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AN INTRODUCTION TOAN INTRODUCTION TO
THE CHEMISTRYTHE CHEMISTRYOF ALCOHOLSOF ALCOHOLS
KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING20152015
SPECIFICATIONSSPECIFICATIONS
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INTRODUCTION
This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.
Individual students may use the material at home for revision purposes or it may be used for classroom teaching.
Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...
www.knockhardy.org.uk/sci.htm
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KNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING
THE CHEMISTRY OF ALCOHOLSTHE CHEMISTRY OF ALCOHOLS
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CONTENTS
• Structure and classification of alcohols
• Nomenclature
• Isomerism
• Physical properties
• Chemical properties of alcohols
• Identification using infra-red spectroscopy
• Industrial preparation and uses of ethanol
• Revision check list
THE CHEMISTRY OF ALCOHOLSTHE CHEMISTRY OF ALCOHOLS
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Before you start it would be helpful to…
• Recall the definition of a covalent bond
• Recall the difference types of physical bonding
• Be able to balance simple equations
• Be able to write out structures for simple organic molecules
• Understand the IUPAC nomenclature rules for simple organic compounds
• Recall the chemical properties of alkanes and alkenes
THE CHEMISTRY OF ALCOHOLSTHE CHEMISTRY OF ALCOHOLS
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CLASSIFICATION OF ALCOHOLSCLASSIFICATION OF ALCOHOLS
Aliphatic • general formula CnH2n+1OH - provided there are no rings
• the OH replaces an H in a basic hydrocarbon skeleton
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CLASSIFICATION OF ALCOHOLSCLASSIFICATION OF ALCOHOLS
Aliphatic • general formula CnH2n+1OH - provided there are no rings
• the OH replaces an H in a basic hydrocarbon skeleton
Aromatic • in aromatic alcohols (or phenols) the OH is attached directly to the ring • an OH on a side chain of a ring behaves as a typical aliphatic alcohol
The first two compounds are classified as aromatic alcohols (phenols) because the OH group is attached directly to the ring.
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CLASSIFICATION OF ALCOHOLSCLASSIFICATION OF ALCOHOLS
Aliphatic • general formula CnH2n+1OH - provided there are no rings
• the OH replaces an H in a basic hydrocarbon skeleton
Aromatic • in aromatic alcohols (or phenols) the OH is attached directly to the ring • an OH on a side chain of a ring behaves as a typical aliphatic alcohol
The first two compounds are classified as aromatic alcohols (phenols) because the OH group is attached directly to the ring.
Structuraldifferences • alcohols are classified according to the environment of the OH group
• chemical behaviour, eg oxidation, often depends on the structural type
PRIMARY 1° SECONDARY 2° TERTIARY 3°
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Alcohols are named according to standard IUPAC rules
• select the longest chain of C atoms containing the O-H group;
• remove the e and add ol after the basic name
• number the chain starting from the end nearer the O-H group
• the number is placed after the an and before the ol ... e.g butan-2-ol
• as in alkanes, prefix with alkyl substituents
• side chain positions are based on the number allocated to the O-H group
e.g. CH3 - CH(CH3) - CH2 - CH2 - CH(OH) - CH3 is called 5-methylhexan-2-ol
NAMING ALCOHOLSNAMING ALCOHOLS
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STRUCTURAL ISOMERISM IN ALCOHOLSSTRUCTURAL ISOMERISM IN ALCOHOLS
Different structures are possible due to...
A Different positions for the OH group and
B Branching of the carbon chain
butan-1-ol butan-2-ol
2-methylpropan-1-ol2-methylpropan-2-ol
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BOILING POINTS OF ALCOHOLSBOILING POINTS OF ALCOHOLS
Increases with molecular size due to increased van der Waals’ forces.
Alcohols have higher boiling points thansimilar molecular mass alkanes
This is due to the added presence ofinter-molecular hydrogen bonding.More energy is required to separate the molecules.
