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A-Level Chemistry > 8 - alkanes > Flashcards

8 - alkanes Flashcards

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1
Q

What are hydrocarbons?

A

They are made of carbon and hydrogen ONLY

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2
Q

What are saturated hydrocarbons?

A

Hydrocarbons with only single bonds

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3
Q

What is a functional group?

A

The part of an organic compound responsible for its chemical reactions

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4
Q

What is a homologous series?

A

A series of organic molecules with the same functional group but with each successive member differing by CH~2

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5
Q

What are alkanes?

A

A homologous series with chains of carbon atoms bonded to hydrogen atoms

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6
Q

What is the general formula for alkanes?

A

C~n H~2n+2

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7
Q

What do alkanes always have to be?

A

Saturated hydrocarbons

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8
Q

How many bonds does carbon always make and why?

A

It makes 4 bonds

Because it’s in group 14 so it needs 4 more electrons for a full outer shell (4 bonded pairs)

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9
Q

What is a molecular formula?

A

It states which elements make up a substances and how many atoms of each element there is

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10
Q

What is a displayed formula?

A

A displayed formula is a type of structural formula and shows all the bonds in the molecule as individual lines, where each line represents a pair of shared electrons.

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11
Q

What is a structural formula?

A

A molecular formula that gives more detail about the arrangement of different atoms (doesn’t show bonds as lines)

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12
Q

What is the structural formula for C~3 H~8?

A

You see what is bonded to each successive carbon and split into these groups

CH~3 CH~2 CH~3 is the structural formula

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13
Q

If a structural formula has multiple of the same group next to each other, what can you do?

A

You can bracket one of them and write how many there are as a subscript to the bracket

E.g CH~3 CH~2 CH~2 CH~2 CH~3

There are 3 CH~2 in the middle so you can write:

CH~3 (CH~2)~3 CH~3

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14
Q

what is a skeletal formula?

A

a way to draw molecules where bonds are represented as lines and each vertex is a carbon with hydrogens attached to those carbons

Anything else except for carbon or hydrogen is drawn as a vertical line coming off the vertex with the written formula at the top

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15
Q

What is the alkane functional group?

A

C-C

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16
Q

How do you show that an organic is an alkane through its name?

A

You add the suffix “ane”

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17
Q

What is the first naming rule?

A

You have to count the number of carbons in the longest chain in the organic and use the prefix for that number

Meth - 1
Eth - 2
Prop - 3
But - 4
Pent - 5
Hex - 6
Hept - 7
Oct - 8
Non - 9
Dec - 10
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18
Q

What is an alkene?

A

An unsaturated hydrocarbon with one or more double bonds between carbons in a chain

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19
Q

What is the general formula for an alkene?

A

C~n H~2n

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20
Q

What is the alkene functional group?

A

C=C

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21
Q

How are numbers used in organic names?

A

A number represents the carbon which something is bonded to

You number the carbons from left to right or right to left (whichever one puts the smallest numbers in the name)

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22
Q

How are numbers used in alkenes?

A

You write which carbon has the double bond on it

If the double bond in butene came off the second carbon (and third)

The name would be but-2-ene

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23
Q

What is an alcohol?

A

The same as an alkane

But one hydrogen is replaced by an OH

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24
Q

What is the alcohol functional group?

A

OH

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25
Q

What is the general formula for alcohols

A

C~n H~(2n + 2) O

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26
Q

How are alcohols named?

A

You add the suffix “-anol”

An alcohol with 4 carbons in the longest chain is:
Butanol

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27
Q

How is numbering used in alcohols?

A

It denotes which carbon the OH group is bonded to

If the OH is bonded to the third carbon of pentanol

It would be called pentan-3-ol

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28
Q

What is a haloalkane?

A

An alkane where a hydrogen is replaced with a halogen atom

Multiple hydrogens can be replaced

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29
Q

How are haloalkanes named?

A

If one hydrogen has been swapped for a chlorine in methane it would be called chloromethane

If 2 hydrogens have been swapped for bromines it would be called dibromomethane

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30
Q

What are all the haloalkane prefixes?

A

Fluoro
Chloro
Bromo
Iodo

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31
Q

How is numbering used in haloalkanes?

A

If there are more than 1 carbons in the base alkane

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32
Q

If there are 4 halogens on an organic, what is the prefix used before the halogen name?

A

Tetra

E.g 1, 2, 3, 4 - tetrafluorobutane

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33
Q

What is an alkyl group?

