Form 5 Chemistry Notes – Ms. R. Buttigieg
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Topic 2 – Organic Chemistry
Organic Chemistry is the chemistry of compounds, which are found in living things or are
derived from living things.
The basis of these molecules is carbon chains or rings.
A Homologous series is a family of compounds.
E.g. Alkanes, alkenes, alkynes, alcohols, fatty acids.
Members of a series have:
1. Same general molecular formula, each member differing from the next by CH2.
2. Similar chemical reactions
3. All possess the same functional group
4. Physical properties of members change gradually in the same direction along the series.
e.g. melting points and boiling points and density rise as molecular mass increases.
Alkanes (CnH2n+1) – See GCSE pg. 175, C4U pg. 159
1. Alkanes are hydrocarbons.
2. They have only single bonds between carbon atoms.
3. b.p and m.p. and density increase with chain length
i. C1 to C4 are gases at room temperature
ii. C5 to C15 are liquids
iii. C16 upwards are solids
Make sure you can draw
Methane to decane
structures
4. They are volatile compared to ionic compounds.
E.g. tar with RMM>500 melts on hot days
While NaCl with RMM of 58.5 melts at 808oC
5. Alkanes are found in natural gas and crude oil.
6. Insoluble in water.
7. Soluble in organic solvents like benzene and tetrachloromethane (CCl4)
8. Very unreactive due to strong C-C and C-H bonds.
9. Do not react with metals, aqueous oxidising agents, acids or alkalis.
10. Burn. Combustion is very exothermic so used as fuels.
CH4 (g) + 2 O2 (g) CO2 (g) + 2H2O (l)
When there is little oxygen, C and CO form as well as CO2. CO is poisonous so a danger.
Form 5 Chemistry Notes – Ms. R. Buttigieg
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11. Alkanes react with halogens by substitution reactions.
This reaction is catalysed by light (U.V.) and is explosive in bright sunlight.
The reaction is slower with bromine.
Nothing happens with iodine.
12. Alkanes readily decompose when they are strongly heated, the higher members splitting more
readily. This is the basis of cracking. Cracking helps to produce more petrol. High temperatures
are used and a catalyst, finely divided aluminium oxide and silicon (IV) oxide speed up the reaction.
Form 5 Chemistry Notes – Ms. R. Buttigieg
The products of crude oil, which are produced by a
specific number of carbon atoms in a hydrogen chain at
a particular temperature, are given in this table:
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Form 5 Chemistry Notes – Ms. R. Buttigieg
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Answer the following:
1. Which one of the following compounds is not a hydrocarbon?
C2H4
C2H6O
C6H6
C4H10
2. Which one of the compounds below isn’t an alkane?
C6H12
C7H16
C10H22
C40H82
3. Write down a use for each of the following alkanes
a. methane: ______________________________________________________________________
b. propane: ______________________________________________________________________
c. octane: _______________________________________________________________________
4. Write down the equations for burning of:
a. methane: ______________________________________________________________________
b. propane: ______________________________________________________________________
Saturated Compounds – contain only single bonds between carbon atoms. So alkanes are
saturated hydrocarbons.
Compounds which have double or triple bonds between carbon atoms are said to be unsaturated.
E.g. alkenes and alkynes. They are called saturated as more atoms can be added to the molecules.
Isomers – compounds which have the same molecular formula but a different structural formula.
E.g.
Butane
Isobutane
Form 5 Chemistry Notes – Ms. R. Buttigieg
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ETHENE
A colourless gas
Insoluble in water.
It has a faint sweet smell,
About the same density as air
Burns with a yellow flame (luminous flame)
Decolourises bromine water and acidified potassium permanganate.
Produced industrially by cracking heavier fractions from crude oil.
Laboratory preparation of ethene
Ethene is produced by the dehydration of ethanol by hot, concentrated sulphuric acid.
100cm3 of concentrated sulphuric acid (excess is used) are mixed with 50cm3 of ethanol.
This is done slowly and while cooling under running cold water as when the sulphuric acid is mixed
with ethanol, a lot of heat is given out.