Mr bp / °C
propane C3H8 44 -42 permanent dipole-dipole interactions
ethanol C2H5OH 46 +78 permanent forces + hydrogen bonding
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BOILING POINTS OF ALCOHOLSBOILING POINTS OF ALCOHOLS
Increases with molecular size due to increased van der Waals’ forces.
Alcohols have higher boiling points thansimilar molecular mass alkanes
This is due to the added presence ofinter-molecular hydrogen bonding.More energy is required to separate the molecules.
Mr bp / °C
propane C3H8 44 -42 permanent dipole-dipole interactions
ethanol C2H5OH 46 +78 permanent forces + hydrogen bonding
Boiling point is higher for “straight” chain isomers.
bp / °Cbutan-1-ol CH3CH2CH2CH2OH 118
butan-2-ol CH3CH2CH(OH)CH3 100
2-methylpropan-2-ol (CH3)3COH 83
Greater branching = lower inter-molecular forces
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BOILING POINTS OF ALCOHOLSBOILING POINTS OF ALCOHOLS
Increases with molecular size due to increased van der Waals’ forces.
Alcohols have higher boiling points thansimilar molecular mass alkanes
This is due to the added presence ofinter-molecular hydrogen bonding.More energy is required to separate the molecules.
Mr bp / °C
propane C3H8 44 -42 just van der Waals’ forces
ethanol C2H5OH 46 +78 van der Waals’ forces + hydrogen bonding
Boiling point is higher for “straight” chain isomers.
bp / °Cbutan-1-ol CH3CH2CH2CH2OH 118
butan-2-ol CH3CH2CH(OH)CH3 100
2-methylpropan-2-ol (CH3)3COH 83
Greater branching = lower inter-molecular forces
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SOLVENT PROPERTIES OF ALCOHOLSSOLVENT PROPERTIES OF ALCOHOLS
SolubilityLow molecular mass alcohols are miscible with water
Due to hydrogen bonding between the two molecules
Heavier alcohols are less miscible
Solventproperties Alcohols are themselves very good solvents
They dissolve a large number of organic molecules
Show the relevant lone pair(s) when drawing hydrogen bonding
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CHEMICAL PROPERTIES OF ALCOHOLSCHEMICAL PROPERTIES OF ALCOHOLS
The OXYGEN ATOM HAS TWO LONE PAIRS; this makes alcohols...
BASES Lewis bases are lone pair donors Bronsted-Lowry bases are proton acceptors
The alcohol uses one of its lone pairs to form a co-ordinate bond
NUCLEOPHILES Alcohols can use the lone pair to attack electron deficient centres
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ELIMINATION OF WATER (DEHYDRATION)ELIMINATION OF WATER (DEHYDRATION)
Reagent/catalyst conc. sulphuric acid (H2SO4) or conc. phosphoric acid (H3PO4)
Conditions reflux at 180°C
Product alkene
Equation e.g. C2H5OH(l) ——> CH2 = CH2(g) + H2O(l)
Mechanism
Step 1 protonation of the alcohol using a lone pair on oxygenStep 2 loss of a water molecule to generate a carbocationStep 3 loss of a proton (H+) to give the alkene
AlternativeMethod Pass vapour over a heated alumina (aluminium oxide) catalyst
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ELIMINATION OF WATER (DEHYDRATION)ELIMINATION OF WATER (DEHYDRATION)
MECHANISM
Step 1 protonation of the alcohol using a lone pair on oxygenStep 2 loss of a water molecule to generate a carbocationStep 3 loss of a proton (H+) to give the alkene
Note 1 There must be an H on a carbon atom adjacent the carbon with the OH
Note 2 Alcohols with the OH in the middle of a chaincan have two ways of losing water.
In Step 3 of the mechanism, a proton can be lostfrom either side of the carbocation. This gives amixture of alkenes from unsymmetrical alcohols...
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OXIDATION OF ALCOHOLSOXIDATION OF ALCOHOLS
All alcohols can be oxidised depending on the conditions
Oxidation is used to differentiate between primary, secondary and tertiary alcoholsThe usual reagent is acidified potassium dichromate(VI)
Primary Easily oxidised to aldehydes and then to carboxylic acids.
