Oxidation of alcohols

Question 1

What chemical is used to oxidise alcohols?

Answer 1

• 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.

Question 2

What can primary alcohols be oxidised into, what can they be further oxidised into?

Answer 2

• Oxidised into aldehydes.
• Further oxidised into carboxylic acids.

Question 3

What can secondary alcohols be oxidised into?

Answer 3

• Can be oxidised into ketones.

Question 4

What can tertiary alcohols be oxidised into?

Answer 4

• TRICK QUESTION!
• Can’t be oxidised using dichromate, only by burning them.

Question 5

What do aldehydes look like?

Answer 5

• CHO functional group at end of chain.

Question 6

What do ketones look like?

Answer 6

• Have C= O in middle of the chain.

Question 7

What do carboxylic acids look like?

Answer 7

• Have COOH group at the end of chain.

Question 8

How do we represent the oxidising agent (potassium dichromate) in oxidation of alcohol equations?

Answer 8

⌊O⌋
(square brackets needed.)

Question 9

What are the conditions required to prouce ONLY an aldehyde from an primary alcohol, using potassium dichromate as oxidising agent?

Answer 9

• 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.)

Question 10

What are the conditions required to prouce carboxylic acid from primary alcohols? What is the equation for this (from formation of aldehyde stage?)

Answer 10

• CONDITIONS: reflux/ excess of oxidising agent.
• Aldehyde + ⌊O⌋ –> carboxylic acid

Question 11

General equation for formation of aldehyde from primary alcohol.

Answer 11

Primary alcohol + ⌊O⌋ –> aldehyde + water

Question 12

Why do we carry out reaction for formation of carboxylic acid from primary alcohol under reflux?

Answer 12

• 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.

Question 13

Write THE OVERALL equation for formation of carboxylic acid from a primary alcohol. What is the condition for this reaction?

Answer 13

1⁰-ol + 2⌊O⌋ –> c.acid + water
Reflux.

Question 14

What is the overall equation for the oxidation of a secondary alcohol? What is the condition needed?

Answer 14

2⁰-ol + ⌊O⌋ –> ketone + water
Reflux

Question 15

What does the ⌊O⌋ in oxidation of alcohols reactions represent?

Answer 15

• Represents gain of oxygen from oxidising agent, potassium dichromate.

Question 16

Symbols used to represent primary, secondary, tertiary alcohols.

Answer 16

1⁰ - primary
2⁰ - secondary
3⁰- tertiary

Question 17

Compare the boiling points of: ethanol, ethanal, ethanoic acid. What IM forces are between them?

Answer 17

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.

Question 18

What 2 solutions can be used to distinguish between aldehydes and ketones?

Answer 18

• Fehling’s solution.
• Tollens’ reagant.

Question 19

What are Fehling’s solution
and Tollens’ reagent known as?

Answer 19

• Known as oxidising agents that oxidise aldehydes, but not ketones.

Question 20

Positive/ negative results for Fehling’s solution.

Answer 20

• 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.

Question 21

Why is Fehling’s solution blue to start off with?

Answer 21

• Blue as it contains Cu2+ ions.

Question 22

What ions does Tollen’s reagant contain?

Answer 22

⌊Ag(NH₃)₂⌋⁺

Question 23

Positive/ negative results for Tollens’ reagant.

Answer 23

• 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.

Question 24

How can we ensure that the Fehling’s solution and Tollens’ reagant added to the aldehyde/ ketone is warm?

Answer 24

• 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.

Question 25

Why is potassium dichromate (VI) written with roman numeral for 6? In terms of oxidation states - include the order you assign oxidation states in?

Answer 25

• K₂Cr₂O₇ (formula)
• Assign oxidation state to K first (because it’s a metal)
• K₂ = +2.
• Then assign to O = (7x -2) = -14.
• Cr₂ needs to SUCK up whatever oxidation state to make. Cr₂= +12.
• Cr = +6.