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Chemistry > The Oxidation Of Phenylmerhanol To Benzoic Acid > Flashcards

The Oxidation Of Phenylmerhanol To Benzoic Acid Flashcards

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1
Q

Identify the two chemicals added that allow separation of the benzoic acid from the other substances after the oxidation was complete

A

HCL

Na2SO3 (Sodium Sulfite)

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2
Q

What changes were observed in the reaction vessel on addition of HCL and Sodium Sulfite and as cooling occured

A

Brown disappears and it becomes colourless, white crystals form

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3
Q

How would recrystallisation have affected the melting point range?

A

Melting point is raised and becomes narrower too

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4
Q

What affects the melting point range of benzoic acid in this experiment?

A

Recrystallisation

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5
Q

Calculate percentage yield

A

Multiply density by volume to get grams
find moles of phenylmethanol and it is equal to benzoic acid
multiply by mr of benzoic acid and get theoretical yield
actual yield / theoretical yield x 100

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6
Q

What is the appearance of phenylmethanol at room temp?

A

Colourless liquid

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7
Q

Structure of phenylmethanol

A

Benzene + CH2OH

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8
Q

What is the phenylmethanol also known as?

A

Benzyl alcohol

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9
Q

Why was excess potassium permanganate used in this reaction?

A

Oxidising agent - will oxidise phenylmethanol to benzoic acid

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10
Q

What is the purpose of sodium carbonate?

A

Provide alkaline conditions

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11
Q

Why are alkaline conditions prefered?

A

Rate of reaction is greatest

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12
Q

Balanced equation for preparation of benzoic acid

A

3C6H5CH2OH + 4KMnO4 - 3C6H5COOH + 4MnO2 +H20 + 4KOH

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13
Q

State and explain the colour change when reaction mixture is heated

A

Changes from purple to brown due to formation of MnO2 (not soluble in water)

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14
Q

Identify brown precipitate

A

Brown residue of MnO2

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15
Q

Identify source of almond smell

A

Formation of benzaldehyde

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16
Q

Draw structure of benzaldehyde

A

Benzene + CHO

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17
Q

How is the benzaldehyde present in mixture?

A

Phenylmethanol oxidised by potassium perm to Benzaldehyde

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18
Q

Why is reaction flask cooled under running water?

A

Benzoic acid is only slightly soluble in cold water, causes benzoic acid to be precipitated out ???

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19
Q

Why is HCL added?

A

Neutralise sodium carbonate
Provides an acidic medium for Mn⁴⁺ ions to be reduced
Mn⁴⁺ + 2e⁻ → Mn²⁺ (in the presence of H⁺)

20
Q

How do you know enough HCL is added?

A

Until no fizzing is apparent

21
Q

What is the intermediate compound formed?

A

Sodium Benzonate

22
Q

What is the structure of the intermediate compound?

A

Benzene + C double bond O, O-Na+

23
Q

What is the purpose of adding sodium sulfite?

A

Reducing agent - Reduces Mn4+ ions that are insoluble to Mn2+ ions which are soluble

24
Q

What is observed when sodium sulfite is added?

A

Brown - Colourless

25
Q

What are the changes in oxidation number of manganese during reaction?

A

Mn+7 - Mn+4 - Mn+2 = +7 - +4 - +2

26
Q

Why is the flask placed in ice?

A

Benzoic acid has a low solubility in cold water, therefore this helps maximise yield of crystals

27
Q

How were crystals seperated from mixture?

A

Suction - filtration with Buchner flask, Buchner funnel and filter paper

28
Q

Why is it preferable to use Suction - Filtration?

A

Speeds up filtration process and also helps dry crystals

29
Q

How could the crystals have been dried?

A

Placing filter paper with crystals on clock glass and allowing to dry overnight

30
Q

Mention three things you could do to maximise yield of benzoic acid

A

Putting conical flask of solution in ice (after addition of sodium sulfite)
Use minimum amount of boiling water (solvent)
Use fluted filter paper

31
Q

What is the overall colour change that occurs in the reaction vessel as phenylmethanol is converted to benzoic acid

A

Purple - brown - colourless

32
Q

Name the transition metal reagent used?

A

MnO4

33
Q

Explain the colour change of purple-brown-colourless by reference to the transition metal reagent used?

A

Mn+7 - Mn+4 - Mn+2

34
Q

What are the bonds that are broken in phenylmethanol?

A
  • H

- H

35
Q

What are the bonds that are formed in benzoic acid?

A

C=O

36
Q

Why is the conical flask rinsed with the filtrate from the Buchner flask?

A

To get rid of any remaining crystals

37
Q

In the recrystallisation of benzoic acid, what is collected at hot filtration?

A

Charcoal

38
Q

In the recrystallisation of benzoic acid, what is collected at filtration?

A

Purified Benzoic acid

39
Q

In the recrystallisation of benzoic acid, how is benzoic acid seperated from salt

A

Both substances soluble in hot water however benzoic acid much less soluble in cold water so comes out of solution
NaCl much more soluble in cold water so remains in solution

40
Q

Describe how benzoic acid is dried

A

Place filter paper with crystals on clock glass and put them in an oven at a low temperature.

41
Q

Give one common use of benzoic acid

A

food preservative / disinfectant

42
Q

Indicate clearly the stage of the recrystallisation process at which insoluble impurities were removed and how it was achieved

A

filtration of hot solution

remained on filter paper (in funnel)

43
Q

Indicate clearly the stage of the recrystallisation process at which soluble impurities were removed and how it was achieved

A

filtration of recrystallised benzoic acid

passed through filter paper (funnel)

44
Q

How could you have ensured that the recrystallisation was complete

A

let solution cool fully (efficiently) / cool in ice-water

45
Q

Why was an excess solution of potassium manganate (VII) used?

A

To ensure there was a sufficient amount of KMnO4 to oxidise all of
the benzyl alcohol to benzoic acid.

46
Q

What reaction occurred when the sodium sulfite was added?

A

A reduction reaction (Mn4+ reduced to Mn2+).

47
Q

What is the structure of Benzoic acid?

A

Benzene ring - C double bond to O and OH

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