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).