Secondary Easily oxidised to ketones
Tertiary Not oxidised under normal conditions.They do break down with very vigorous oxidation
PRIMARY 1° SECONDARY 2° TERTIARY 3°
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OXIDATION OF PRIMARY ALCOHOLSOXIDATION OF PRIMARY ALCOHOLS
Primary alcohols are easily oxidised to aldehydes
e.g. CH3CH2OH(l) + [O] ——> CH3CHO(l) + H2O(l)
ethanol ethanal
it is essential to distil off the aldehyde before it gets oxidised to the acid
CH3CHO(l) + [O] ——> CH3COOH(l)
ethanal ethanoic acid
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OXIDATION OF PRIMARY ALCOHOLSOXIDATION OF PRIMARY ALCOHOLS
Primary alcohols are easily oxidised to aldehydes
e.g. CH3CH2OH(l) + [O] ——> CH3CHO(l) + H2O(l)
ethanol ethanal
it is essential to distil off the aldehyde before it gets oxidised to the acid
CH3CHO(l) + [O] ——> CH3COOH(l)
ethanal ethanoic acid
Practical details
• the alcohol is dripped into a warm solution of acidified K2Cr2O7
• aldehydes have low boiling points - no hydrogen bonding - they distil off immediately• if it didn’t distil off it would be oxidised to the equivalent carboxylic acid• to oxidise an alcohol straight to the acid, reflux the mixture
compound formula intermolecular bonding boiling point
ETHANOL C2H5OH HYDROGEN BONDING 78°C
ETHANAL CH3CHO DIPOLE-DIPOLE 23°C
ETHANOIC ACID CH3COOH HYDROGEN BONDING 118°C
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OXIDATION OF PRIMARY ALCOHOLSOXIDATION OF PRIMARY ALCOHOLS
Controlling the products
e.g. CH3CH2OH(l) + [O] ——> CH3CHO(l) + H2O(l)
then CH3CHO(l) + [O] ——> CH3COOH(l)
Aldehyde has a lower boiling point so distils off before being oxidised further
OXIDATION TO ALDEHYDESDISTILLATION
OXIDATION TO CARBOXYLIC ACIDSREFLUX
Aldehyde condenses back into the mixture and gets oxidised to the acid
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OXIDATION OF SECONDARY ALCOHOLSOXIDATION OF SECONDARY ALCOHOLS
Secondary alcohols are easily oxidised to ketones
e.g. CH3CHOHCH3(l) + [O] ——> CH3COCH3(l) + H2O(l)
propan-2-ol propanone
The alcohol is refluxed with acidified K2Cr2O7. However, on prolonged treatment with a powerful oxidising agent they can be further oxidised to a mixture of acids with fewer carbon atoms than the original alcohol.
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OXIDATION OF SECONDARY ALCOHOLSOXIDATION OF SECONDARY ALCOHOLS
Secondary alcohols are easily oxidised to ketones
e.g. CH3CHOHCH3(l) + [O] ——> CH3COCH3(l) + H2O(l)
propan-2-ol propanone
The alcohol is refluxed with acidified K2Cr2O7. However, on prolonged treatment with a powerful oxidising agent they can be further oxidised to a mixture of acids with fewer carbon atoms than the original alcohol.
OXIDATION OF TERTIARY ALCOHOLSOXIDATION OF TERTIARY ALCOHOLS
Tertiary alcohols are resistant to normal oxidation
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OXIDATION OF ALCOHOLSOXIDATION OF ALCOHOLS
Why 1° and 2° alcohols are easily oxidised and 3° alcohols are not
For oxidation to take place easily you must have two hydrogen atoms on adjacent C and O atoms.
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OXIDATION OF ALCOHOLSOXIDATION OF ALCOHOLS
Why 1° and 2° alcohols are easily oxidised and 3° alcohols are not
For oxidation to take place easily you must have two hydrogen atoms on adjacent C and O atoms.
