ALKENES (14-16 GCSE
level notes)
structure & chemical properties
5. ALKENES – unsaturated hydrocarbons –
their structure and chemical reactions e.g. addition of bromine and
polymerisation
All my
GCSE level chemistry revision
notes
All my GCSE level oil and organic chemistry revision
notes
All my advanced A level organic chemistry notes
Advanced A level organic chemistry revision notes on
alkenes
email doc
brown - comments - query?
Use your
mobile phone or ipad etc. in 'landscape'
orientation
This is a BIG
website, you need to take time to explore it [ SEARCH
BOX]
Sub-index
for this page
5a.
The structure and
names of alkenes (unsaturated hydrocarbons)
5b.
Examples of the molecular formula and
molecular structure of alkenes - various representations
5c.
The covalent bonding diagram for
the alkene
ethene
5d.
A laboratory demonstration of cracking a saturated
hydrocarbon into alkenes
5e.
Explaining the addition reactions of alkenes
5f.
The addition of
bromine - used as a simple test for alkenes
5g.
The addition of
hydrogen to form a saturated alkane
5h.
The addition of water to alkenes to make alcohols
5i.
Summary of alkene
chemistry and a few extra points
Also
For
details of polymer formation –
the polymerisation of alkenes See Part 7
Revision notes on alkenes in
chemistry, physical properties of alkenes, uses of alkenes, chemical reactions of
alkenes, help when revising for AQA
GCSE chemistry, Edexcel GCSE chemistry, OCR GCSE gateway science chemistry,
OCR 21st century science chemistry GCSE 9-1 chemistry examinations.
5a.
The structure and names of the ALKENE
hydrocarbons (unsaturated)
Examples of ALKENES
The lower
displayed formulae
are a more accurate representation of the structure of the alkene molecules
-
Alkenes
are
a family of hydrocarbons containing at least one covalent carbon...carbon double bond (>C=C<)
as well as the single bonds formed between carbon and carbon (C-C) and
carbon and hydrogen (C-H).
-
The C=C double bond is called
the functional group of the homologous series we call
alkenes.
-
Note that the name of all
alkene hydrocarbons ends in
...ene eg , ethene, propene, butene etc.
-
All alkenes have single C–H
bonds, and from propane onwards, C–C single bonds as well as the
characteristic functional group C=C bond.
-
Reminder: A hydrocarbon, e.g.
an alkene, can only consist of carbon and hydrogen atoms.
-
Alkenes
are another homologous series of organic compounds
with the general formula CnH2n where n = 2, 3, 4
etc. giving the formulae C2H4, C3H6,
C4H8 etc.
-
A
homologous series is a family of
compounds which have a general formula (CnH2n
for alkenes) and have similar chemical
properties because they have the same functional group.
-
In the general formula n =
number of carbon atoms in the alkane molecule (n = 2, 3, 4 etc.) and
from the general formula you can deduce the number of hydrogen atoms,
hence the complete molecular formula for ANY alkene with a carbon chain
containing one double bond only.
-
This general formula only
applies to hydrocarbon alkenes with one C=C double bond.
-
The carbon - carbon double
bond (>C=C<) is referred to as the functional group of the
alkenes.
-
The functional group is a
group atoms common to all members of a homologous series that confer a
particular set of characteristic chemical reactions on each member of
the series.
-
This means you can not only
predict the formula of an alkene, but you can also predict their
possible chemical reactions and the outcome i.e. the products.
-
The three alkene formula quoted
above match the three names above.
-
As with naming all organic
molecule series in alkenes eth.. means 2 carbon atoms in the chain,
prop... means 3 and but.. means 4 etc.
-
In the general formula, n =
number of carbon atoms in the alkene molecule (n = 2, 3, 4 etc.) and
from the general formula you can deduce the number of hydrogen atoms,
hence the complete molecular formula for ANY alkene.
-
See section 8. or more on
homologous series and the variety of organic
compounds
-
Alkenes are called
unsaturated molecules because two atoms can join
onto half of the carbon = carbon double bond when it opens up. In other
words they potentially have spare bonds to link up with other atoms and
alkanes cannot do this.
