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A-Level Chemistry > 8 - alkanes > Flashcards

8 - alkanes Flashcards

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
What is the general formula for alcohols
C~n H~(2n + 2) O
26
How are alcohols named?
You add the suffix "-anol" An alcohol with 4 carbons in the longest chain is: Butanol
27
How is numbering used in alcohols?
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
28
What is a haloalkane?
An alkane where a hydrogen is replaced with a halogen atom Multiple hydrogens can be replaced
29
How are haloalkanes named?
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
30
What are all the haloalkane prefixes?
Fluoro Chloro Bromo Iodo
31
How is numbering used in haloalkanes?
If there are more than 1 carbons in the base alkane
32
If there are 4 halogens on an organic, what is the prefix used before the halogen name?
Tetra E.g 1, 2, 3, 4 - tetrafluorobutane
33
What is an alkyl group?
When a hydrogen is replaced with a whole group similar to an alkane
34
What is the general formula for alkyl groups?
C~n H~(2n + 1)
35
How are the alkyl groups named?
Methyl - 1 carbon Ethyl - 2 carbons Propyl - 3 carbons Etc
36
How do alkyl groups fit into other organics?
A hydrogen is replaced with an entire alkyl group
37
If an alkyl group is present in an organic, how do you name it?
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
38
How is numbering used for alkyl groups?
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
39
What are the rules for separating words and numbers in an organic name?
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
What are cyclics?
Where the chain of carbons joins up into a ring
41
How do you denote a cyclic in the name?
You add the "cyclo" prefix
42
What are aliphatic alkanes?
What are aliphatic alkanes? | When the alkane is a chain which doesn't join up into a ring
43
What is an alicyclic alkane?
A cyclic alkane which is NOT AROMATIC
44
If there are multiple different groups added to the alkane How do you have to name it?
You have to write the different groups in alphabetical order Separate the different groups with hyphens
45
How do you draw a methyl group on a skeletal diagram?
Just a vertical line off the correct carbon That's it since the hydrogens are assumed to be off every remaining bond
46
What is the general formula of a cyclic?
C~n H~2n
47
What is are structural isomers?
Molecules with the same molecular formula but with different structural arrangement of atoms
48
How do you write branches (alkyl groups) in structural formulas?
Just put brackets around it and put it after the part it is bonded to
49
What are the 3 structural isomers of C~5 H~12? Give their names and structural formulas
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
Where can you attach the different alkyl groups
Never on carbon 1 Methyl on carbon 2 or higher Ethyl on carbon 3 or higher Propyl on carbon 4 or higher Etc
51
How do you draw alkyl groups in skeletal?
Draw a line (no letters) for methyl Draw 2 connected lines for ethyl Etc
52
What are the 5 structural isomers of C~6 H~14?
Hexane 2-methylpentane 3-methylpentane 2, 2-dimethylbutane 2, 3-dimethylbutane
53
What is the technique for finding all of the isomers?
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
What are the 2 structural isomers for C~3 H~7 Cl?
1-chloropropane 2-chloropropane
55
How are hydrocarbon molecules held together and why?
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
How are hydrocarbons obtained and why does this work?
Crude oil is fractionally distilled The fractions have different boiling points so the crude oil is spoilt into its constitue
57
How does length of chain affect boiling point and why?
For a longer carbon chain: Larger surface area More London forces Boiling points are higher
58
How do branches affect boiling point and why?
For more branches on chain: Molecules are held further away from each other (due to shapes) Less London forces Boiling point is lower
59
How reactive are hydrocarbons and why?
Not very reactive Only held together by London forces so not polar
60
What makes a hydrocarbon a cleaner fuel?
the chain is shorter
61
What are combustion equations?
Chemical equations that show a fuel (hydrocarbon) reacting with oxygen in combustion
62
What is unusual about combustion equations?
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
What is complete combustion?
When there is plenty of oxygen so carbon dioxide and water is produced
64
What is incomplete combustion?
where there is not enough oxygen to allow the fuel to react completely to produce carbon dioxide and water. Carbon monoxide is produced
65
What happens when oxygen is very limited?
Carbon itself is produced It's a black powder (soot)
66
What do you have to always remember in combustion of alcohols?
Don't forget that there is always one oxygen on the LHS when you try to balance it
67
What does fractional distillation tend to produce more of?
Longer chain molecules
68
Which molecules are desirable products of crude oil and why?
Short chain alkanes, cycloalkanes and short branched alkanes More efficient fuels
69
In what 2 ways can covalent bonds be broken?
Homolytic fission Heterolytic fission
70
Describe homolytic fission
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
How is homolytic fission shown in an equation?
H~3 C-CH~3 --> H~3 C• + •CH~3 The 2 products are radicals
72
Describe heterolytic fission
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
How do you write an equation for heterolytic fission?
``` H~3 C-Cl --> H~3 C+ + Cl- ```
74
How do you use curly arrows in bond fission equations?
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
What are addition reactions?
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
What is substitution reaction?
When one group of atoms is swapped with another group CH~3 Br + OH --> CH~4 O + Br Is an example of this
77
What are the 3 steps of radical substitution reactions?
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
Define homolytic fission
A chemical reaction in which the breaking of bonds yields radicals each having 1 unpaired electron
79
What are the 3 reasons for alkanes being unreactive?
C-C and C-H sigma bonds are very strong C-C bonds are non-polar C-H bonds are non-polar
80
What can NOT be a radical?
You can't have a hydrogen radical H•
81
What causes bond fission?
UV radiation In the initiation stage, UV radiation causes homolytic fission etc.
82
What does the ozone layer do?
Doesn't let through harmful UV-a rays But it does let through UV-b and UV-c
83
When is ozone formed?
When oxygen gas absorbs high energy UV radiation
84
Show how oxygen gas forms ozone
O~2 --> 2O• O~2 + O• O~3
85
What is the general equation for how ozone is removed?
O~3 + O --> 2O~2
86
What causes the ozone to be removed?
CFCs going up to the stratosphere from fridges NO• from lightning and aircraft engines
87
How long do CFCs take to reach the ozone?
25 years
88
What is the equation for CFCs?
CF~2 Cl~2
89
Show how CFCs remove ozone
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
How many ozone molecules can 1 CFC remove?
100,000
91
Show how NO• removes ozone
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
What are all these reactions called? Ones like O~3 + O --> 2O~2
Radical substitution reactions
93
What is the reaction from hexane to cyclohexane?
C~6 H~14 --> | C~6 H~12 + H~2
94
What is the main reaction mechanism in this topic?
Free radicals substitution The one with the initiation, propagation etc
95
what are radical substitution reactions?
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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