Alcohols

Question 1

How do you apply IUPAC rules for nomenclature to alcohols, aldehydes, ketones and carboxylic acids limited to chains with up to 6 carbon atoms?

Answer 1

Alcohols

• Alcohol = suffix ‘ol’; e.g. methanol, ethanol, propanol.
• Chains longer than ethanol (2 C’s), number where -OH group is:
  -Primary alcohols: e.g. Propan-1-ol
  = bonded to 1 alkyl group (end of the chain)
  -Secondary alcohols: e.g. Propan-2-ol
  = bonded to 2 alkyl groups (in body of chain; middle-ish)
  -Tertiary alcohols: e.g. Propan-3-ol
  = bonded to 3 alkyl groups. (branch in the chain)
• If there are more than one -OH groups present, di-, tri-, terta- then number to say where they are:
  e.g. butane- 1, 4-diol, propane- 1, 2, 3-triol (glycerol).

Question 2

Why do alcohols have higher melting points than alkanes?

Answer 2

The -OH group means tha hydrogen bonding occurs between the molecules, also making shorter chain alcohols soluble in water due to the formation of hydrogen bonds between the -OH groups and the water molecules.

Question 3

How is ethanol produced industrially by fermentation?

Answer 3

1. Carbohydrates from plants (sugar cane, sugar beet) are broken down into sugars.
2. Sugars converted into ethanol by the action of enzymes from yeast.
3. When the solution reaches 15% ethanol, the enzymes are unable to function, yeast dies, fermentation stops. (fractional distillation is used to increase concenctration of ethanol)

• Anaerobic conditions (prevents oxidation of ethanol to ethanoic acid)
• 35°C (too cold = too slow, too high =denatured)

glucose > ethanol + carbon dioxide

Question 4

What are the economic and environmental advantages and disadvantages of this process compared with the industrial production from ethene?

Answer 4

Advantages

• Uses gentle temperatures and atmospheric pressure; lower cost to run.
• Uses renewable resources (sugars) based on plant material.

Disadvantages

• Very slow
• Batch process; inefficient, high labour costs.
• Quality of product is very low; very impure, needs further processing. (aqueous solution of ethanol produced)

Question 5

What is the meaning of the term biofuel?

Answer 5

A fuel that’s derived/produced from renewable biological sources (made from biological material that has recently died).

Question 6

Define ‘carbon neutral’.

Answer 6

An activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere.

Question 7

What is the extent to which ethanol, produced by
fermentation, can be considered to be a carbon-neutral
biofuel?

Answer 7

• CO₂ released when it is burnt is balanced by the carbon dioxide absorbed by the plant from which it was originally obtained during photosynthesis.
• HOWEVER, there are other carbon costs associated with the energy needed to transport crops and the fuel, processing the crops, making the fertilisers for plant growth, powering agricultral machinery etc.

Question 8

Show the oxidation of a primary alcohol to an aldehyde, state the conditions.

Answer 8

Primary alcohols are oxidised to aldehydes, and then to carboxylic acids.

Aldehydes: (-al; RCHO; =O and -H attached to C₁)

• Heat (gently) an excess of alcohol with controlled amount of oxiding agent (acidified potassium dichromate;
  Cr₂O₇^2-/H^+; dilute sulfuric aid) in distillation apparatus; so the aldehyde is distilled off immediately.
• E.g. ethanol + [O] > ethanal
• ethanal vaporises as soon as it forms hence distilled off (preventing further oxidation to ethanoic acid)
• Potassium dichromate solution turns from orange to green (orange dichromate ions(VI) reduced to green chromium(III) ions).

The excess of the alcohol means that there isn’t enough oxidising agent present to carry out the second stage. Removing the aldehyde as soon as it is formed means that it doesn’t hang around waiting to be oxidised anyway!

Question 9

Show the oxidation of a primary alcohol to a carboxylic acid, state the conditions.

Answer 9

Primary alcohols are vigorously oxidised to carboxylic acids.

