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AQA Organic Chemistry
Unit 2
AS Summary
Formulae of organic molecules
• Empirical formulae – the simplest whole
number ratio of atoms of each element found
in the compound. Often deduced form
combustion analysis data
• Molecular formulae – the actual number of
atoms of each element found in a molecule
Formulae of organic molecules
• Structural formulae – the minimal detail that shows
the arrangement of atoms, e.g. CH3CH3 for ethane
• Displayed formulae – shows the relative position of
all atoms and bonds between them, e.g. ethene
• Skeleton formulae – the simplest representation of
organic molecules, removing all hydrogen atoms,
leaving only the outline of the carbon skeleton and
adding only any associated functional groups,
e.g.1-chlorobutane
Nomenclature and Isomerism
• Homologous series – compounds that
differ by a CH2 unit, sharing the same
functional group and exhibiting trends in
their properties
• Know the first ten alkanes – methane to
decane
• Functional group – (group of) atoms that
are chemically reactive in molecules
Nomenclature
• Locate longest
chain
• Identify
functional
groups and
positions
• Identify side
groups and
their positions
CH3CH2CH3
propane
CH3CH2CH2OH
propan-1-ol
CH3CHCH3
I
CH3
2-methyl
propan-1-ol
Structural Isomerism
Structural isomers are molecules with the same molecular
formulae but with different structural formulae.
1-chlorobutane
2-chlorobutane
Both these molecules have the same molecular formula
(C4H9Cl) but one cannot be converted to the other without
breaking and making bonds.
These are examples of positional isomers
More structural isomerism
CH3CH2CH2CHO
and
CH3CH2COCH3
CH3CH2CH2CH3
and
(CH3)2CHCH3
butanal
and
butanone
Functional group
isomers of
C4H8O
butane
and 2-methyl
propane
chain isomers of
C4H10
Stereoisomerism
• Same structural arrangement, but different
spatial arrangement
• E/Z isomers e.g. of alkenes
Z-isomer
(groups on one
side of C=C
bond)
E-isomer (groups
on opposite sides
of C=C bond)
Where the groups on both sides of the double bond are the
same – the terms cis and trans apply
Petroleum and Alkanes
• Petroleum = Crude oil
• A mixture consisting
mainly of alkanes
• Fractional distillation
used to separate into
component fractions
with different boiling
ranges
• Temperature gradient
in fractionating tower
enables this to
happen
Cracking Alkanes
Involves breaking c-c bonds in alkane molecules
•Converts heavy fractions of oil into higher value products,
increasing profits for oil companies
Produces alkenes for the petrochemical industry e.g. to
make poly(ethene)
Combustion of Alkanes
Crude Oil fractions contain sulphur as an impurity.
If not removed, this burns to form SO2 – causes acid rain
Complete Combustion
• Only CO2 and H2O
produced
Incomplete combustion
• CO, NOx and unburnt
hydrocarbons produced
in car engines
• Can be removed by
catalytic converters
• CO2 is a greenhouse
gas causing global
warming
•
•
•
•
Toxic fumes
Global dimming
Photochemical smog
Acid rain
Chlorination of Alkanes
• Free radical substitution
i.e. homolytic breaking of covalent bonds
Overall reaction equation
CH4 + Cl2
CH3Cl + HCl
Conditions
ultra violet light
excess methane to reduce further substitution
Free radical substitution mechanism
ultra-violet
Cl2
Cl + Cl
initiation step
two
propagation
steps
CH4 + Cl
CH3 + HCl
CH3 + Cl2
CH3Cl + Cl
CH3 + Cl
CH3Cl
termination step
CH3 +
CH3CH3
minor
termination step
CH3
Further free radical substitutions
Further reaction equations
CH3Cl + Cl2
CH2Cl2 + HCl
CH2Cl2 + Cl2
CHCl3 + HCl
CHCl3 + Cl2
CCl4 + HCl
Conditions
ultra-violet light
excess chlorine
Alkenes
Bonds are regions of high electron density
p
H
C
s
C
s
s
H
s
H
s
H
p
p bonds are exposed
and are therefore more vulnerable to attack
by electrophiles
Planar alkene molecules
• With only three areas of bonding electrons
around the double bonded C atoms, the
shape of atoms around each carbon atom
is trigonal planar
• The π electrons repel the C-H bonding
electrons effectively preventing the C-H
bonds from rotating about the C=C
double bonds
• This causes cis-trans isomerism
Hydrogenation of Alkenes
• Ethene (example) reacts with hydrogen in the presence of
a finely divided nickel catalyst at a temperature of
about 150°C. Ethane is produced.
