Chemistry - 3.3.5: Alcohols

What conditions are used for fermentation and why (4)?

- Yeast (or zymase, the enzyme)
- 30-42°C: if too low, too slow RoR - if too high, enzymes denature
- Anaerobic: prevent oxidation of alcohol to ethanoic acid (also no anaerobic respiration)
- Aqueous / in solution

Functional group of alcohols

-OH

When heating under reflux, do you need a thermometer?

No

General formula of alcohols

Cn H (2n+1) OH - shortened to ROH

Are alcohols reactive?

Yes - they are relatively reactive

How do you name alcohols?

- Use -ol as a suffix; use hydroxy- as a prefix if there are other functional groups present
- Use di, tri, tetra if there are multiple -OH groups

Primary alcohol

The carbon with the -OH group has is bonded to one carbon atom (so has one R group)

Secondary alcohol

The carbon with the -OH group has is bonded to 2 carbon atoms (so has two R groups)

Tertiary alcohols

The carbon with the -OH group has is bonded to 3 carbon atoms (so has three R groups)

Describe the physical properties of alcohols

1. Higher MP/BP than alkenes: has hydrogen bonds between molecules due to -OH group
2. Ones with short hydrocarbon chains are soluble in water (hydrogen bonding predominates), while those with longer chains are insoluble (the non-polar hydrocarbon chain predominates)

Why are alcohols very important in industrial chemistry?

They are used as intermediates since they are easily made and converted into other compounds

What is ethanol used for (4)?

Intermediate to make other organic chemicals, aftershave, perfumes, drugs, inks

2 ways of producing ethanol

Industrially (from crude oil) or by fermentation

How is ethanol produced industrially? (include the equation and the catalyst needed)

Ethene is reacted with steam (hydration reaction) - phosphoric acid catalyst
CH2=CH2 + H2O -> C2H5OH [phosphoric acid catalyst]

What type of reaction occurs when ethanol is produced industrially and what happens?

Hydration reaction - water is added across the double bond

How is ethanol produced by fermentation?

Anaerobic respiration of glucose by yeast:
C6H12O6 (aq) -> 2C2H5OH + 2CO2

Why is ethene important?

It is the starting material for poly(ethene) and other chemicals

How is ethene produced?

Dehydrating ethanol made from sugar (renewable)

Carbon-neutral

There is no net CO2 emission to the atmosphere - the carbon dioxide released when a fuel is burnt is balanced by the carbon dioxide absorbed from the atmosphere by the plant from which it was originally obtained (during photosynthesis)

Biofuel

A fuel derived or produced from renewable biological sources

Elimination reactions

Reactions where a small molecule leaves the parent molecule (e.g. water in dehydration reactions) - this forms a double bond

What happens during an elimination/dehydration reaction of alcohols?

Step 1: The lone pair of electrons on O of -OH is transferred to H+ ion from acid catalyst, making the O have a positive charge (O+)
Step 2: The electrons from the C-O bond are transferred to the O+. The electrons from the C-H bond of the adjacent carbon is transferred to the C-C bond (between the carbon of C-O and the adjacent carbon), producing an alkene, water molecule and H+ ion

What conditions are needed for elimination reactions to dehydrate alcohols?

Excess hot concentrated sulfuric acid / phosphoric acid or by passing vapours over heated aluminium oxide

What forms after a dehydration reaction?

An alkene and a molecule of water

What are primary alcohols oxidised to?

Aldehydes [RCHO]

What are aldehydes oxidised to?

Carboxylic acids [RCOOH]

What are secondary alcohols oxidised to?

Ketones (both have the letter O) [R2CO]

What are tertiary alcohols oxidised to?

None - they aren't easily oxidised

Why can't tertiary alcohols or ketones be oxidised?

Oxidation would need a C-C bond to break, rather than C-H bond

What oxidising agent is used to oxidise alcohols?

Acidified potassium dichromate (acidified with dilute sulfuric acid)

What happens when acidified potassium dichromate is used to oxidise alcohols?

Colour change from orange to green: orange dichromate (VI) ions are reduced to green chromium (III) ions

How do you oxidise a primary alcohol to an aldehyde? (2)

- Dilute acid + less potassium dichromate than is needed for complete oxidation
- Distillation used as it vaporises as soon as it is formed and distils off, stopping it from being oxides further, while unreacted ethanol remains in the flask

What symbol is used to represent oxygen from the oxidising agent?

[O]

Equation for formation of ethanal from ethanol

CH3CH2OH (l) + [O] -> CH3CHO (g) + H2O (l)

How do you oxidise a primary alcohol to a carboxylic acid? (2)

- Concentrated sulfuric acid and excess potassium dichromate to ensure complete oxidation
- Reflux used so any ethanol/ethanal condenses and drips back into the flask - then later distils off any ethanoic acid (not needed)

Describe what happens when a reaction mixture is refluxed, explain why it is necessary and draw a diagram (3)

- Mixture of liquids is heated to boiling point for prolonged time
- Vapour forms; escapes from liquid mixture, condenses back into a liquid and returns to liquid mixture
- Any initial products that evaporates at the start can then be oxidised

How do you oxidise a secondary alcohol to a ketone? (2)

- Concentrated sulfuric acid + excess potassium dichromate
- Reflux use

Aldehydes vs ketones

Both have C=O group (carbonyl) - for aldehydes, C=O at end of hydrocarbon chain + -al suffix, while in ketones, C=O in body of hydrocarbon chain + -one suffix

Equation for formation of ethanoic acid from ethanol

CH3CH2OH (l) + 2[O] -> CH3COOH (g) + H2O (l)

Two tests for aldehydes and ketones

Tollens' reagent and Fehling's solution

How to test for tertiary alcohols?

