3.3.5 Alcohols
3.3.5.1 Alcohol productionAlcohols are produced industrially by hydration of alkenes in the presence of an acid catalyst. Ethanol is produced industrially by fermentation of glucoseEthanol has the formula CH3CH2OH
C
H
H
H
C OH
H
H
Ethanol can be made by two processes:• Direct hydration of ethene• Fermentation
Direct hydration of etheneEthene + Steam → Ethanol
CH2=CH2 + H2O → CH3CH2OHConditions required:Temperature of 300°CHigh pressure of 6.5 x 103 kPa (expensive)Phosphoric acid catalyst
MechanismYou must learn the mechanism for the formation of an alcohol by the reaction of an alkene with steam in the presence of an acid catalyst (phosphoric acid)Example: Ethanol formed by the reaction of ethene with steam (and phosphoric acid catalyst)Name of mechanism: Electrophilic addition
Electrophilic Addition of Steam to Ethene
FermentationPlants contain sugars such as glucose (C6H12O6). Fermentation converts sugars such as glucose into ethanol and carbon dioxide using yeast. Glucose → Ethanol + Carbon dioxide
C6H12O6 → 2CH3CH2OH + 2CO2
Conditions required:• Yeast• Anaerobic conditions (absence of oxygen)• Temperature of 35°C
FermentationThe mixture is left at 35°C for several days in the absence of air. Yeast is killed by about 15% of ethanol in the mixture. The ethanol is purified by fractional distillation (water boils at 100 °C and ethanol boils at 78 °C).
Fermentation Hydration of ethene
Raw materials Sugars from plants(Renewable)
Ethene from oil(Non-renewable)
Speed of reaction Slow Fast
Yield Low (15%) High (95%)
Quality of product Impure ethanol(needs distilling) Pure ethanol
Atom economy Low, 51.1% High, 100%
Type of process Batch (stop start)Expensive on manpower
Continuous (24 hours)Cheap on manpower
Equipment Cheap Expensive
Energy used Low (35°C and atmospheric pressure)
High(300°C and 6.5 x 103
kPa)
BiofuelsDefinition: A biofuel is a fuel produced from renewable living things such as plantsEthanol produced by fermentation comes from plants which are renewable. Ethanol can be burned (combusted) to release energy
CH3CH2OH + 3O2 → 2CO2 + 3H2O
BiofuelsAs the demand for biofuels increases so will the demand to grow sugar rich plants. This causes problems for developing countries as it leads to competition for land which is used for growing crops. Land area used to grow plants may increase leading to deforestation. Trees are good at absorbing carbon dioxide
Carbon neutralDefinition: There is no change in the total amount / level of carbon dioxide present in the atmosphere.Carbon neutral - the carbon dioxide released when the fuel (ethanol from plants) is burnt is the same as the carbon dioxide taken in from the air by the plant by photosynthesis. By a series of equations we can prove that ethanol made by fermentation is carbon neutral.
Carbon dioxide taken in Carbon dioxide released
1) Photosynthesis in plants produces sugars such as glucose:
Carbon dioxide + water → glucose + oxygen
6CO2 + 6H2O → C6H12O6 + 6O2
1) Fermentation produces ethanol:
C6H12O6 → 2C2H5OH + 2CO2
2) Combustion (burning) of
ethanol:C2H5OH + 3O2 → 2CO2 + 3H2O
2C2H5OH + 6O2 → 4CO2 + 6H2O
6 molecules of CO2 taken in 6 molecules of CO2 released
Even though it may be imagined that the production of biofuels such as ethanol, and their use, is carbon neutral, closer inspection reveals that overall it is not.
The sugar cane is probably grown on land that otherwise would probably have forests capturing and holding carbon dioxide. The care, irrigation and harvesting requires machinery and the installations themselves need a supply of electricity and other facilities. The ethanol needs to be transported to the point of sale, which also uses fuel. However, that said, biofuels reduce the carbon footprint of countries that would otherwise rely on fossil fuels for their energy supply.