H H
R C O + [O] R C O + H2O
H H
1°
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OXIDATION OF ALCOHOLSOXIDATION OF ALCOHOLS
Why 1° and 2° alcohols are easily oxidised and 3° alcohols are not
For oxidation to take place easily you must have two hydrogen atoms on adjacent C and O atoms.
H H
R C O + [O] R C O + H2O
H H
H H
R C O + [O] R C O + H2O
R R
1°
2°
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OXIDATION OF ALCOHOLSOXIDATION OF ALCOHOLS
Why 1° and 2° alcohols are easily oxidised and 3° alcohols are not
For oxidation to take place easily you must have two hydrogen atoms on adjacent C and O atoms.
H H
R C O + [O] R C O + H2O
H H
H H
R C O + [O] R C O + H2O
R R
This is possible in 1° and 2° alcohols but not in 3° alcohols.
1°
2°
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OXIDATION OF ALCOHOLSOXIDATION OF ALCOHOLS
Why 1° and 2° alcohols are easily oxidised and 3° alcohols are not
For oxidation to take place easily you must have two hydrogen atoms on adjacent C and O atoms.
H H
R C O + [O] R C O + H2O
H H
H H
R C O + [O] R C O + H2O
R R
R H
R C O + [O]
R
This is possible in 1° and 2° alcohols but not in 3° alcohols.
1°
2°
3°
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ESTERIFICATION OF ALCOHOLSESTERIFICATION OF ALCOHOLS
Reagent(s) carboxylic acid + strong acid catalyst (e.g conc. H2SO4 )
Conditions reflux
Product ester
Equation e.g. CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l)
ethanol ethanoic acid ethyl ethanoate
Notes Concentrated H2SO4 is a dehydrating agent - it removes water
causing the equilibrium to move to the right and increases the yield
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ESTERIFICATION OF ALCOHOLSESTERIFICATION OF ALCOHOLS
Reagent(s) carboxylic acid + strong acid catalyst (e.g conc. H2SO4 )
Conditions reflux
Product ester
Equation e.g. CH3CH2OH(l) + CH3COOH(l) CH3COOC2H5(l) + H2O(l)
ethanol ethanoic acid ethyl ethanoate
Notes Concentrated H2SO4 is a dehydrating agent - it removes water
causing the equilibrium to move to the right and increases the yield
Uses of esters Esters are fairly unreactive but that doesn’t make them uselessUsed as flavourings
Naming esters Named from the alcohol and carboxylic acid which made them...
CH3OH + CH3COOH CH3COOCH3 + H2O
from ethanoic acid CH3COOCH3 from methanol
METHYL ETHANOATE
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OTHER REACTIONS OF ALCOHOLSOTHER REACTIONS OF ALCOHOLS
OXYGEN Alcohols make useful fuels
C2H5OH(l) + 3O2(g) ———> 2CO2(g) + 3H2O(l)
Advantages have high enthalpies of combustion do not contain sulphur so there is less pollution can be obtained from renewable resources
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OTHER REACTIONS OF ALCOHOLSOTHER REACTIONS OF ALCOHOLS
OXYGEN Alcohols make useful fuels
C2H5OH(l) + 3O2(g) ———> 2CO2(g) + 3H2O(l)
Advantages have high enthalpies of combustion do not contain sulphur so there is less pollution can be obtained from renewable resources
SODIUMConditions room temperature
Product sodium alkoxide and hydrogen
Equation 2CH3CH2OH(l) + 2Na(s) ——> 2CH3CH2O¯ Na + + H2(g) sodium ethoxide
Notes alcohols are organic chemistry’s equivalent of waterwater reacts with sodium to produce hydrogen and so do alcoholsthe reaction is slower with alcohols than with water.
