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A-level Chemistry > CbD - Carbonyl compounds > Flashcards

CbD - Carbonyl compounds Flashcards

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1
Q

How can aldehydes and ketones be made?

A

By oxidising alcohols.

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2
Q

Examples of carbonyl compounds

A

Aldehydes and ketones.

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3
Q

How do you make an aldehyde?

A

Gently heat a primary alcohol with an oxidising agent.

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4
Q

What oxidising agent is usually used when making an aldehyde?

A

Acidified potassium dichromate(VI).

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5
Q

What do aldehydes oxidise further to make?

A

Carboxylic acids.

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6
Q

What do you have to do make an aldehyde not oxidise further into carboxylic acids?

A

You’ve got to distil the aldehyde out of the reaction as soon as it’s formed to stop it oxidising further.

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7
Q

How do you make a ketone?

A

Reflux a secondary alcohol with an oxidising agent.

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8
Q

What can you use to test for aldehydes and ketones?

A

Fehling’s solution and Tollens reagent.

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9
Q

What can Fehling’s solution be used to test for?

A

Aldehydes and ketones.

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10
Q

What is Fehling’s solution?

A

A blue solution of complexed copper(II) ions dissolved in sodium hydroxide. Fehling’s A and Fehling’s B.

(Fehling’s A = Cu2+ weak oxidising agent. Fehling’s B = alkaline solution containing a bidentate ligand.)

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11
Q

What does Fehling’s solution do to aldehydes?

A

Oxidises them to carboxylic acids.

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12
Q

Result when Fehling’s solution is mixed with an aldehyde

A

Blue solution –> Brick red precipitate

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13
Q

Result when Fehling’s solution is mixed with a ketone

A

Nothing happens

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14
Q

Explain how you’d use Fehling’s solution to test for aldehydes and ketones

A

1) Add 2cm3 of Fehling’s solution to a test tube (or Benedict’s solution). It should be a clear blue solution.
2) Add 5 drops of the aldehyde or ketone to the test tube.
3) Put the test tube in a hot water bath to warm it for a few minutes.

Aldehyde - blue solution to brick red precipitate.
Ketone - nothing happens.

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15
Q

Explain what is happening when you add Fehling’s solution to an aldehyde

A

Blue copper(II) ions (Cu^2+) in the Fehling’s solution are reduced (the electrons come from the oxidation of the aldehyde) forming copper(I) ions (Cu+) which then form a brick red precipitate of copper(I) oxide (Cu2O).

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16
Q

How do you make Tollens reagent?

A

1) Put 2cm3 of 0.1 mol dm-3 (colourless) silver nitrate solution in a test tube.
2) Add a few drops of dilute sodium hydroxide solution. A light brown precipitate should form (silver oxide).
3) Add drops of dilute ammonia solution until the brown precipitate dissolves completely. This is now Tollens reagent.

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17
Q

What is Tollens reagent?

A

Dissolved silver(I) oxide (Ag2O) in ammonia solution to form the [Ag(NH3)2]+ (aq) complex ion.

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18
Q

What can Tollens reagent be used to test for?

A

Aldehydes and ketones.

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19
Q

How do you use Tollens reagent to test for aldehydes and ketones?

A
  • Place the test tube with Tollens reagent in a hot water bath and add 10 drops of aldehyde or ketone.
  • Wait for a few minutes.

Aldehyde - a silver mirror (a thin coating of silver) forms on the walls of the test tube.
Ketone - nothing happens.

20
Q

Explain what is happening when you add Tollens reagent to an aldehyde

A

Diamminesilver ions in the Tollens reagent (the silver(I) ions) are reduced (the electrons come from the oxidation of the aldehyde) to elemental silver, which appears as a silvery layer on the inner surface of the test tube.

21
Q

Why must the test tubes be heated in a water bath rather than using a bunsen when testing for the aldehydes and ketones?

A

Aldehydes and ketones are flammable.

22
Q

Equation of what is happening when testing for aldehydes using Fehling’s solution

A

Cu^2+ (aq) + e- –> Cu+ (s)

23
Q

What is the aldehyde oxidised into when it reacts with Fehling’s solution/Tollens reagent?

A

RCHO is oxidised to the carboxylate ion RCOO-.

24
Q

Equation of what is happening when testing for aldehydes using Tollens reagent

A

[Ag(NH3)2]+ (aq) + e- –>Ag (s) + 2NH3 (aq)

25
Q

Colour change when an aldehyde reacts with acidified dichromate

A

orange –> green

26
Q

Colour change when an aldehyde reacts with Fehling’s solution

A

blue –> brick-red

27
Q

Colour change when an aldehyde reacts with Tollens reagent

A

clear –> silver (silver mirror effect)

28
Q

Colour change when a ketone reacts with acidified dichromate

A

stays orange

29
Q

Colour change when a ketone reacts with Fehling’s solution

A

stays blue

30
Q

Colour change when a ketone reacts with Tollens reagent

A

no change (colourless still)

31
Q

What is the original colour of acidified dichromate?

A

orange

32
Q

What is the original colour of Fehling’s solution?

A

blue

33
Q

What is the original colour of Tollens reagent?

A

colourless

34
Q

Through what reaction will hydrogen cyanide react with carbonyls?

A

Nucleophilic addition

35
Q

What is produced when hydrogen cyanide reacts with carbonyl compounds?

A

Cyanohydrins

36
Q

What are cyanohydrins?

A

Molecules with a -CN and an -OH group.

37
Q

What is hydrogen cyanide?

A

A weak acid

38
Q

What does it mean that hydrogen cyanide is a weak acid?

A

It partially dissociates in water to form H+ ions and CN- ions.

39
Q

How does hydrogen cyanide react with carbonyls by nucleophilic addition?

A

1) The CN- group attacks the partially positive carbon atom and donates a pair of electrons. Two electrons from the double bond transfer to the oxygen.
2) H+ (from either hydrogen cyanide or water) bonds to the oxygen to form the hydroxyl group (-OH).

40
Q

Why is hydrogen cyanide bad?

A

It is a highly poisonous gas.

41
Q

How does the nucleophilic addition of hydrogen cyanide with carbonyl compounds need to be carried out?

A

In a fume cupboard - as it is a highly poisonous gas.

42
Q

What are cyanohydrins also known as?

A

Hydroxynitriles.

43
Q

What is the cyano- functional group (nitrite group)?

A

N≡C^-

44
Q

What is the cyano- group as an ion in an inorganic salt called?

A

Cyanide

45
Q

What is the cyano- group in an organic compound called?

A

A cyano- group or nitrile group

A-level Chemistry (51 decks)

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