A

When a hydrogen is replaced with a whole group similar to an alkane

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34
Q

What is the general formula for alkyl groups?

A

C~n H~(2n + 1)

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35
Q

How are the alkyl groups named?

A

Methyl - 1 carbon

Ethyl - 2 carbons

Propyl - 3 carbons

Etc

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36
Q

How do alkyl groups fit into other organics?

A

A hydrogen is replaced with an entire alkyl group

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37
Q

If an alkyl group is present in an organic, how do you name it?

A

The alkyl name is added as a prefix

E.g if a methane has an ethyl group instead of one of its hydrogens

It would be called ethylmethane

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38
Q

How is numbering used for alkyl groups?

A

You put the carbon number to which the alkyl is bonded before the alkyl

If there are 2 propyls on the second carbon of 4 carbon chain

It will be called:

2, 2 - propylbutane

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39
Q

What are the rules for separating words and numbers in an organic name?

A

A number and a word are always separated by a hyphen

2 numbers are separated by a comma

Words aren’t separated, they make a combined word

40
Q

What are cyclics?

A

Where the chain of carbons joins up into a ring

41
Q

How do you denote a cyclic in the name?

A

You add the “cyclo” prefix

42
Q

What are aliphatic alkanes?

A

What are aliphatic alkanes?

When the alkane is a chain which doesn’t join up into a ring

43
Q

What is an alicyclic alkane?

A

A cyclic alkane which is NOT AROMATIC

44
Q

If there are multiple different groups added to the alkane

How do you have to name it?

A

You have to write the different groups in alphabetical order

Separate the different groups with hyphens

45
Q

How do you draw a methyl group on a skeletal diagram?

A

Just a vertical line off the correct carbon

That’s it since the hydrogens are assumed to be off every remaining bond

46
Q

What is the general formula of a cyclic?

A

C~n H~2n

47
Q

What is are structural isomers?

A

Molecules with the same molecular formula but with different structural arrangement of atoms

48
Q

How do you write branches (alkyl groups) in structural formulas?

A

Just put brackets around it and put it after the part it is bonded to

49
Q

What are the 3 structural isomers of C~5 H~12?

Give their names and structural formulas

A

Pentane
CH~3 (CH~2)~3 CH~3

2-methylbutane
CH~3 CH(CH~3) CH~2 CH~3

2,2-dimethylpropane
C(CH~3)~4

50
Q

Where can you attach the different alkyl groups

A

Never on carbon 1

Methyl on carbon 2 or higher

Ethyl on carbon 3 or higher

Propyl on carbon 4 or higher

Etc

51
Q

How do you draw alkyl groups in skeletal?

A

Draw a line (no letters) for methyl

Draw 2 connected lines for ethyl

Etc

52
Q

What are the 5 structural isomers of C~6 H~14?

A

Hexane

2-methylpentane

3-methylpentane

2, 2-dimethylbutane

2, 3-dimethylbutane

53
Q

What is the technique for finding all of the isomers?

A

Always use skeletal diagrams to help you

Start with the straight chain

Then try it where you only add 1 methyl group in multiple ways

Then try with 2

Etc

54
Q

What are the 2 structural isomers for C~3 H~7 Cl?

A

1-chloropropane

2-chloropropane

55
Q

How are hydrocarbon molecules held together and why?

A

London forces only

There are no dipoles because C-H and C-C don’t have enough electronegativity difference

So hydrocarbons are always non-polar

56
Q

How are hydrocarbons obtained and why does this work?

A

Crude oil is fractionally distilled

The fractions have different boiling points so the crude oil is spoilt into its constitue

57
Q

How does length of chain affect boiling point and why?

A

For a longer carbon chain:

Larger surface area

More London forces

Boiling points are higher

58
Q

How do branches affect boiling point and why?

A

For more branches on chain:

Molecules are held further away from each other (due to shapes)

Less London forces

Boiling point is lower

59
Q

How reactive are hydrocarbons and why?

A

Not very reactive

Only held together by London forces so not polar

60
Q

What makes a hydrocarbon a cleaner fuel?

A

the chain is shorter

61
Q

What are combustion equations?

A

Chemical equations that show a fuel (hydrocarbon) reacting with oxygen in combustion

62
Q

What is unusual about combustion equations?

A

The big numbers which normally state the number of molecules actually relate to the amount of that molecule

So you can have decimal numbers like 3.5 before a molecule in an equation

63
Q

What is complete combustion?

A

When there is plenty of oxygen so carbon dioxide and water is produced

64
Q

What is incomplete combustion?