• Margarine is made by hydrogenating C=C double bonds in
animal or vegetable fats and oils. Temperatures of only
60°C are needed.
• Vegetable oils often contain high proportions of
polyunsaturated and mono-unsaturated fats, and as a
result are oils at room temperature. That makes them
messy to spread on your bread or toast, so many
producers use hydrogenation to raise the melting point of
these fats.
Halogenation of Alkenes
• Alkenes react easily with the halogens in
an addition reaction. This has led to
bromine water being used as a test for
alkenes.
• Alkenes decolourise bromine water.
CH2=CH2 + Br2  CH2BrCH2Br
Brown  colourless
Electrophilic addition mechanism
bromine with propene
H
H
C C
CH3
H
+
Br
H
H
carbocation
CH3 C
+
C
H
Br
Bromine molecule is
spontaneously
polarised as it
approaches the
electron rich π bond
Br
Br-
H
H
CH3 C
C
Br Br
1,2-dibromopropane
H
Alkenes with hydrogen Halides
• Alkenes react with gaseous hydrogen
halides at room temperature. If the alkene
is also a gas, you can simply mix the
gases. If the alkene is a liquid, you can
bubble the hydrogen halide through the
liquid.
• This is not done in aqueous solution due
to the polar nature of the water molecules,
which would ionise the H-X molecule to H+
and X- and side reactions could occur.
Electrophilic addition mechanism
hydrogen bromide with trans but-2-ene
CH3
H
C C
CH3
H
H
+
Br
-
H
H
carbocation
CH3 C
+
C
CH3
Br-
H
H
H
CH3 C
C
Br
H
CH3
2-bromobutane
Summary of Electrophilic addition
bromine with propene
CH3CH=CH2 + Br2
CH3CHBrCH2Br
1,2-dibromopropane
hydrogen bromide with but-2-ene
CH3CH=CHCH3+ HBr
CH3CH2CHBrCH3
2-bromobutane
Polymerisation of Alkenes
• Addition polymerisation – very many monomer
molecules add together, forming chemical bonds
making a very long chain polymer molecule
ETHENE
PROPENE
CHLOROETHENE
TETRAFLUORO
-ETHANE
POLY(ETHENE)
POLY(PROPENE)
PVC
PTFE
Processing of waste polymers
• Collection , sorting and separating into different
types and then recycling
• Combustion for energy production
– Need to remove toxic waste gases i.e. HCl during
combustion of PVC and other halogenated plastics
• Feedstock for cracking
• Also aiming to develop more biodegradable
polymers e.g. from maize, starch
Halogenoalkanes
• Polar molecules with δ+ve carbon centres
• Susceptible to attack from nucleophiles –
species able to donate lone pairs able to
make covalent bonds
• Reaction with aqueous alkali – hydrolysis
via nucleophilic substitution reaction
Haloalkanes with potassium
hydroxide
CH3CH2Br + KOH
CH3CH2OH + KBr
• Conditions: boil under reflux with aqueous
potassium hydroxide
• The bromoethane has been hydrolysed to make
ethanol
• This is a nucleophilic substitution mechanism
Nucleophilic substitution mechanism
Aqueous hydroxide ion with bromoethane
H
+
CH3 C
H
OH-
Br
H
CH3 C
OH
H
ethanol
Br
Identification of the halide group
1.
2.
3.
Warm the substance with aqueous sodium hydroxide
Add dilute nitric acid
Add silver nitrate solution (AgNO3)
Ag+ (aq) + X- (aq)
AgX (s)
Results: A silver halide precipitate forms (AgX),
•
White precipitate indicates chloride ions
•
Cream precipitate indicates bromide ions
•
Yellow precipate indicates iodide ions
The halide groups can also be identified by adding ammonia solution to
the silver halide precipitates:
•
Chloride ions dissolve in dilute NH3 (aq)
•
Bromide ions dissolve in concentrated NH3 (aq)
•
Iodide ions are insoluble in concentrated NH3 (aq)
Alcohols
• An alcohol is a compound where an OH
functional group has replaced one or more
H atoms on an alkane
• Alcohols can be separated into 3 groups
– Primary (1°)
– Secondary (2°)
– Tertiary (3°)
Differences between Alcohols
• In a primary alcohol, the
carbon bonded to the OH
group is only attached to
one alkyl group.