-
The word unsaturation
implies the fact that the carbon atoms are
not bonded to the maximum number of atoms they can be.
-
Alkenes have two fewer hydrogen atoms
than alkanes.
-
Alkanes are described as saturated
because they have no C=C double bond and atoms cannot add to them via a
double bond, like
in the bromine water test described below for alkenes.
-
ie 2 atoms can join onto the
two atoms of the carbon–carbon double bond in alkenes.
TOP OF PAGE
and sub-index
5b. Examples of the molecular formula
and
molecular structure of ALKENES in various styles
-
The first three in the
alkenes series are shown
in the section
below and are colourless smelly gases.
-
Examples of alkene
structure
-
(1) is the molecular formula: a
summary of the totals of
each atoms of each element in one molecule
-
(2)
is are 'shorthand' or 'condensed' versions of the full structural formula or
displayed formula (3)
-
(3)
is called the full structural formula or displayed formula:
-
The displayed/structural
formula shows how all the atoms are
linked with the covalent bonds (the dashes –) ie the C–C bonds and the C–H
atom bonds.
-
Note that carbon must
form four bonds (C–C single bond or a C=C double bond) and hydrogen forms one bond (C–H).
-
It is the presence of
the covalent carbon = carbon double bond (C=C) which makes alkenes
unsaturated hydrocarbon molecules.
-
More notes on
molecules and covalent bonding
|
(1) ,
(2) ,
(3)
the C=C is referred to as
the carbon – carbon 'double bond' |
ethene
|
(1) ,
(2a) , (2b) ,
(3)
|
propene
|
(1) , (2)
and
(butene
has 2
possible structures)
better shown as but-1-ene
and
but-2-ene
These two different molecular structures for the same molecular
formula are called isomers. |
butene
|
(1)
,
(2)
|
pentene |
The full
displayed formula for the first four members of the homologous series of
ALKENES
These
diagrams show ALL the covalent bonds (single C-H, C-C and double C=C) in alkene
molecules
The formulae can also be written as:
CH2=CH2, CH3CH=CH2, CH3CH2CH=CH2
and (i) CH3CH2CH2CH=CH2
Note: Just like butene above, there is another molecule
of pentene where the double bond is in another alternative position in the
carbon chain: (ii)
CH3CH2CH=CHCH3
that is, as well as (i)
there
is also (ii)
Like with butene, this is another example of what we call
isomers, molecules of different structure with the same molecular formula,
in this case C5H10. You might not need to know
the word isomer, but you may need to know these different structures exist,
and, be able to spot them with a spot of atom counting! |
another
form of C5H10 and below, two examples of C6H12
and
The point I'm making here is that they all have the carbon
- carbon double bond
(>C=C<) somewhere in the molecule making them an alkene molecule!
TOP OF PAGE
and sub-index
|
other alkenes
names not important |
5c. The covalent bonding diagram for
the alkene ETHENE
Two atoms of carbon (2.4) combine with four atoms of hydrogen (1) to form ethene C2H4
(only the outer shell of carbon's electrons are shown). So two electrons from
each carbon atom are shared to form the carbon = carbon double bond. The other
carbon electrons pair up with a hydrogen electron to make the single carbon -
hydrogen bonds.
simplified 'dot and cross' electronic diagram for the covalently bonded
ethene molecule
Electronically, hydrogen (1
outer electron)
becomes like helium (2 outer electrons, full outer shell) and carbon (2.4, 4
outer electrons) becomes like neon (2.8, full outer shell of 8 electrons), so ALL the
hydrogen and carbon atoms effectively have full outer shells in forming the
covalent bonds when the atoms share their outer electrons.
With only four hydrogen atoms in the
ethene molecule, two carbon atoms must share four electrons to form a double
covalent bond (C=C).
The molecule can be shown as with one carbon = carbon double bond and four carbon
– hydrogen single covalent bonds.
is
the full 'dot and x' electronic diagram for ethene.
|
TOP OF PAGE
and sub-index
5d.
A laboratory demonstration of cracking a saturated hydrocarbon into alkenes
-
Alkenes are extremely reactive and important compounds in the chemical
industry and are converted into very useful compounds e.g. plastics and
alcohols.