Alcohols contain the functional group O-HWhen naming alcohols:• Name the carbon skeleton first (e.g. butane)• Remove the letter ‘e’ from the end of the name and
replace with ol (e.g. Butanol)• Numbers are used to show which carbon atom the
OH group is attached to• The number goes in the middle of the name with a
dash either side (e.g. butan-1-ol)• The numbering is always done so you get the lowest
total number (e.g. butan-1-ol NOT butan-4-ol)
Alcohols are classified as primary, secondary and tertiaryCount the number of carbon atoms only the carbon of the C-OH bond is attached to• Primary alcohol (1°) - one carbon atom• Secondary alcohol (2°) - two carbon
atoms• Tertiary alcohol (3°) - three carbon atoms
Primary alcohol (1°) Secondary alcohol (2°) Tertiary alcohol (3°)
Homologous series Name: prefix or suffix Functional group Example
Aldehydes Suffix – al
CH3CHO
Ethanal
Ketones Suffix – one
CH3COCH3
Propanone
Carboxylic acids Suffix – oic acid
CH3COOH
Ethanoic acid
CHR
O
CR
O
R
C
O
OHR
When naming aldehydes:• Name the carbon skeleton first including the carbon attached to
the oxygen atom (i.e. propane)• Remove the letter ‘e’ from the end of the name and replace with -
al (i.e. propanal)• No numbers are needed since the functional group is always at
the end of the chain
When naming ketones:• Name the carbon skeleton first including the carbon attached to
the oxygen atom (i.e. propane)• Remove the letter ‘e’ from the end of the name and replace with -
one (i.e. propanone)• The number goes in the middle of the name with a dash either
side. Only applies to ketones with 4 or more carbons (i.e. pentan-2-one)
When naming carboxylic acids:Name the carbon skeleton first including the carbon attached to the oxygen atom (i.e. ethane)Remove the letter ‘e’ from the end of the name and replace with -oic acid (i.e. ethanoic acid)No numbers are needed since the functional group is always at the end of the chain
Testing for aldehydes and ketones
Test substance Tollen’s reagent Fehling's solution
Aldehyde Silver mirrorBlue solution
changes to brick red precipitate
Ketone No observable change
No observable change
We can distinguish between primary, secondary and tertiary alcohols using oxidising agents such as acidified potassium dichromate (VI)We use the symbol [O] to represent the oxidising agent. If oxidation of the alcohol occurs the solution changes colour from ORANGE to GREEN.
Oxidation of primary alcoholsPrimary alcohols are first oxidised to aldehydesPrimary alcohol + [O] → Aldehyde + H2O (Removes two hydrogen atoms)
Example: Ethanol + [O] → Ethanal + water CH3CH2OH + [O] → CH3CHO + H2O
The aldehyde produced can be either separated by distillation or further oxidised into a carboxylic acid under reflux.Aldehyde + [O] → Carboxylic acid (Adds oxygen atom)Example: Ethanal + [O] → Ethanoic acid CH3CHO + [O] → CH3COOH
Aldehyde or carboxylic acid from a primary alcohol
Oxidation of secondary alcoholsSecondary alcohol + [O] → Ketone + H2O (Removes two hydrogen atoms)
Example: Propan-2-ol + [O] → Propanone + water CH3CH(OH)CH3 + [O] → CH3COCH3 + H2O
Tertiary alcohols cannot be oxidised by acidified potassium dichromate(VI).
Alcohol
Colour change with acidified
potassium dichromate(VI
)
Product with acidified
potassium dichromate(VI
)
Test with Tollen’s reagent
Test with Fehling’s solution
Primary Orange to green
Aldehyde first then
carboxylic acid
Silver mirror with
aldehyde
Brick-red precipitate
with aldehyde
Secondary Orange to green Ketone No change No change
Tertiary Stays orange None No change No change
3.3.5.3 EliminationAlkenes can be formed from alcohols by acid-catalysed elimination reactions (dehydration). Water is removed from the alcohol (dehydrated) to form an alkeneConditions required:• Temperature of 180°C• Concentrated sulfuric acid (acts as a catalyst) or
concentrated phosphoric acid.
Example: Acid-catalysed elimination of ethanol in the presence of concentrated sulfuric acid
ethanol → ethene + water CH3CH2OH → C2H4 + H2O
Example: Acid-catalysed elimination of propan-2-ol in the presence of concentrated sulfuric acid
Propan-2-ol → Propene + water CH3CH(OH)CH3 → CH3CH=CH2 + H2O
Mechanism for (acidic) elimination
Alkenes produced by this method can be used to produce addition polymers without using monomers derived from crude oilThe alcohol (ethanol) is formed by fermentation (renewable). Ethene is formed from ethanol by acid-catalysed elimination. The alkene is used to make a polymer by addition polymerisation (polyethene).