Oxidation of alcohols Flashcards
What chemical is used to oxidise alcohols?
- Acidified potassium dichromate (VI) (K₂Cr₂O₇)
- Mild oxidising agent (it is reduced itself) Will turn from orange (Cr₂O₇²- dichromate ion) to green (Cr³⁺- chromium ion) after it has oxidised the alcohol.
What can primary alcohols be oxidised into, what can they be further oxidised into?
- Oxidised into aldehydes.
- Further oxidised into carboxylic acids.
What can secondary alcohols be oxidised into?
- Can be oxidised into ketones.
What can tertiary alcohols be oxidised into?
- TRICK QUESTION!
- Can’t be oxidised using dichromate, only by burning them.
What do aldehydes look like?
- CHO functional group at end of chain.
What do ketones look like?
- Have C= O in middle of the chain.
What do carboxylic acids look like?
- Have COOH group at the end of chain.
How do we represent the oxidising agent (potassium dichromate) in oxidation of alcohol equations?
⌊O⌋
(square brackets needed.)
What are the conditions required to prouce ONLY an aldehyde from an primary alcohol, using potassium dichromate as oxidising agent?
- Needs to be distilled.
- This is because we need to make sure no carboxylic acid is in our product from any further oxidation that occurred.
- Use disilation to separate aldehyde from alcohol/ any carboxylic acid formed (lower bp than the alcohol/c. acid.)
What are the conditions required to prouce carboxylic acid from primary alcohols? What is the equation for this (from formation of aldehyde stage?)
- CONDITIONS: reflux/ excess of oxidising agent.
- Aldehyde + ⌊O⌋ –> carboxylic acid
General equation for formation of aldehyde from primary alcohol.
Primary alcohol + ⌊O⌋ –> aldehyde + water
Why do we carry out reaction for formation of carboxylic acid from primary alcohol under reflux?
- Reflux allows strong heating without losing volatile reactants/ products. Aldehydes evaporate, condense and fall back into the flask so they can be further oxidised to produce carboxylic acid.
Write THE OVERALL equation for formation of carboxylic acid from a primary alcohol. What is the condition for this reaction?
1⁰-ol + 2⌊O⌋ –> c.acid + water
Reflux.
What is the overall equation for the oxidation of a secondary alcohol? What is the condition needed?
2⁰-ol + ⌊O⌋ –> ketone + water
Reflux
What does the ⌊O⌋ in oxidation of alcohols reactions represent?
- Represents gain of oxygen from oxidising agent, potassium dichromate.
Symbols used to represent primary, secondary, tertiary alcohols.
1⁰ - primary
2⁰ - secondary
3⁰- tertiary
Compare the boiling points of: ethanol, ethanal, ethanoic acid. What IM forces are between them?
In order of boiling point:
- Ethanoic acid (hydrogen bonding)
- Ethanol (hydrogen bonding)
- Ethanal (dipole-dipole attractions)
- Ethanoic acid has higher boiling point than ethanol because it has a larger Mr, so has increased/ stronger Van der Waals forces.
What 2 solutions can be used to distinguish between aldehydes and ketones?
- Fehling’s solution.
- Tollens’ reagant.
What are Fehling’s solution
and Tollens’ reagent known as?
- Known as oxidising agents that oxidise aldehydes, but not ketones.
Positive/ negative results for Fehling’s solution.
- Positive: Fehling’s solution added to aldehyde/ ketone and placed in hot water bath.
- In aldehydes, Fehling’s solution will go from blue SOLUTION to brick-red precipitate - Cu₂O.
- Ketones: Fehling’s solution remains blue.
Why is Fehling’s solution blue to start off with?
- Blue as it contains Cu2+ ions.
What ions does Tollen’s reagant contain?
⌊Ag(NH₃)₂⌋⁺
Positive/ negative results for Tollens’ reagant.
- Tollens’ reagant is addded to aldehydes/ ketones and placed in water bath.
- Positive: Added to aldehydes; Tollens’ reduced to silver precipitate - which coats inside of test-tube.
- Ketones: no silver precipitate is formed.
How can we ensure that the Fehling’s solution and Tollens’ reagant added to the aldehyde/ ketone is warm?
- Place the solution with aldehyde/ ketone in test-tube in water bath, add Fehling’s solution OR Tollens’ reagant to the test-tube in the water bath.