• In a secondary alcohol,
the carbon is attached to
two alkyl groups
• In a Tertiary alcohol the
carbon is attached to
three alkyl groups
Physical properties
• Like the alkanes the boiling points of the
alcohols increase as the carbon chain length
increases.
• However the boiling points of the alcohols are
significantly higher than the alkanes due to the
OH group.
• This is because of Hydrogen bonding between
the OH groups!
– More energy is required to overcome the bonds
between the molecules  higher boiling points
Solubility
• Small chain alcohols dissolve easily into water
• Longer chain alcohols are less soluble, there is
a decrease in solubility as the chain length
increases.
• Whilst smaller alcohols form hydrogen bonds
with the water molecules, compensating for the
hydrogen bonds broken, larger alcohols break
more H-bonds than they replace due to their
long hydrocarbon tail which is unable to form Hbonds.
Manufacture of Alcohols
• Alcohols can be manufactured by reacting the
corresponding alkene with steam over a
catalyst. The catalyst and reaction conditions
vary between the alcohols.
– Eg
– This is carried out at 60-70atm, 300°C
• Ethanol can also be made by fermentation of
sugars with a yeast catalyst.
– Eg
Comparison of methods
Fermentation
Hydration of ethene
Type of
process
Batch process
Continuous process
Rate of
reaction
Very slow
Very rapid
Purity
Impure - requires
further processing
Very pure
Reaction
conditions
Gentle temperatures,
atmospheric pressure
High Temperatures and
pressures
Atom
economy
Low – waste products
made BE ABLE TO
CALC THIS!!
High – only 1 product
made BE ABLE TO
CALC THIS!!
Use of
resources
Renewable resources
Finite resources
(mainly crude oil)
Dehydration of Alcohols
• Alcohols may be dehydrated to form
alkenes
• This, as the name implies, involves the
removal of a water molecule from the
alcohol
• The reaction takes place using conc.
Phosphoric acid or conc sulfuric acid as a
catalyst
• Alkene gases are collected over water
Oxidation of Alcohols
• Only primary and secondary alcohols may be
oxidised.
• Primary alcohols are oxidised to aldehydes, and
depending upon the conditions may be further
oxidised to carboxylic acids.
• Secondary alcohols may only be oxidised to
form ketones.
• Tertiary alcohols cannot normally be oxidised
Formation of Aldehydes
• A suitable oxidising agent is Cr2O7 2-
• An acid catalyst is used and the mixture is heated
gently.
• During the reaction orange dichromate(VI) ions are
reduced to blue-green chromium(III) ions
Formation of Carboxylic acids
• Aldehydes are further oxidised to Carboxylic
acids when heated under reflux
•To do this you need to use an excess of the
oxidising agent
Formation of Ketones
• Secondary alcohols form ketones under an
oxidation reaction
• The ketones do not undergo further oxidation
• The secondary alcohol is oxidised under the
same conditions as the primary alcohol
• The same colour change from orange to bluegreen is displayed
Uses
• Alcohols have three main uses
– Drinks
– Solvents
– Fuel (gasohol in poorer countries)
– Methylated spirits
• Alcohols are also used in other reactions
which have not been mentioned
– Formation of esters
– Triiodomethane (Iodoform) reaction
Carbonyls - Aldehydes and Ketones
Aldehydes
R
C=O
H
Ketones
Where R are alkyl or
aryl groups, and may or
may not be different
The aldehydes are named
using the suffix -al
E.g.
CH2O methanal
R
C=O
R1
The ketones are named using the
suffix -one
E.g.
CH3COCH3 propanone
CH3CH2COCH3 butanone
CH3CHO ethanal
CH3CH2CH2COCH3 pentan-2-one
Carboxylic acids and Esters
Where R are alkyl or
aryl groups, and may or
may not be different
• Named as –oic acids,
assuming the –COOH carbon
to be carbon 1 in the chain
e.g. CH3CH(CH3)CH2COOH is
named as 3-methylbutanoic
acid
• Named as –yl –oates
e.g. CH3COOCH2CH2CH3 is
named as propyl-ethanoate
Esterification
• Heat a carboxylic acid with an alcohol in
the presences of a strong acid catalyst
(usually conc. sulphuric acid).
• This can also be known as condensation
reactions as a water molecule is lost.
Uses of esters
• Food flavourings – sweet smells and nice
fragrances and flavours (flowers and fruits)
• Plasticisers – because they make plastic more
flexible.
• Solvents – Esters are polar liquids so dissolve
polar compounds, they have a low boiling point
(therefore evaporate easily), making them good
solvents in glue and printing inks.