3.3.5 Alcohols

Alcohol production Alcohols are produced industrially by hydration of alkenes in the presence of an acid catalyst. Ethanol is produced industrially by fermentation of glucose Ethanol has the formula CH 3 CH 2 OH

Ethanol can be made by two processes: Direct hydration of ethene Fermentation

Direct hydration of ethene Ethene + Steam → Ethanol CH 2 =CH 2 + H 2 O → CH 3 CH 2 OH Conditions required: Temperature of 300°C High pressure of 6.5 x 10 3 kPa (expensive) Phosphoric acid catalyst

Mechanism You 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

Fermentation Plants contain sugars such as glucose (C 6 H 12 O 6 ). Fermentation converts sugars such as glucose into ethanol and carbon dioxide using yeast. Glucose → Ethanol + Carbon dioxide C 6 H 12 O 6 → 2CH 3 CH 2 OH + 2CO 2 Conditions required: Yeast Anaerobic conditions (absence of oxygen) Temperature of 35°C

Fermentation The 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).

FermentationHydration of ethene Raw materials Sugars from plants (Renewable) Ethene from oil (Non-renewable) Speed of reactionSlowFast YieldLow (15%)High (95%) Quality of product Impure ethanol (needs distilling) Pure ethanol Atom economyLow, 51.1%High, 100% Type of process Batch (stop start) Expensive on manpower Continuous (24 hours) Cheap on manpower EquipmentCheapExpensive Energy used Low (35°C and atmospheric pressure) High (300°C and 6.5 x 10 3 kPa)

Biofuels Definition: A biofuel is a fuel produced from renewable living things such as plants Ethanol produced by fermentation comes from plants which are renewable. Ethanol can be burned (combusted) to release energy CH 3 CH 2 OH + 3O 2 → 2CO 2 + 3H 2 O

Biofuels As 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 neutral Definition: 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 inCarbon dioxide released 1) Photosynthesis in plants produces sugars such as glucose: Carbon dioxide + water → glucose + oxygen 6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2 1) Fermentation produces ethanol: C 6 H 12 O 6 → 2C 2 H 5 OH + 2CO 2 2) Combustion (burning) of ethanol: C 2 H 5 OH + 3O 2 → 2CO 2 + 3H 2 O 2C 2 H 5 OH + 6O 2 → 4CO 2 + 6H 2 O 6 molecules of CO 2 taken in6 molecules of CO 2 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-H When 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 tertiary Count 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°)

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 substanceTollen’s reagent Fehling's solution AldehydeSilver mirror Blue solution changes to brick red precipitate Ketone No observable change

Oxidation of primary alcohols Primary alcohols are first oxidised to aldehydes Primary alcohol + [O] → Aldehyde + H 2 O (Removes two hydrogen atoms) Example: Ethanol + [O] → Ethanal + water CH 3 CH 2 OH + [O] → CH 3 CHO + H 2 O 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 CH 3 CHO + [O] → CH 3 COOH

Aldehyde or carboxylic acid from a primary alcohol

Oxidation of secondary alcohols Secondary alcohol + [O] → Ketone + H 2 O (Removes two hydrogen atoms) Example: Propan-2-ol + [O] → Propanone + water CH 3 CH(OH)CH 3 + [O] → CH 3 COCH 3 + H 2 O 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 KetoneNo change TertiaryStays orangeNoneNo change

Elimination Alkenes can be formed from alcohols by acid- catalysed elimination reactions (dehydration). Water is removed from the alcohol (dehydrated) to form an alkene Conditions 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 CH 3 CH 2 OH → C 2 H 4 + H 2 O Example: Acid-catalysed elimination of propan-2-ol in the presence of concentrated sulfuric acid Propan-2-ol → Propene + water CH 3 CH(OH)CH 3 → CH 3 CH=CH 2 + H 2 O

Mechanism for (acidic) elimination

Alkenes produced by this method can be used to produce addition polymers without using monomers derived from crude oil The 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).