Polymer Revolution Organic Functional Group. Learning Outcomes (d) recognise and write formulae for...
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Transcript of Polymer Revolution Organic Functional Group. Learning Outcomes (d) recognise and write formulae for...
Learning Outcomes
• (d) recognise and write formulae for alkenes and use systematic nomenclature to name and interpret the names of alkenes;
(e) recognise members of the following homologous series: aldehydes, ketones, carboxylic acids;
(f) recall the difference between primary, secondary and tertiary alcohols from their structures and identify examples of them;
CLASSIFICATION OF ALCOHOLS
Aliphatic • general formula CnH2n+1OH - provided there are no rings
• the OH replaces an H in a basic hydrocarbon skeleton
CLASSIFICATION 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.
CLASSIFICATION 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°
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 ALCOHOLS
STRUCTURAL 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
BOILING 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 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
BOILING 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
SOLVENT 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
CHEMICAL 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
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
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...
OXIDATION 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°
OXIDATION 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
OXIDATION 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
OXIDATION 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
OXIDATION 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 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 ALCOHOLS
Tertiary alcohols are resistant to normal oxidation
OXIDATION 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.
OXIDATION 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°
OXIDATION 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°
OXIDATION 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°
OXIDATION 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°
ESTERIFICATION 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
ESTERIFICATION 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
OTHER 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
OTHER 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+
BROMINATION 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
INFRA-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
INFRA-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
INDUSTRIAL 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
INDUSTRIAL 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
INDUSTRIAL 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
INDUSTRIAL 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
USES 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
LABORATORY 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.
REVISION 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? YES NO