Then some sand (acid washed) is placed in the flask so that the ethene forms on the sand and so
avoid frothing.
The contents are then heated on a sand-bath to about 180oC.
The ethene given off is passed through sodium hydroxide solution to remove impurities (sulphur
dioxide and carbon dioxide) and collected over water.
The liquid darkens in the flask due to carbon formed by charring.
C2H5OH (aq) + H2SO4 (aq) C2H5HSO4 (aq) + H2O (l)
C2H5OH - H2O C2H4
Form 5 Chemistry Notes – Ms. R. Buttigieg
JL Annual 2006 - Section 1
Pg. 25
Form 5 Chemistry Notes – Ms. R. Buttigieg
Pg. 26
Properties of Alkenes (taking ethene as example)
1. More reactive than alkanes – due to unstable Carbon-Carbon double bond.
2. Undergo addition reactions – due to their double bond.
a. With Hydrogen
A nickel catalyst at 150oC.
Called Sabatier’s Reaction
Used in hardening oils to make margerine
Oils are solidified by reacting them with hydrogen.
As their RMM
increases they solidify.
This is also called a hydrogenation reaction as hydrogen is added.
Ethene
Hydrogen
Ethane
b. With water - e.g. Ethene
With water ethanol is formed.
This is the industrial preparation of ethanol.
Takes place at 300oC, 60 atm and over a catalysts of phosphoric acid.
Ethene
water
ethanol
3. Ethene is oxidised with acidified or alkaline potassium permanganate
C2H4 + H2O + [O] CH2OH. CH2OH
Ethan-1,2-diol is used as a coolant in aircraft engines, an antifreeze solution for motor car radiators
and in the production of terylene.
4. Ethene burns. C2H4 (g) + 3 O2 (g) 2 CO2 (g) + 2 H2O (g)
Form 5 Chemistry Notes – Ms. R. Buttigieg
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5. Ethene can undergo polymerisation. This happens when molecules of ethene add on to each
other to form giant molecules of poly-ethene or polythene.
Polythene is a white solid.
High temperatures and pressure are needed and a catalyst (trace of oxygen)
6. Ethene is used: to make plastics (polythene, terylene, polystyrene, PVC), ethanol (in methylated
spirits), and 1,2-dibromoethane (an important additive in petrol).
JL2006 annual
Form 5 Chemistry Notes – Ms. R. Buttigieg
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Alkynes
These are hydrocarbons which have a triple
bond between 2 of the carbon atoms.
Physical properties
1. They are insoluble in water.
2. They are quite soluble in the usual organic solvents of low polarity (e.g. ligroin, ether, benzene,
carbon tetrachloride, etc.).
3. They are less dense than water.
4. Their boiling points show the usual increase with increasing carbon number.
5. They are very nearly the same as the boiling points of alkanes or alkenes with the same carbon
skeletons.
Chemical properties
They burn in air to produce carbon dioxide and water . The ethynes are highly dangerously
explosives when mixed with air or oxygen.
They undergo addition reactions.
Naming alkynes
Number of Carbons
Chemical formula
Structural formula
Ethyne
Propyne
Butyne
The alkynes are said to be unsaturated because of the existence of a multiple bond in the
molecule. The general structure of the alkyne series of hydrocarbons is CnH2n-2.
Form 5 Chemistry Notes – Ms. R. Buttigieg
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1. Copy and complete the following on the HW copybook.
Hydrocarbons are compounds containing the elements _______________ and ______________. If
they are saturated, the atoms of carbons are bound together by _________________ bonds. On the
other hand, unsaturated hydrocarbons include at least one _______________ or _______________
bond. One example of unsaturated hydrocarbons is _________________. This compound burns in
a plentiful supply of air to produce ________________________ and _________________. It also
reacts with bromine water changing the colour of the solution from _________________ to
_________________. (5 marks)
2. The statements below describe the chemistry of carbon and some of its compounds. Write brief
notes to show that you understand the meaning of these statements with particular reference to the
underlined terms.
