Alcohols

Difference in boiling temperatures of alcohols compared to their corresponding alkanes

Higher in alcohols

Why do alcohols have higher boiling temperatures than their corresponding alkanes?

Addition of hydrogen bonding adds to the VdW forces and increases overall intermolecular forces of attraction

Trend in solubility of alcohols with increasing carbon chain length

Solubility decreases because the large, insoluble hydrocarbon portion of the molecule outweighs the influence of the -OH group.

Are alcohols more reactive than alkanes?

Yes

Why are alcohols more reactive than alkanes?

The C-O and O-H bond in the molecules are polar and easier to break than C-C or C-H bonds.

How do alcohols burn in air?

Clean, colourless flame

Equation for alcohol reaction with sodium

Alcohol + sodium → sodium alkoxide salts + hydrogen

Properties of alkoxides

Ionic salts (CH₃CH₂O^-Na⁺)

What happens in a dehydration reaction?

The loss of a water molecule from an alcohol through an elimination reaction forming an alkene.

An -OH group leaves the molecule from one carbon and a hydrogen atom leaves from an adjacent carbon atom.

How does a dehydration reaction work?

Heating an alcohol with concentrated phosphoric acid (H₃PO₄) as a catalyst OR heating the alcohol to 600K with Al₂O₃ as a catalyst.

Uses for producing alkenes from alcohols

Provides a possible route to polymers without having to use monomers derived from crude oil.

Oxidising agent used in oxidation of alcohol

Dichromate ion. Sulphuric acid is used to provide H⁺ ions. Therefore it is called acidified potassium dischromate represented as [o].

Colour change of dichromate ions when they are reduced

Orange to green

Which alcohols cannot be oxidised?

Tertiary alcohols.

Equation for oxidation of primary alcohols

Primary alcohol --_^[o]>> aldehyde --_^[o]>> carboxylic acid

Partial oxidation

Secondary alcohol → Aldehyde

Why is it hard to obtain an aldehyde from the oxidation of a primary alcohol?

As soon as the aldehyde forms, it instantly oxidises to form a carboxylic acid.

How is an aldehyde obtained from the oxidation of a primary alcohol?

Distil off the aldehyde from the reaction mixture.

What are the products when a secondary alcohol is oxidised?

A ketone and water

How is a secondary alcohol oxidised?

The secondary alcohol and acidified potassium dichromate (VI) are heated under reflux.

What are the four tests for an aldehyde?

Acidified potassium dichromate

Fehling’s solution

Benedict’s solution

Tollen’s reagent

What is Fehling’s solution?

Copper (II) with tartarate ions in sodium hydroxide solution

What is Benedict’s solution?

Copper (II) with citrate ions in sodium carbonate solution

What is Tollen’s reagent?

A solution of silver nitrate in aqueous ammonia which oxidses aldehydes to carboxylic acids.

What happens when acidified potassium dichromate is added to an aldehyde?

Colour change from orange to green

What happens when Fehling’s solution is added to an aldehyde in a warm water bath?

Colour change from blue solution to brick red precipitate

What happens when Benedict’s solution is added to an aldehyde in a warm water bath?

Colour change from blue solution to brick red precipitate

Why is there a colour change when Fehling’s or Benedict’s solution is added to an aldehyde?

The copper ions are reduced (Cu²⁺ to Cu¹⁺)

What happens when Tollen’s reagent is added to an aldehyde in a warm water bath?

Formation of a silver mirror.

Uses of ethanol

Alcoholic drinks, solvent in cosmetics, burnt as fuel,

What are the two main methods of producing ethanol?

Fermentation of sugar using yeast

Reacting ethene with steam using a catalyst of phosphoric acid

Steps to producing ethanol from ethene

1. Produce ethene from cracking of crude oil

2. Hydrate the ethene by addition of water across the double bond

Equation for hydration of ethene to form ethanol

CH₂=CH₂ + H₂O → C₂H₅OH

Conditions for hydration of ethene to form ethanol

Acid Catalyst (hydrated H₃PO₄/phosphoric acid)

300C temperature

High pressure

What type of addition reaction is hydration of ethene?

Acid catalysed addition reaction

Advantages of hydrating ethene as a method to form ethanol

• Faster reaction

• Purer product

• Continuous process → Cheaper manpower

Disadvantages of hydrating ethene as a method to form ethanol

• High technology equipment needed (expensive initial cost)

• Ethene is a non-renewable resource

• High energy costs for pumping to produce high pressures

Process of producing ethanol by the fermentation of sugar

The sugars (glucose) in plants (sugar cane, sugar beet) are broken down and converted into ethanol by the action of enzymes from yeast.

What is the fermentation of sugar process also known as?

Anaerobic respiration

Equation for fermentation of sugar

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

Conditions for fermentation of sugar

Enzyme from yeast

No air

35-38C temperature

What is the optimum temperature for fermentation of sugar

38C

Why is fermentation carried out in an absence of air?

To prevent the oxidation fo ethanol to form ethanoic acid (vinegar)

When does fermentation stop?

When the fermenting solution contains 15% ethanol, the enzymes are unable to function.

The ethanol is then distilled from the mixture by fractional distillation

Advantages of fermenting sugar as a method to form ethanol

• Sugar is a renewable resource

• Production uses low level technology (cheaper equipment)

Disadvantages of fermenting sugar as a method to form ethanol

• Batch process which is slow and has high production costs

• Ethanol isn’t pure and needs purifying by fractional distillation

• Depletes land use for growing food crops

What is a biofuel?

A fuel produced from plants

Characteristics of ethanol

Biofuel when produced from fermentation

Therefore, carbon neutral

What does carbon neutral mean?

Has no net annual carbon emissions to the atmosphere

Equation for removal of CO₂ by photosynthesis

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Carbon analysis of photosynthesis

Six CO₂ molecules are removed from the atmosphere when plants undergo photosynthesis to produce 1 molecule of glucose

Equation for production of CO₂ by fermentation

C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂

Equation for production of CO₂ by combustion

2CH₃CH₂OH + 6O₂ → 4CO₂ + 6H₂O

Carbon analysis of fermentation and combustion

When 1 molecule of glucose is fermented it produces 2 molecules of ethanol

When these 2 molecules of ethanol are burned, they produce 4 moles of CO₂

Overall carbon analysis of production of ethanol by fermentation of sugar

For every 6 molecules of CO₂ absorbed, 6 molecules of CO₂ are emitted.

Therefore, there is no net contribution of CO₂ to the atmosphere.

Why might production of ethanol by fermentation of sugar not always be regarded as carbon neutral?

Any CO₂ emissions associated with the energy needed to transport the crops or fuel to process the crops into ethanol is not taken into account.