Alkoxides are white, ionic crystalline solids e.g. CH3CH2O¯ Na+
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BROMINATION OF ALCOHOLSBROMINATION OF ALCOHOLS
Reagent(s) conc. hydrobromic acid HBr(aq) orsodium (or potassium) bromide and concentrated sulphuric acid
Conditions reflux
Product haloalkane
Equation C2H5OH(l) + conc. HBr(aq) ———> C2H5Br(l) + H2O(l)
Mechanism The mechanism starts off similar to that involving dehydration(protonation of the alcohol and loss of water) but the carbocation(carbonium ion) is attacked by a nucleophilic bromide ion in step 3
Step 1 protonation of the alcohol using a lone pair on oxygen
Step 2 loss of a water molecule to generate a carbocation (carbonium ion)
Step 3 a bromide ion behaves as a nucleophile and attacks the carbocation
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INFRA-RED SPECTROSCOPYINFRA-RED SPECTROSCOPY
Chemical bonds vibrate at different frequencies. When infra red (IR) radiation is passed through a liquid sample of an organic molecule, some frequencies are absorbed. These correspond to the frequencies of the vibrating bonds.
Most spectra are very complex due to the large number of bonds present and each molecule produces a unique spectrum. However the presence of certain absorptions can be used to identify functional groups.
BOND COMPOUND ABSORBANCE RANGE
O-H alcohols broad 3200 cm-1 to 3600 cm-1
O-H carboxylic acids medium to broad 2500 cm-1 to 3500 cm-1
C=O ketones, aldehydes strong and sharp 1600 cm-1 to 1750 cm-1
esters and acids
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INFRA-RED SPECTROSCOPYINFRA-RED SPECTROSCOPY
IDENTIFYING ALCOHOLS USING INFRA RED SPECTROSCOPY
Differentiation Compound O-H C=O
ALCOHOL YES NO
ALDEHYDE / KETONE NO YES
CARBOXYLIC ACID YES YES
ESTER NO YES
ALCOHOL ALDEHYDE CARBOXYLIC ACID PROPAN-1-OL PROPANAL PROPANOIC ACID O-H absorption C=O absorption O-H + C=O absorption
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INDUSTRIAL PREPARATION OF ALCOHOLSINDUSTRIAL PREPARATION OF ALCOHOLS
FERMENTATION
Reagent(s) GLUCOSE - produced by the hydrolysis of starch
Conditions yeastwarm, but no higher than 37°C
Equation C6H12O6 ——> 2 C2H5OH + 2 CO2
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INDUSTRIAL PREPARATION OF ALCOHOLSINDUSTRIAL PREPARATION OF ALCOHOLS
FERMENTATION
Reagent(s) GLUCOSE - produced by the hydrolysis of starch
Conditions yeastwarm, but no higher than 37°C
Equation C6H12O6 ——> 2 C2H5OH + 2 CO2
Advantages LOW ENERGY PROCESSUSES RENEWABLE RESOURCES - PLANT MATERIALSIMPLE EQUIPMENT
Disadvantages SLOWPRODUCES IMPURE ETHANOLBATCH PROCESS
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INDUSTRIAL PREPARATION OF ALCOHOLSINDUSTRIAL PREPARATION OF ALCOHOLS
HYDRATION OF ETHENE
Reagent(s) ETHENE - from cracking of fractions from distilled crude oil
Conditions catalyst - phosphoric acid high temperature and pressure
Equation C2H4 + H2O ——> C2H5OH
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INDUSTRIAL PREPARATION OF ALCOHOLSINDUSTRIAL PREPARATION OF ALCOHOLS
HYDRATION OF ETHENE
Reagent(s) ETHENE - from cracking of fractions from distilled crude oil
Conditions catalyst - phosphoric acid high temperature and pressure
Equation C2H4 + H2O ——> C2H5OH
Advantages FASTPURE ETHANOL PRODUCEDCONTINUOUS PROCESS
Disadvantages HIGH ENERGY PROCESSEXPENSIVE PLANT REQUIREDUSES NON-RENEWABLE FOSSIL FUELS TO MAKE ETHENE
Uses of ethanol ALCOHOLIC DRINKSSOLVENT - industrial alcohol / methylated spiritsFUEL - petrol substitute in countries with limited oil reserves
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USES OF ALCOHOLSUSES OF ALCOHOLS
ETHANOL
DRINKSSOLVENT industrial alcohol / methylated spirits (methanol is added)FUEL used as a petrol substitute in countries with limited oil reserves
METHANOL
PETROL ADDITIVE improves combustion properties of unleaded petrolSOLVENTRAW MATERIAL used as a feedstock for important industrial processesFUEL
Health warning Methanol is highly toxic
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LABORATORY PREPARATION OF ALCOHOLSLABORATORY PREPARATION OF ALCOHOLS
from haloalkanes - reflux with aqueous sodium or potassium hydroxide
from aldehydes - reduction with sodium tetrahydridoborate(III) - NaBH4
from alkenes - acid catalysed hydration using concentrated sulphuric acid
Details of the reactions may be found in other sections.