3. This question concerns the laboratory preparation of ethene and its properties.
a) Ethene can be prepared in the laboratory by dehydration of ethanol. Explain this statement,
including essential conditions and an equation for the reaction. (A diagram is not required). (6
marks)
b) Ethene undergoes a number of addition reactions such as halogenation, hydration and
hydrogenation. For each of these addition reactions,
(i) name the substance that adds on the ethene,
(ii) give an equation for the reaction. (9 marks)
c) Ethene burns with a smoky flame. Give a reason for this observation and write an equation for
the reaction. (3 marks)
d) Give two uses of ethene. (2 marks)
Form 5 Chemistry Notes – Ms. R. Buttigieg
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Alcohols (see Chemistry for You pg. 178 – 182, GCSE pg. 203)
These contain the –OH group.
The OH group – hydroxyl group does not form hydroxide ions
in solution.
Alcohols are NOT alkaline.
Naming alcohols
Number of Carbons
Chemical formula
Structural formula
Methanol
Ethanol
Propanol
Alcoholic drinks contain ethanol. That’s what makes them alcoholic.
So beer, wine and whisky all contain ethanol.
If alcohol is taken in excess it:
Slows down our reactions
Makes us overconfident
Damages the liver
Is addictive
Laboratory Preparation of Ethanol - by fermentation
Sugar is dissolved in water. Some yeast is added. The mixture is put in a conical flask. A delivery
tube leads to a test-tube containing calcium hydroxide solution (lime-water). The flask is left in a
warm place for several days.
The contents in the flask begin to froth. A gas is given out. When it is bubbled through the calcium
hydroxide solution, this turns milky, indicating it is carbon dioxide.
C12H22O11 (s) + H2O (l) 2 C6H12O6 (aq)
sugar
glucose
C6H12O6 (aq) 2 C2H5OH (aq) + 2 CO2 (g)
Form 5 Chemistry Notes – Ms. R. Buttigieg
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These two reactions happen through the action of yeast. After several days the contents are found
to contain between 5 and 10% ethanol.
Pure ethanol can be obtained by fractional distillation.
Ethanol can be tested by burning as it burns easily with a pale yellow flame.
Industrial Preparation of Ethanol
Ethene is reacted with steam to form ethanol.
C2H4 (g) + H2O (l) C2H5OH (aq)
o
Temp: 300 C
Pressure: 65 atm
Catalyst: Phosphoric Acid
Properties of Ethanol
1. Miscible with water
2. Colourless liquid (b.p. 78oC)
3. Very good solvent for molecular substances
4. Burns very easily with a pale yellow flame
C2H5OH (l) + 3 O2 (g) 2 CO2 (g) + 3 H2O (l)
5. Ethanol reacts with sodium. Hydrogen gas is given off.
2 C2H5OH (l) + Na (s) 2 C2H5ONa (aq) + H2 (g)
If the excess ethanol is evaporated, sodium ethoxide is obtained as a white solid. Sodium ethoxide
is ionic: C2H5O- Na+
6. Ethanol reacts with phosphorus pentachloride, PCl5
C2H5OH (l) + PCl5 (l) C2H5Cl (g) + POCl3 (l) + HCl (g)
It is a vigorous reaction in which steamy fumes of hydrogen chloride are formed.
7. Ethanol reacts with concentrated sulphuric acid that acts as a dehydrating agent.
a. If it is in excess at a temperature of 180oC
C2H5OH - H2O C2H4
b. If it is in excess at a temperature of 145oC only 1 molecule of water is extracted from
2 molecules of ethanol.
2 C2H5OH - H2O C2H5OC2H5
ethoxyethanoate
Work these out on your copybook - Chemistry For You
Pg. 185 numbers 1, 2
Pg. 254 numbers 11, 14, 15; Pg. 255 numbers 16, 19
Form 5 Chemistry Notes – Ms. R. Buttigieg
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8. Ethanol reacts with ethanoic acid to form an ESTER
C2H5OH (l) + CH3COOH (l)
CH3COOC2H5 (l) + H2O (l)
To make an ester, we use:
4 cm3 of ethanol
2 cm2 ethanoic acid,
some concentrated sulphuric acid
some antibumping granules.