A

where there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water.

Carbon monoxide is produced

65
Q

What happens when oxygen is very limited?

A

Carbon itself is produced

It’s a black powder (soot)

66
Q

What do you have to always remember in combustion of alcohols?

A

Don’t forget that there is always one oxygen on the LHS when you try to balance it

67
Q

What does fractional distillation tend to produce more of?

A

Longer chain molecules

68
Q

Which molecules are desirable products of crude oil and why?

A

Short chain alkanes, cycloalkanes and short branched alkanes

More efficient fuels

69
Q

In what 2 ways can covalent bonds be broken?

A

Homolytic fission

Heterolytic fission

70
Q

Describe homolytic fission

A

Each of the bonded atoms takes one electron from the shared pair of electrons

Each of the 2 new groups of atoms are called radicals and have a single unpaired electron

71
Q

How is homolytic fission shown in an equation?

A

H~3 C-CH~3
–>
H~3 C• + •CH~3

The 2 products are radicals

72
Q

Describe heterolytic fission

A

One of the bonded atoms takes both of the electrons in the pair

The atom that takes the electrons (most electronegative) is now a negative ion and the atom that doesn’t take the electrons becomes a positive ion

73
Q

How do you write an equation for heterolytic fission?

A 
H~3 C-Cl 
-->
H~3 C+
\+
Cl-
74
Q

How do you use curly arrows in bond fission equations?

A

The arrow comes from the middle of the bond and goes up and curls over and down

It points at the atom which receives the electron pair

75
Q

What are addition reactions?

A

When a molecule is added to an unsaturated organic

The double bond is broken and the new molecule is bonded by a single bond

All bonds are now single and the compound is saturated

76
Q

What is substitution reaction?

A

When one group of atoms is swapped with another group

CH~3 Br + OH –> CH~4 O + Br

Is an example of this

77
Q

What are the 3 steps of radical substitution reactions?

A

Initiation (radicals are made)(both on RHS)

Propagation (radicals react with non-radicals)(one radical on either side)

Termination (radicals react with each other to form reaction products) (both on LHS)

78
Q

Define homolytic fission

A

A chemical reaction in which the breaking of bonds yields radicals each having 1 unpaired electron

79
Q

What are the 3 reasons for alkanes being unreactive?

A

C-C and C-H sigma bonds are very strong

C-C bonds are non-polar

C-H bonds are non-polar

80
Q

What can NOT be a radical?

A

You can’t have a hydrogen radical

H•

81
Q

What causes bond fission?

A

UV radiation

In the initiation stage, UV radiation causes homolytic fission etc.

82
Q

What does the ozone layer do?

A

Doesn’t let through harmful UV-a rays

But it does let through UV-b and UV-c

83
Q

When is ozone formed?

A

When oxygen gas absorbs high energy UV radiation

84
Q

Show how oxygen gas forms ozone

A

O~2 –> 2O•

O~2 + O• O~3

85
Q

What is the general equation for how ozone is removed?

A

O~3 + O –> 2O~2

86
Q

What causes the ozone to be removed?

A

CFCs going up to the stratosphere from fridges

NO• from lightning and aircraft engines

87
Q

How long do CFCs take to reach the ozone?

A

25 years

88
Q

What is the equation for CFCs?

A

CF~2 Cl~2

89
Q

Show how CFCs remove ozone

A

Initiation:
CF~2 Cl~2 –> Cl• + CF~2 Cl

Propagation:
Cl• + O~3 –> ClO• + O~2
ClO• + O –> Cl• + O~2

Overall:
O~3 + O –> 2O~2

90
Q

How many ozone molecules can 1 CFC remove?

A

100,000

91
Q

Show how NO• removes ozone

A

We don’t need to know initiation here

Propagation:
NO• + O~3 –> NO~2• + O~2

NO~2• + O –> NO• + O~2

Overall:
O~3 + O –> 2O~2

92
Q

What are all these reactions called?

Ones like O~3 + O –> 2O~2

A

Radical substitution reactions

93
Q

What is the reaction from hexane to cyclohexane?

A

C~6 H~14 –>

C~6 H~12 + H~2

94
Q

What is the main reaction mechanism in this topic?

A

Free radicals substitution

The one with the initiation, propagation etc

95
Q

what are radical substitution reactions?

A

a radical-substitution reaction is a substitution reaction involving free radicals as a reactive intermediate. The reaction always involves at least two steps, and possibly a third.

A-Level Chemistry (25 decks)

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