8 - alkanes Flashcards
What are hydrocarbons?
They are made of carbon and hydrogen ONLY
What are saturated hydrocarbons?
Hydrocarbons with only single bonds
What is a functional group?
The part of an organic compound responsible for its chemical reactions
What is a homologous series?
A series of organic molecules with the same functional group but with each successive member differing by CH~2
What are alkanes?
A homologous series with chains of carbon atoms bonded to hydrogen atoms
What is the general formula for alkanes?
C~n H~2n+2
What do alkanes always have to be?
Saturated hydrocarbons
How many bonds does carbon always make and why?
It makes 4 bonds
Because it’s in group 14 so it needs 4 more electrons for a full outer shell (4 bonded pairs)
What is a molecular formula?
It states which elements make up a substances and how many atoms of each element there is
What is a displayed formula?
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.
What is a structural formula?
A molecular formula that gives more detail about the arrangement of different atoms (doesn’t show bonds as lines)
What is the structural formula for C~3 H~8?
You see what is bonded to each successive carbon and split into these groups
CH~3 CH~2 CH~3 is the structural formula
If a structural formula has multiple of the same group next to each other, what can you do?
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
what is a skeletal formula?
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
What is the alkane functional group?
C-C
How do you show that an organic is an alkane through its name?
You add the suffix “ane”
What is the first naming rule?
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
What is an alkene?
An unsaturated hydrocarbon with one or more double bonds between carbons in a chain
What is the general formula for an alkene?
C~n H~2n
What is the alkene functional group?
C=C
How are numbers used in organic names?
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)
How are numbers used in alkenes?
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
What is an alcohol?
The same as an alkane
But one hydrogen is replaced by an OH
What is the alcohol functional group?
OH
What is the general formula for alcohols
C~n H~(2n + 2) O
How are alcohols named?
You add the suffix “-anol”
An alcohol with 4 carbons in the longest chain is:
Butanol
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
What is a haloalkane?
An alkane where a hydrogen is replaced with a halogen atom
Multiple hydrogens can be replaced
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
What are all the haloalkane prefixes?
Fluoro
Chloro
Bromo
Iodo
How is numbering used in haloalkanes?
If there are more than 1 carbons in the base alkane
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
What is an alkyl group?
When a hydrogen is replaced with a whole group similar to an alkane
What is the general formula for alkyl groups?
C~n H~(2n + 1)
How are the alkyl groups named?
Methyl - 1 carbon
Ethyl - 2 carbons
Propyl - 3 carbons
Etc
How do alkyl groups fit into other organics?
A hydrogen is replaced with an entire alkyl group
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
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
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
What are cyclics?
Where the chain of carbons joins up into a ring
How do you denote a cyclic in the name?
You add the “cyclo” prefix
What are aliphatic alkanes?
What are aliphatic alkanes?
When the alkane is a chain which doesn’t join up into a ring
What is an alicyclic alkane?
A cyclic alkane which is NOT AROMATIC
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
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
What is the general formula of a cyclic?
C~n H~2n
What is are structural isomers?
Molecules with the same molecular formula but with different structural arrangement of atoms
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
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
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
How do you draw alkyl groups in skeletal?
Draw a line (no letters) for methyl
Draw 2 connected lines for ethyl
Etc
What are the 5 structural isomers of C~6 H~14?
Hexane
2-methylpentane
3-methylpentane
2, 2-dimethylbutane
2, 3-dimethylbutane
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
What are the 2 structural isomers for C~3 H~7 Cl?
1-chloropropane
2-chloropropane
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
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
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
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
How reactive are hydrocarbons and why?
Not very reactive
Only held together by London forces so not polar
What makes a hydrocarbon a cleaner fuel?
the chain is shorter
What are combustion equations?
Chemical equations that show a fuel (hydrocarbon) reacting with oxygen in combustion
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
What is complete combustion?
When there is plenty of oxygen so carbon dioxide and water is produced
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
What happens when oxygen is very limited?
Carbon itself is produced
It’s a black powder (soot)
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
What does fractional distillation tend to produce more of?
Longer chain molecules
Which molecules are desirable products of crude oil and why?
Short chain alkanes, cycloalkanes and short branched alkanes
More efficient fuels
In what 2 ways can covalent bonds be broken?
Homolytic fission
Heterolytic fission
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
How is homolytic fission shown in an equation?
H~3 C-CH~3
–>
H~3 C• + •CH~3
The 2 products are radicals
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
How do you write an equation for heterolytic fission?
H~3 C-Cl --> H~3 C+ \+ Cl-
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
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
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
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)
Define homolytic fission
A chemical reaction in which the breaking of bonds yields radicals each having 1 unpaired electron
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
What can NOT be a radical?
You can’t have a hydrogen radical
H•
What causes bond fission?
UV radiation
In the initiation stage, UV radiation causes homolytic fission etc.
What does the ozone layer do?
Doesn’t let through harmful UV-a rays
But it does let through UV-b and UV-c
When is ozone formed?
When oxygen gas absorbs high energy UV radiation
Show how oxygen gas forms ozone
O~2 –> 2O•
O~2 + O• O~3
What is the general equation for how ozone is removed?
O~3 + O –> 2O~2
What causes the ozone to be removed?
CFCs going up to the stratosphere from fridges
NO• from lightning and aircraft engines
How long do CFCs take to reach the ozone?
25 years
What is the equation for CFCs?
CF~2 Cl~2
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
How many ozone molecules can 1 CFC remove?
100,000
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
What are all these reactions called?
Ones like O~3 + O –> 2O~2
Radical substitution reactions
What is the reaction from hexane to cyclohexane?
C~6 H~14 –>
C~6 H~12 + H~2
What is the main reaction mechanism in this topic?
Free radicals substitution
The one with the initiation, propagation etc
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.