React with acidified potassium dichromate - if no colour change (stays orange), then no oxidation occurred and tertiary alcohols are present (colour change to green means primary/secondary alcohols are present)

How to carry out the Tollens' test?

Warm sample with Tollens' reagent: if aldehydes are present, a silver mirror forms - oxidises aldehydes but has no effect on ketones

How to carry out the Fehling's test?

Warm sample with Fehling's reagent; if aldehydes are present, colour change from blue to brick red + red precipitate - oxidises aldehydes but has no effect on ketones

What is Tollens' reagent?

A solution of silver nitrate in aqueous ammonia

What is in Fehling's reagent?

Blue copper (II) ions

Advantages and disadvantages of using fermentation instead of hydration to produce ethanol

ADVANTAGES (4):
1. Low technology
2. Renewable feedstock/resource
3. Allowed for use in drinks or perfume
4. Considered to be 'green'

DISADVANTAGES (5):
1. Slow
2. Low yield
3. Significant land use
4. Has to be distilled
5. Labour intensive

Dehydration

The elimination of water from a compound / molecule

Equation for formation of ethanoic acid from ethanal

CH3CH2CH2CH2CHO + [O] -> CH3CH2CH2CH2COOH

What is the role of OH- ions in elimination reactions?

Base

Why are small glass beads added when producing carboxylic acids?

To prevent 'bumping' (prevent large bubbles, ensure only small bubbles)

When producing an alkene by a dehydration reaction, why should you use sodium carbonate solution to wash the distillate?

To neutralise and remove the phosphoric acid

Describe how to use distillation apparatus to produce a sample of ethanol and draw a diagram (6)

1. Heat the flask containing water and ethanol with a Bunsen burner, producing water and ethanol vapours
2. The vapours pass up the fractionating column
3. The water and ethanol are separated as the water condenses back into the flask so only ethanol vapour passes into the condenser
4. Observe the thermometer to keep the temperature at or slightly below the boiling point of ethanol
5. Use the condenser to cool the vapours and condense the ethanol back into a liquid
6. Can put collection vessel in ice bath to reduce evaporation of the sample

Why might ethanol produced from fermentation not be carbon-neutral?

Planting / harvesting / distillation of the ethanol solution uses fossil fuels, which releases CO2

Why is nucleophilic substitution not used to produce alcohols?

- Lower yield
- Slow rate of reaction
- Need to produce the halogenoalkane first

What are the conditions used for the hydration of ethene to form ethanol? (4)

1. High temperature
2. Pressure of 50atm </= P </= 100atm
3. Excess ethene
4. Excess steam
5. Concentrated phosphoric acid

In a condenser, where does water go in/out?

Water goes in at the bottom and goes out at the top

When carrying out distillation, why do you need a bung and thermometer?

To prevent other gases from escaping from the reaction mixture so only your desired product is formed and to ensure that the temperature is within a suitable range

When carrying out distillation / oxidation, why is it better to use a water bath rather than directly heat with a bunsen burner?

The alcohol is flammable

Outline the mechanism for the formation of ethanol by the reaction of ethene with steam in the presence of an acid catalyst (3)

1. Pair of electrons from C=C is transferred to H+ from acid catalyst
2. This forms a carbocation. A water molecule with a lone pair of electrons transfers its electrons to it.
3. The oxygen atom then has a positive charge, so one of the hydrogen atoms bonded to it transfers its electrons to it (forming an alcohol and a H+ ion)

Equation for the formation of propanone from propan-2-ol.

CH3CH(OH)CH3 + [O] -> CH3C(O)CH3 + H2O

What do Tollens' reagent and Fehling's reagent do?

They oxidise aldehydes

What type of reaction is a hydration reaction?

Electrophilic addition

Write equations to support the idea that ethanol is a carbon-neutral biofuel

Production of CO2 with fermentation:
C6H12O6 -> 2C2H5OH + 2CO2

Production of CO2 with combustion:
2C2H5OH + 7 O2 -> 4CO2 + 6H2O

Production of glucose and removal of CO2 with photosynthesis:
6CO2 + 6H2O -> C6H12O6 + 6O2

Do aldehydes or ketones have hydrogen bonding?

No - there are no hydrogen atoms bonded directly to the oxygen atom

What type of intermolecular bonding is present between aldehydes and ketones?

Permanent dipole-dipole forecs and Van der Waal forces

What happens if the condenser in reflux/distillation is in the wrong direction?

If in the wrong direction, condenser is not full of water and is not cool enough, so the product may not condense

Write a half-equation for the overall oxidation of ethanol into ethanoic acid.

CH3CH2OH + H2O → CH3COOH + 4H⁺ + 4e^-

2 ways to maximise the yield after distillation

1. Keep the temperature above the desired product’s boiling point but below the other compounds’ boiling points

2. Cool the distillate / collecting vessel

When carrying out distillation, how should the thermometer be positioned?

Its bulb should be level with the side arm