Carboxylic acids: (-oic acid; RCOOH; =O and -OH attached to C₁)

• Heat (under reflux; vapour condenses and drips back into reaction flask) a controlled amount of alcohol with an excess of oxiding agent (acidified potassium dichromate; Cr₂O₇^2-/H⁺; concentrated sulfuric aid) under reflux, so increasing temperature to boiling means no loss of volatile (evaporating readily at normal temperatures) solvents, reactants or products.

E.g. ethanol + [O] > ethanal + [O] > ethanoic acid

• While the reaction is refluxing, any ethanol or ethanal vapour condenses and drips back into the flask until eventually it is all oxidised to ethanoic acid
• After refluxing for 20 minutes, the ethanoic acid is distilled off (boiling temperature 391K/118°C; along with any water) by rearranging the apparatus.

ethanol + 2[O] > ethanoic acid + water

Question 10

Show the oxidation of a secondary alcohol to a ketone, state the conditions.

Answer 10

• Secondary alcohols are oxidised to a ketone when heated with acidified (dilute sulfuric acid) potassium dichromate (H⁺ ions and Cr₂O₇^2- ions).
• No further oxidation is possible as the ketone has no hydrogens to be lost (no H bonded to a Carbon).
• If you look back at the second stage of the primary alcohol reaction, you will see that an oxygen “slotted in” between the carbon and the hydrogen in the aldehyde group to produce the carboxylic acid. In this case, there is no such hydrogen - and the reaction has nowhere further to go.

e.g. propan-2-ol + [O] > propanone + water
CH₃HCOHCH₃ + [O] > CH₃COCH₃ + H₂O

Question 11

Are tertiary alcohols oxidised?

Answer 11

• No, they are not easily oxidised; don’t react with acidified potassium dichromate(VI) at all; solution stays orange.
• Only oxidised through combustion.
• If you look at what is happening with primary and secondary alcohols, you will see that the oxidising agent is removing the hydrogen from the -OH group, and a hydrogen from the carbon atom attached to the -OH. Tertiary alcohols don’t have a hydrogen atom attached to that carbon
• You need to be able to remove those two particular hydrogen atoms in order to set up the carbon-oxygen double bond.

Question 12

How do you distinguish between aldehydes and ketones?

(simple chemical test)

Answer 12

Tollens’ test (silver mirror) [Ag(NH₃)₂]⁺

• Gentle oxidising agent; solution of silver nitrate in aqueous ammonia.
• (Colourless silver(I) complex ions) are reduced to silver when warmed with an aldehyde; silver coats inside of the apparatus to form a silver mirror.
• Has no effect with ketone.
• Once used commericially for making mirrors.

Benedict’s (Fehling’s) test

• Gentle oxidising agents; both contain blue copper(II) Cu²⁺ complex ions.
• These oxidise aldehydes (reduced themselves) turning brick-red from deep blue, but not with ketones, staying blue.

Cu²⁺ + e^- > Cu⁺

Question 13

How are alkenes formed from alcohols with sulfuric acid?

Answer 13

• Alcohols are dehydrated with excess hot concentrated sulfuric acid (H₂SO₄) in an elimination reaction (water is eliminated).
• Ethanol is heated with an excess of concentrated sulphuric acid at a temperature of 170°C under reflux.
• The gases produced are passed through sodium hydroxide solution to remove the carbon dioxide and sulphur dioxide produced from side reactions.
• The ethene is collected over water.
• The concentrated sulphuric acid is a catalyst.

This reaction allows you to produce alkenes from renewable resources (ethanol produced by fermentation of glucose extracted from plants).
Importance lies in production of polymers without the need for oil.

Question 14

How are alkenes formed from alcohols with Al₂O₃?

Answer 14

• Alcoholds are dehydrated by passing their vapours over heated aluminium oxide.
• An alkene is formed.
• Simple way of making gaseous alkenes like ethene. If ethanol vapour is passed over heated aluminium oxide powder, the ethanol is essentially cracked to give ethene and water vapour.
• 600K
• Dehydration of longer chain/branched alcohols may produce a mixture of alkenes, including geometric isomers.