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REVISION CHECKREVISION CHECK
What should you be able to do?
Recall and explain the physical properties of alcohols
Recall the different structural types of alcohols
Recall the Lewis base properties of alcohols
Recall and explain the chemical reactions of alcohols
Write balanced equations representing any reactions in the section
Understand how oxidation is affected by structure
Recall how conditions and apparatus influence the products of oxidation
Explain how infrared spectroscopy can be used to differentiate between functional groups
CAN YOU DO ALL OF THESE? CAN YOU DO ALL OF THESE? YES YES NONO
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You need to go over the You need to go over the relevant topic(s) againrelevant topic(s) again
Click on the button toClick on the button toreturn to the menureturn to the menu
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WELL DONE!WELL DONE!Try some past paper questionsTry some past paper questions
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© 2015 JONATHAN HOPTON & KNOCKHARDY PUBLISHING© 2015 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
THE ENDTHE END
AN INTRODUCTION TOAN INTRODUCTION TO
THE CHEMISTRYTHE CHEMISTRYOF ALCOHOLSOF ALCOHOLS
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BIOFUELSBIOFUELS
What are they?What are they?Liquid fuels made from plant material and recycled elements of the food chain
BiodieselBiodieselAn alternative fuel which can be made from waste vegetable oil or from oil produced from seeds. It can be used in any diesel engine, either neat or mixed with petroleum diesel.
It is a green fuel, does not contribute to the carbon dioxide (CO2) burden and produces drastically reduced engine emissions. It is non-toxic and biodegradable.
vegetable oil glycerol biodiesel
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BIOFUELSBIOFUELS
AdvantagesAdvantages • renewable - derived from sugar beet, rape seed• dramatically reduces emissions• carbon neutral• biodegradable• non-toxic• fuel & exhaust emissions are less unpleasant• can be used directly in unmodified diesel engine• high flashpoint - safer to store & transport• simple to make• used neat or blended in any ratio with petroleum diesel
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BIOFUELSBIOFUELS
AdvantagesAdvantages • renewable - derived from sugar beet, rape seed• dramatically reduces emissions• carbon neutral• biodegradable• non-toxic• fuel & exhaust emissions are less unpleasant• can be used directly in unmodified diesel engine• high flashpoint - safer to store & transport• simple to make• used neat or blended in any ratio with petroleum diesel
DisadvantagesDisadvantages • poor availability - very few outlets & manufacturers
• more expensive to produce• poorly made biodiesel can cause engine problems
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BIOFUELSBIOFUELS
AdvantagesAdvantages • renewable - derived from sugar beet, rape seed• dramatically reduces emissions• carbon neutral• biodegradable• non-toxic• fuel & exhaust emissions are less unpleasant• can be used directly in unmodified diesel engine• high flashpoint - safer to store & transport• simple to make• used neat or blended in any ratio with petroleum diesel
DisadvantagesDisadvantages • poor availability - very few outlets & manufacturers
• more expensive to produce• poorly made biodiesel can cause engine problems
FutureFutureproblemsproblems • there isn’t enough food waste to produce large amounts
• crops grown for biodiesel use land for food crops• a suitable climate is needed to grow most crops• some countries have limited water resources