dumb-bell shaped.

48
Q

Why do pi bonds have a relatively low bond enthalpy?

A

Pi bonds are much weaker than sigma bonds because the electron density is spread out above and below the nuclei. This means that the electrostatic attraction between the nuclei and the shared pair of electrons is weaker, so pi bonds have a relatively low bond enthalpy,

49
Q

Why are alkanes less reactive than alkenes?

A

Alkanes only contain C-C and C-H sigma bonds, which have a high bond enthalpy and so are difficult to break. The bonds are also non-polar so they don’t attract nucleophiles or electrophiles. This means alkanes don’t react easily.

50
Q

Why are alkenes more reactive than alkanes?

A

> The C=C bond contains both sigma and pi bonds.
The C=C bond contains 4 electrons so it has a high electron density and pi bond sticks out above and below the rest of the molecule. Makes it more likely to be attracted to electrophiles.
The low bond enthalpy of the pi bond also contributes to the reactivity of alkenes.
The double bond is so reactive, alkenes are handy starting points for making other organic compounds and for making petrochemicals.

51
Q

What is the structure of carbon atoms in a C=C bond and the atoms bonded to these carbons?

A

All lie in the same plane, they’re planar.

52
Q

What can single bonds do that double bonds cannot?

A

In C=C double bonds the atoms can’t rotate around them like they can around single bonds, in fact double bonds are fairly rigid and don’t bend much either.

53
Q

What causes alkenes to be able to form stereoisomers?

A

The restricted rotation around the C=C double bond.

54
Q

What are stereoisomers?

A

Have the same structural formula but a different arrangement in space.

55
Q

When do stereoisomers occur?

A

When the 2 double-bonded carbon atoms each have 2 different atoms or groups attached to them.

56
Q

What makes a Z isomer?

A

The Z isomer has the same groups either both above or below the double bond.

57
Q

What makes an E isomer?

A

The carbons in the double bond are attached to the same group on opposite sides of the double bond.

58
Q

What is the set of rules that help work out E/Z stereoisomerism?

A

Cahn-Ingold-Prelog (CIP) rules.

59
Q

How is higher priority worked out?

A

The atom attached to the carbon in the C=C bond with the higher atomic number is given the higher priority.

60
Q

What do you do if the atoms directly bonded to the carbon in the double bond are the same?

A

You have to look further along the chain and the next atom in the groups to work out which has the higher priority.

61
Q

What can E/Z isomerism be called if the carbon atoms have at least one group in common?

A

Cis-trans Isomers.

62
Q

What does cis mean?

A

Means the same groups are on the same side of the double bond.

63
Q

What does trans mean?

A

Means the same groups are on opposite sides of the double bond.

64
Q

What is a nucleophile?

A

An electron pair donor.

65
Q

What is an electrophile?

A

An electron pair acceptor.

66
Q

What happens in electrophilic addition to alkenes?

A

The alkene double bond opens up and atoms are added to the carbon atoms.

67
Q

Why does electrophilic addition happen?

A

Because the double bond has plenty of electrons and is easily attacked by electrophiles.

68
Q

Describe what kind of molecules are electrophiles?

A

They are usually a bit short of electrons, so are attracted to areas where there are lots of them about. Electrophiles include positively charged ions like H+ and NO2+, and polar molecules.

69
Q

What happens when you add hydrogen to a C=C bond?

A

Ethene will react with hydrogen gas in an addition reaction to produce ethane. It needs a nickel catalyst and a temperature of 50 degrees to occur.

70
Q

What forms when you react halogens with alkenes?

A

Dihaloalkanes. The halogens add across the double bond and each of the C atoms ends up bonded to one halogen atom, it’s an electrophilic addition.

71
Q

How can bromine water be used to test for carbon double bonds?

A

> When you shake an alkene with orange bromine water, the solution quickly decolourises.
This is because bromine is added across the double bond to form a colourless dibromoalkane.

72
Q

What conditions are needed for the steam hydration of alkenes?

A

> At 300 degrees and a pressure of 60-70atm

>The reaction needs a solid phosphoric acid catalyst.

73
Q

What forms when alkenes are hydrated with steam?

A

Alcohols, such as ethanol from ethene.

74
Q

Is the steam hydration of alkenes reaction reversible and how is it’s reaction yield?

A

The reaction is reversible and the reaction yield is low. However, you can recycle the unreacted alkene gas, making the overall yield much better.

75
Q

What does a curly arrow in electrophilic addition show?

A

The movement of a pair of electrons.

76
Q

How else are haloalkanes formed?

A

In an addition reaction between hydrogen halides and alkenes.

77
Q

How many products are formed when hydrogen halides react with unsymmetrical alkenes?

A

Two.

78
Q

What does the amount of each product in the reaction of hydrogen halides with unsymmetrical alkanes depend on?

A

The amount of each product depends on how stable the carbocation formed in the middle of the reaction is.

79
Q

What does the stability of carbocations depend on?

A

Carbocations with more alkyl groups are more stable because the alkyl groups feed electrons towards the positive charge making the electrons have a greater attraction. The more stable carbocation is much more likely form.

80
Q

What is the product that has a larger quantity called?

A

The major product.

81
Q

What does the Markownikoff’s rule state?

A

The major product from addition of a hydrogen halide to an unsymmetrical alkene is the one where hydrogen adds to the carbon with the most hydrogens already attached.

82
Q

What is it called when the double bonds in alkenes open up and join together to make long chains of polymers?

A

Addition polymerisation.

83
Q

What are the individual small alkanes in the repeat unit called?

A

Monomers.

84
Q

Whats a property of polymers that makes it both useful and is a problem?

A

They are unreactive -
>Good for everyday uses such as: The PTFE coating on pans doesn’t react with food and plastic windows don’t rot.
>Bad - Most polymers aren’t biodegradable, so they’re really difficult to dispose of.

85
Q

Give three ways of waste disposal?

A

Waste plastics can be buried (landfills) , reused or burned.

86
Q

Give two ways plastics can be reused?

A

> Some plastics (polypropene) can be recycled by melting and remoulding them.
Some plastics can be cracked into monomers and used as an organic feedstock to make more plastics or other chemicals.

87
Q

Why can burning plastics be dangerous?

A

It needs to be controlled to reduce toxic gases being produced. For example, polymers that contain chlorine (PVC) produce HCI when they’re burned, this has to be removed.

88
Q

How can toxic gases that are produced from the combustion of plastics be removed?

A

Waste gases from the combustion are passed through scrubbers which can neutralise gases such as HCI by allowing them to react with a base.

89
Q

What does it mean for a material to be biodegradable?

A

It can be broken down by environmental conditions or microorganisms (living things).

90
Q

What does it mean for a material to be photodegradable?

A

These are polymers that decompose when exposed to sunlight.

91
Q

Why are biodegradable polymers preferred?

A

> They decompose relatively quickly in the right conditions.
Biodegradable polymers can be made from renewable raw materials such as starch or oil fractions.
There are various potential uses.

AS Chemistry (6 decks)

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