3.3.5 Alcohols

What is the functional group of an alcohol?

Hydroxyl group -OH

What is the general formula of an alcohol?

C_nH_2n+1OH

What kind of intermolecular forces do alcohols have & why?

Hydrogen bonding, due to the electronegativity difference in the O-H bond

How do alcohols’ M.P. & B.P. compare to other hydrocarbons of similar carbon chain length?

Higher, as they have hydrogen bonding which is stronger than Van der Waals forces

When are alcohols soluble & insoluble in water?

• Soluble when short chain: O-H bonds to the hydrogen bond in water

• Insoluble when long chain: non-polarity of C-H bonds takes precedence

How do you work out whether an alcohol is primary, secondary or tertiary?

• Primary alcohols (1º): C bonded to O-H is only bonded to one other C atom

• Secondary alcohols (2º): C bonded to O-H is bonded to two other C atoms

• Tertiary alcohol (3º): C bonded to O-H is bonded to three other C atoms

What is the functional group & suffix of aldehydes?

• Functional group: -CHO (C=O group at end of carbon chain)

• Suffix: -al (e.g. propanal)

What is the functional group & suffix of carboxylic acids?

• Functional group: -COOH (at the end of carbon chain)

• Suffix: -oic acid (e.g. propanoic acid)

What is the functional group & suffix of ketones?

• Functional group: C=O (in the middle of carbon chain)

• Suffix: -one (e.g. propanone)

How can alkenes be made from alcohols?

Eliminating water from alcohols in a dehydration reaction

What is the condition for a dehydration reaction?

Hot, concentrated H₂SO₄ catalyst

What are the two methods used to produce ethanol & other alcohols?

• Fermentation of glucose (can get from plants)

• Hydrating alkenes

What are the conditions for fermentation?

• Enzymes in yeast (catalyst)

• 35ºC

• Anaerobic conditions

What is the word & symbol equation for fermentation?

• Glucose → ethanol + carbon dioxide

• C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

What is the condition for the hydration of alkenes to produce alcohols?

Concentrated H₂SO₄ catalyst & room temperature

What happens to ethanol produced by fermentation?

It is separated by fractional distillation & can be used as a biofuel

What are the advantages & disadvantages of hydrating alkenes?

Advantage:

• very fast & pure

Disadvantages:

• ethene from crude oil is a finite resource

• continuous process, meaning expensive equipment is needed (but low labour costs)

What are the advantages & disadvantages of fermentation?

Advantages:

• sugars (glucose) are a renewable resource

• batch process, meaning cheap equipment is needed (but high labour costs)

Disadvantage:

• very slow & impure, meaning it needs further processing

Write equations to support the statement that ethanol produced by fermentation is a carbon neutral fuel

• 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (photosynthesis)

• C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

• 2C₂H₅OH + 6O₂ → 4CO₂ + 6H₂O

What is a biofuel?

A fuel that is made from recently deceased biological material

What are the advantages of biofuels?

• They are a renewable energy source (more sustainable)

• They are carbon neutral

What are the disadvantages of biofuels?

• Land being used to grow crops for fuel can’t be used to grow food

• Trees cut down to create more land to grow crops for biofuels destroys habitats & reduces photosynthesis

• Fertilisers can pollute waterways & release nitrous oxide

Define carbon neutral

No net addition of carbon dioxide into the atmosphere

Why is making ethanol using fermentation not entirely carbon neutral?

Fossil fuels need to be burned to power the machinery used to make fertilisers for crops & the machinery to harvest the crops → burning fuels will produce carbon dioxide

What is the oxidising agent [O] used to oxidise alcohols?

Acidified potassium dichromate (K₂Cr₂O₇):

• reduces itself & will turn from orange (dichromate ion) to green (chromium ion)

What is the difference between the oxidation of primary & secondary alcohols?

• 1º alcohols can be partially oxidised to aldehydes & then fully oxidised then to carboxylic acids

• 2º alcohols can only be oxidised to ketones

What are the conditions needed for oxidation of aldehydes & carboxylic acids?

• Aldehydes: heat with acidified potassium dichromate & distil off immediately

• Carboxylic acids: heat with excess acidified potassium dichromate & reflux

Why can’t ketones be oxidised further & why can’t tertiary alcohols be oxidised at all?

A C-C bond would have to break

What are the conditions needed for the oxidation of a secondary alcohol?

Acidified potassium dichromate & reflux

How is Fehling’s solution used to test for aldehydes & ketones?

Fehling’s solution is blue, as it contains Cu²⁺ ions:

• if added warm to aldehydes, Fehling’s solution turns from a blue solution to a brick red precipitate

• if added warm to ketones, Fehling’s solution remains blue

How is Tollens’ reagent used to test for aldehydes & ketones?

Colourless [Ag(NH₃)₂]⁺ complex:

• if added warm to aldehydes, Tollens’ is reduced to a silver mirror

• if added warm to ketones, Tollens’ remains colourless

12.0 cm³ of 2,3,3-trimethylbutan-1-ol (Mr = 116.0) produces 6.12g of 2,3,3-trimethylbut-1-ene.

Calculate the percentage yield. (5 marks)

density of 2,3,3-trimethylbutan-1-ol = 0.818 g cm^-3

Draw the structure of an alcohol that is an isomer of pentan-2-ol that is not dehydrated when heated with concentrated sulfuric acid