-
You can demonstrate cracking
in the laboratory by heating paraffin grease over an aluminium oxide
catalyst at 400–700oC, and collecting the smaller gaseous
hydrocarbon molecules over water – easily shown to be flammable!
-
This experiment needs to be
done as a teacher demonstration – most carefully!
-
Any hydrocarbon liquids
collected in the bottle or gases in the inverted test tube should decolourise bromine water – the test for
unsaturated alkene molecules.
-
The main products of cracking are smaller molecules of
the larger alkane and can be saturated alkanes or unsaturated alkenes.
-
For more on cracking and its
importance see
CRACKING - a problem of supply and demand
TOP OF PAGE
and sub-index
5e. Explaining the addition reactions of alkenes
IMPORTANT comparison: Unsaturated alkenes are much more
reactive than alkanes because of the reactivity of the carbon = carbon double
bond (saturated alkanes have no double bonds).
In these addition reactions unsaturated alkenes form a new
bonds either side of the original double bond and therefore form a saturated
compound (no longer has a C=C double bond)
You can think of the double bond as half opening up and a
bond breaking in the added molecule and two new bonds form e.g.
Once two atoms are joined by the two new single
covalent bonds, the double C=C bond no longer exists, it would then be described
as a saturated
molecule - cannot add any more atoms to it.
Three addition reactions of alkenes are described
next
TOP OF PAGE
and sub-index
|
5f. Reaction 1.
Addition of bromine - used as a test for alkenes
A
test to distinguish between ALKANE and ALKENE
hydrocarbons |
|
What is a simple test for an alkene?
Hydrocarbons are colourless.
Bromine
dissolved in water or trichloroethane solvent forms an orange
(yellow/brown) solution.
When orange bromine solution (bromine
water) is added to both an alkane or an alkene the
result is quite different.
The alkane solution remains orange
– no
reaction.
However, the alkene decolourises the bromine as it forms
a colourless dibromo–alkane compound – see the word and balanced symbol equations below. |
ethene + bromine ====> 1,2–dibromoethane
 .... or
CH2=CH2 + Br2
====> Br–CH2CH2–Br or better
colour of mixture changes from orange to
colourless
|
Alkenes are
unsaturated molecules, atoms can add
to them via the C=C double bond, so a
reaction occurs.
The double bond opens up and
new carbon – bromine bonds (C–Br) are formed.
This double bond makes
alkenes much more reactive than alkanes, the bromine water test for
alkenes is just one example.
Alkanes are saturated –
no double bond – and atoms cannot add –
so no reaction. |
propene + bromine ====> 1,2–dibromopropane
....
or
CH3CH=CH2 + Br2
====> CH3–CHBr–CH2Br
or
better
|
2nd example of bromine addition to a
double bond giving a saturated dibromo compound.
The decolourisation of
bromine is a simple and effective chemical test for an alkene – an
unsaturated hydrocarbon. The same reaction happens with chlorine (just but Cl
instead of Br)
This reaction
is NOT given by alkanes because they
do NOT have a carbon = carbon double bond. |
a butene + bromine ====> a dibromobutane
+ Br2 ====>

+ Br2 ====>
 |
The addition of bromine to the two butenes giving two
slightly different dibromobutanes. Note again you have gone from an
unsaturated alkene (can add atoms to it) to a saturated derivative of
an alkane (cannot add atoms to it) |
5g.
Reaction 2. Alkenes can add hydrogen
to form a saturated alkane molecule |
CH3–CH=CH2 + H2
====> CH3–CH2–CH3
H2

propene + hydrogen ===> propane |
|
5h.
Reaction 3.
The addition of water to alkenes to make alcohols |
ethene +water ===> ethanol
CH2=CH2 + H2O
===> CH3–CH2–OH
+ H2O
===>
TOP OF PAGE
and sub-index
|
eg the alcohol ethanol can be made by
passing ethene gas and water vapour (steam) over an acid catalyst at
300oC. The higher boiling ethanol and unreacted water can be
condensed out as liquids and very low boiling ethene gas recycled through
the reactor and the ethanol purified by fractional distillation.
