3.3.5 - alcohols

what is the functional group in alcohols? (2)

• -OH

• hydroxyl group

what is the general formula for an alcohol? (1)

C_nH_(2n+1)OH

how are alcohols classified? (3)

• primary

• secondary

• tertiary

what are the two ways of producing ethanol? (2)

• fermentation of glucose

• direct hydration of ethene

what is the equation for the fermentation of glucose to produce ethanol? (1)

C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂

what conditions are required for the fermentation of glucose to produce ethanol? (4)

• yeast provides enzymes

• temperature of 35°C

• oxygen-free environment (anaerobic respiration)

• 15% yield

what is a biofuel, and how is ethanol from fermentation separated? (2)

• a biofuel is a fuel derived from biological materials, such as plants or animals

• ethanol is separated using fractional distillation

what is the equation for the direct hydration of ethene to produce ethanol? (1)

C₂H₄ + H₂O → CH₃CH₂OH

what are the essential conditions for the direct hydration of ethene? (2)

• high temperature (300°C)

• high pressure (70 atm)

what catalyst is used in the direct hydration of ethene? (2)

concentrated phosphoric acid (H₃PO₄) or concentrated sulfuric acid (H₂SO₄)

what type of reaction mechanism is involved in the direct hydration of ethene? (1)

electrophilic addition

draw the mechanism for the direct hydration of ethene to produce ethanol (4)

draw a table to compare the two techniques of producing ethanol (10)

why is ethanol made from ethene not considered renewable? (1)

because it originates from crude oil

why is ethanol made by fermentation considered renewable? (2)

• sugars used in fermentation come from plants like sugar cane

• which can be grown annually, making it a biofuel

what is the environmental concern with current carbon-based fuels? (1)

they release carbon dioxide, contributing to global warming and climate change

why is ethanol made by fermentation sometimes called a carbon-neutral fuel? (2)

• the carbon dioxide released during combustion

• is balanced by the carbon dioxide absorbed by the plant during photosynthesis

is ethanol made by fermentation truly carbon neutral? and why or why not? (2)

no, because additional carbon dioxide is released during transportation, harvesting, and other processes

write the equation for photosynthesis in the growing plant (1)

6H₂O + 6CO₂ → C₆H₁₂O₆ + 6O₂

write the equation for the combustion of ethanol (1)

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

what is the elimination reaction of alcohols also referred to as? (2)

• dehydration reaction

• turning alcohol into an alkene

what catalyst is required for the elimination reaction of alcohols? (2)

concentrated sulfuric acid (H₂SO₄) or concentrated phosphoric acid (H₃PO₄) catalyst

what are the three stages of the mechanism for the elimination reaction of alcohols? (3)

1. a lone pair of electrons on the alcohol oxygen is donated to a proton (H+) from the acid catalyst

2. oxygen gains the proton and becomes positively charged

3. hydrogen is lost from an adjacent carbon, forming a double bond (C=C), and water is eliminated

draw the mechanism of the elimination reaction for propan-2-ol, showing the formation of products (3)

what are the possible alkene products formed in the elimination reaction of pentan-2-ol? (2)

• CH₃CH₂CH₂CH=CH₂ (pentene)

• minor product: E or Z isomers depending on conditions

what reaction can cyclohexanol undergo to form cyclohexene? (2)

• cyclohexanol can undergo dehydration using concentrated phosphoric acid

• to form cyclohexene and water

what are the possible impurities in the product of cyclohexanol dehydration? (3)

• acid catalyst (H₃PO₄)

• unreacted cyclohexanol

• water

how is a separating funnel used to purify cyclohexene after dehydration of cyclohexanol? (1)

used to separate the organic layer (cyclohexene) from the aqueous layer containing impurities

draw and label the apparatus for using a separating funnel (3)

what is the apparatus used to separate two immiscible liquids? (1)

a separating funnel

why is sodium hydrogen carbonate solution added to the impure product? write an equation for the reaction that occurs (2)

• to neutralise the acid catalyst

• 3NaHCO₃ + H₃PO₄ → Na₃PO₄ + 3CO₂ + 3H₂O

why is it important to remove the stopper after shaking the mixture in the practical method? (1)

to prevent the build up of pressure due to CO₂ being produced

how does re-distillation remove unreacted cyclohexanol from the product? (2)

• cyclohexanol and cyclohexene have different boiling points

• distillation separates the two based on their boiling points

how is the purified organic product dried to remove water? (3)

1. pour the organic liquid into a beaker and add a spatula of drying agent (e.g., calcium chloride or magnesium sulphate)

2. swirl and add more drying agent until the liquid changes from cloudy to clear

3. filter to remove the drying agent

what oxidising agent is used to oxidise alcohols? (1)

acidified potassium dichromate (K₂Cr₂O₇/H⁺)

what colour change indicates oxidation when using potassium dichromate (VI)? (1)

orange to green

what new bond is formed when alcohols are oxidized? (1)

a carbonyl bond (C=O)

what are the oxidation products of a primary alcohol, including their functional groups? (2)

1. aldehyde (RCHO)

2. then carboxylic acid (RCOOH)

what is the oxidation product of a secondary alcohol, including its functional group? (1)

ketone (RCOR)

can tertiary alcohols be oxidised using acidified potassium dichromate(VI)? (1)

no, tertiary alcohols cannot be oxidised

draw the oxidation flowchart showing the products of oxidation for primary, secondary, and tertiary alcohols (6)

what are primary alcohols oxidised to, and what apparatus is used? (2)

• primary alcohols are oxidised to aldehydes (-CHO)

• the apparatus used is distillation

write the equation for the oxidation of ethanol to ethanal (1)

CH₃CH₂OH + [O] → CH₃CHO + H₂O

what are aldehydes oxidised to, and what apparatus is used? (2)

• aldehydes are oxidised to carboxylic acids (-COOH)

• the apparatus used is reflux

write the equation for the oxidation of ethanal to ethanoic acid (1)

CH₃CHO + [O] → CH₃COOH

what is the overall equation for the formation of ethanoic acid from ethanol (primary alcohol)? (1)

CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

what happens to acidified potassium dichromate (VI) when primary and secondary alcohols are oxidised? (1)

the solution turns from orange to green

how do tertiary alcohols affect acidified potassium dichromate (VI)? (1)

tertiary alcohols have no effect - the solution remains orange

rank ethanal, ethanol, and ethanoic acid from lowest to highest boiling point (3)

ethanal < ethanol < ethanoic acid

why does ethanoic acid have the highest boiling point? (2)

• ethanoic acid forms more hydrogen bonds than ethanol and is a larger molecule

• leading to stronger van der Waals forces between molecules

why does ethanol have a higher boiling point than ethanal? (2)

• ethanol forms hydrogen bonds

• which are stronger than the permanent dipole-dipole forces present in ethanal

why does ethanal have the lowest boiling point among ethanol, ethanal, and ethanoic acid? (2)

• ethanal only has permanent dipole-dipole forces

• which are weaker than hydrogen bonds present in ethanol and ethanoic acid

draw and label the distillation apparatus to form aldehyde / ketone (6)

draw and label the reflux apparatus to form carboxylic acid (6)