Chemistry 3.5- Alcohols

How can we classify alcohols?

• Primary alcohols- the carbon atom bonded to the -OH group is attached to one other carbon atom

• Secondary alcohols- the carbon atom bonded to the -OH group is attached to two other carbon atoms

• Tertiary alcohols- the carbon atom bonded to the -OH group is attached to three other carbon atoms

Which alcohols can be oxidised and what to?

• Primary alcohols can be oxidised into aldehydes, which can be further oxidised into carboxylic acids

• Secondary alcohols can be oxidised into ketones

• Tertiary alcohols can’t be oxidised

What can be used to oxidise alcohols and what change occurs?

• Acidified potassium dichromate (VI)

• The orange Cr 6+ ions are reduced to green Cr 3+ ions when alcohols are oxidised

• So the solution turns from orange to green

What colour flame do alcohols burn with?

Blue, but is often hard to see

What are the physical properties of alcohols?

• The polar hydroxyl group can form hydrogen bonds, so they have higher melting and boiling points than alkanes with similar RFMs

• However, the non-polar hydrocarbon chain can’t form hydrogen bonds, so shorter chain alcohols are soluble in water due to the hydroxyl group, while longer chains aren’t, as the non-polar end takes dominance

How can we produce aldehydes from alcohols? Why does this work?

• Distillation- the alcohol can be heated gently with a small amount of potassium chromate (oxidising agent) and dilute sulfuric acid, and the aldehyde can be distilled out, without collecting a carboxylic acid

• This is because aldehydes have no hydroxyl group, so they don’t form H bonds, so they have much lower boiling points than alcohols and carboxylic acids, and will evaporate first

• To prevent the aldehyde evaporating off, the receiver is cooled in an ice bath

How can we produce carboxylic acids from alcohols? Why does this work?

• Reflux- the alcohol can be heated with an excess of potassium chromate (oxidising agent) and dilute sulfuric acid, and allowed to oxidise until the mixture is fully carboxylic acid

• This works because the aldehyde produced evaporates, cools in the condenser and falls back into the flask to be further oxidised to a carboxylic acid

How can we test to distinguish between aldehydes and ketones?

• Fehling’s solution- turns from clear blue to a red precipitate when heated gently with an aldehyde (as the blue Cu 2+ ions oxidise the aldehyde and are themselves reduced to Cu + in the red precipitate Cu2O)

• Tollen’s solution- turns from clear and colourless to a silver mirror when heated gently with an aldehyde (as the dissolved Ag + ions oxidise the aldehyde and are themselves reduced to insoluble Ag atoms)

How can we test for carboxylic acids?

• Carboxylic acids will neutralise sodium carbonate solution to produce bubbles of carbon dioxide

What is the general oxidation equation for alcohols into aldehydes?

RCH2OH + [O] → RCHO + H2O

What is the general oxidation equation for alcohols into carboxylic acids?

RCH2OH + 2[O] → RCOOH + H2O

What is the general oxidation equation for aldehydes into carboxylic acids?

RCHO + [O] → RCOOH

What is the general oxidation equation for alcohols into ketones?

RCHOHR + [O] → RCOR + H2O

Outline the mechanism for the dehydration of alcohols into alkenes by elimination

What are the conditions for the dehydration of alcohols into alkenes?

• Heating the alcohol and passing the vapours over a hot catalyst such as concentrated phosphoric acid or aluminium oxide

Why is the dehydration of alcohols to alkenes useful?

• It produces alkenes that can be used to form addition polymers, without using monomers derived from crude oil

How can we produce ethanol by fermentation? Include the formula

• Fermenting glucose (usually from sugarcane) with yeast

• 35°C

• Anaerobic conditions (prevents oxidation to carboxylic acid)

• At around 15% ethanol the yeast dies, so fractional distillation is used to produce higher concentrations

• C₆H₁₂O₆ —> 2C₂H₅OH + 2CO₂

How can we produce ethanol from crude oil?

• Long hydrocarbon chains can be cracked to produce ethene

• Ethene can undergo hydration (addition of water) to form ethanol, in the conditions:

• 300-600

• 50-100 atm

• Concentrated phosphoric acid catalyst

• Steam

What are the positives and negatives of producing ethanol by fermentation?

Pros

• It is renewable as sugarcane is very quick to grow- crude oil is not renewable as it takes millions of years to form

• It can be viewed as carbon neutral as the carbon dioxide released when ethanol is combusted is balanced by the carbon dioxide absorbed through photosynthesis of the sugarcane

Cons

• In practice it isn’t fully carbon neutral, considering transportation, irrigation and processing (fractional distillation)

• The monoculture production of sugarcane takes up lots of deforested land in rainforests and reduces biodiversity

What are biofuels?

Fuels produced from biomass eg. plants or algae