The contents are heated under reflux in a water bath for 25 minutes, then
poured in a brine solution. Oily drops are seen on the surface smelling of
pear drops. This is the ester ethyl ethanoate.
9. Ethanol can be oxidised to ethanoic acid, if we use acidified potassium dichromate as an
oxidising agent.
The ethanol is oxidised to ethanoic acid.
C2H5OH (l) + 2 [O] CH3COOH (l) + H2O (l)
The orange potassium dichromate turns to green.
The experiment is performed in the following way:
Mix ethanol with acidified potassium dichromate.
Heat under reflux for 15 minutes.
Fractionally distillate the contents to obtain ethanoic acid.
Wine goes sour as the ethanol in it is oxidised to ethanoic acid (vinegar). The oxidation is
catalysed by enzymes in the wine, using oxygen from the air
Esters have a pleasant fruity smell. They are found in fruit, flowers and are used in perfumes.
Fats are complex esters.
See GCSE Chemistry, page 208, 209
pg. 208 no. 2
Work out
Pg. 276 no. 76
Pg. 277 no. 78, 80
Form 5 Chemistry Notes – Ms. R. Buttigieg
Number of Carbons
Pg. 33
Alkanes
Alkenes
Alkynes
1
2
3
4
Organic Acids
An organic compound is an acid if it has the –COOH group.
This –COOH is called a carboxyl group.
Acid
Chemical formula
Chemical Structure
Ethanoic acid
Propanoic Acid.
Properties
1. These acids are weak acids. i.e. they are not fully
dissociated in solution.
CH3COOH (aq)
CH3COO- (aq) + H+ (aq)
2. When pure they are not acidic, since it is in water that they
break up into ions.
3. Ethanoic acid is a weak electrolyte. Like all acids, it reacts with metals, alkalis and carbonates.
2 CH3COOH (aq) + Ca (s) (CH3COO)2 (aq) + H2 (g)
CH3COOH (aq) + NaOH (aq) CH3COONa (aq) + H2O (g)
2 CH3COOH (aq) + Na2CO3 (s) 2CH3COONa (aq) + CO2 (g) + H2O (g)
Laboratory preparation of ethanoic Acid – see previous page – oxidation of ethanol
Form 5 Chemistry Notes – Ms. R. Buttigieg
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Synthetic Polymers
These can be either addition or condensation polymers.
Addition Polymers
Polystyrene – poly(phenyl ethene)
Perspex
Note: The above are addition polymers. Units are added together as the monomers are unsaturated.
The polymer than gives a saturated product.
Condensation Polymers
Nylon
Terylene
Note: i. Formation of the above polymers requires two types of monomer molecules, each with
two reactive groups. ii. Each time a bond is formed, a small molecule such as water is removed.
Form 5 Chemistry Notes – Ms. R. Buttigieg
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Answer the following questions:
1. The following represent two monomers, which react together to form a polymer.
H2N -
- NH2
COOH -
- COOH
a. Name the type of polymerisation ______________________________
b. What is the substance lost during polymerisation ________________________
c. Draw a diagram to show the structure of the polymer formed from, at least three monomers.
d. Give the name of one example of this type of polymer. ______________________
e. Another type of polymer, poly(chloroethene), is formed by the polymerisation of chloroethene.
The following diagram represents part of the poly(chloroethene chain).
- CH2 – CH – CH2 – CH – CH2 – CH – CH2 – CH –
|
|
|
|
Cl
Cl
Cl
Cl
Draw a diagram to represent a single molecule of chloroethane
JL Annual 2001 – Past paper questions related to organic chemistry
2
Form 5 Chemistry Notes – Ms. R. Buttigieg
JL Annual 2002 – Past paper questions related to organic chemistry
3
JL Annual 2001 – Past paper questions related to organic chemistry
4
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