This is an example of
an addition reaction and a hydration reaction because it
involves the addition of water to another molecule.
see Alcohols,
Ethanol, manufacture for more details of the process |
5i. More on ALKENES
– unsaturated hydrocarbons
– and quick summary too
- Alkenes readily burn, just like
alkanes, to give carbon dioxide and water if combustion is complete
e.g.
- complete oxidation = complete combustion
- alkene hydrocarbon + oxygen ===> carbon dioxide +
water
- ethene + oxygen ====> carbon dioxide +
water
-
C2H4 + 3O2
====> 2CO2 + 2H2O
- propene + oxygen ====> carbon dioxide + water
-
C3H6 + 41/2O2
====> 3CO2 + 3H2O
- or
2C3H6 +9O2
====> 6CO2 + 6H2O
- However, they are NOT used as fuels for
two reasons.
- They are far too valuable for use to
make plastics, anti–freeze and numerous other useful compounds.
- They burn with a more smoky flame
than alkanes due to less efficient, and more polluting incomplete combustion,
so the heat energy release is lower than for alkanes.
- Free unburned carbon might be released or only
half-burned to carbon monoxide.
- The hydrogen will always be oxidised to
water.
- So, for example, pentene might partially burn as
follows ..
- C5H10 + 5O2
===> 2C + CO +
2CO2 + 5H2O
- ... giving two complete combustion products
and two incomplete combustion products,
smokey!
- Alkenes are isomeric with cycloalkanes
- Isomers have the same molecular formula, but have different molecular
structures.
- e.g. the molecular formula C6H12
can represent hexene or cyclohexane
- hexene CH3–CH2–CH2–CH2–CH=CH2
or cyclohexane
- and note that ....
- hexene is an unsaturated
hydrocarbon, a typical alkene with a double bond, and, readily
decolourises bromine water
- the isomeric cyclohexane does not
have a double bond and is a saturated hydrocarbon, and does not
react with bromine water.
-
Test to distinguish an alkane from an alkene
- so a simple bromine test could
distinguish the two similar colourless liquids,
- because only the hexene would
decolorize the bromine water test reagent.
INDEX of Advanced A Level revision notes on the chemistry ALKENES including reactions and polymers
Multiple Choice Quizzes and Worksheets
KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products
(easier–foundation–level)
KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products
(harder–higher–level)
KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry
and
3 linked easy Oil Products gap–fill quiz worksheets
ALSO gap–fill ('word–fill') exercises
originally written for ...
... AQA GCSE Science
Useful products from
crude oil AND
Oil, Hydrocarbons
& Cracking
etc.
... OCR 21st C GCSE Science
Worksheet gap–fill C1.1c Air
pollutants etc ...
... Edexcel GCSE Science
Crude Oil and its Fractional distillation
etc ...
... each set are interlinked,
so clicking on one leads to a sequence of several quizzes
[SEARCH
MY WEBSITE]
|
revision study notes for 14-16 school chemistry AQA Edexcel OCR IGCSE/GCSE
9-1 chemistry science topics modules for studying the chemistry of
alkenes, unsaturated hydrocarbons that undergo addition reactions, molecular
structure of alkenes e.g. ethene C2H4, propene C3H6, butene C4H8, chemical
reaction, describe some uses of alkenes, reactions of alkenes with bromine
hydrogen, online fashion brands,
Abercrombie & Fitch, Old Navy, 14-16 gcse organic chemistry, Free People, Rue 21, Pacsun, Ralph Lauren,
Gini & Jony, United Colors of Benetton, 14-16 gcse organic
14-16
gcse organic chemistry,
INDEX of Advanced A Level revision notes on the chemistry ALKENES including reactions and polymers
Website content © Dr
Phil Brown 2000+. All copyrights reserved on revision notes, images,
quizzes, worksheets etc. Copying of website material is NOT
permitted.
Exam revision summaries & references to science course specifications
are unofficial.
Doc Brown's revision notes on
organic chemistry for UK GCSE level and US grade 9 and grade 10
examinations
INDEX of GCSE level notes on Organic
Chemistry |
|