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A-Level Chemistry > 26 Carbonyls > Flashcards

26 Carbonyls Flashcards

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1
Q

What is the carbonyl functional group?

A

C=O

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2
Q

What do aldehydes end with?

A

-Al

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3
Q

What do ketones end with?

A

-One

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4
Q

What can aldehydes be oxidised to?

A

Carboxylic acids (when acidified with potassium dichromate ions)

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5
Q

Why do carbonyl compounds react differently compared to alkenes?

A

Due to the nature of the double bond.

The C=C bond in alkenes is non-polar whereas the C=O bond in carbonyl compounds is polar.

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6
Q

Why is the C=O double bond polar?

A
  • Oxygen is more electronegative than carbon.
  • The electron density in the double bond lies closer to the oxygen atom than the carbon atom.
  • This makes the carbon end of the bond slightly positive and the oxygen end slightly negative.
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7
Q

Why can aldehydes and ketones react with some nucleophiles?

A

Due to the polarity of the C=O bond.

- The nucleophile is attracted to and attacks the slightly positive carbon atom resulting in addition across C=O bond

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8
Q

What reducing agent is used to reduce aldehydes and ketones to alcohol?

A

Sodium tetrahydridoborate, NaBH4

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9
Q

What are aldehydes & ketones reduced to?

A

Aldehydes- Primary alcohol

Ketones- Secondary alcohol

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10
Q

Why can hydrogen cyanide not be used safely in an open laboratory?

A

HCN is an extremely poisonous liquid that boils slightly above room temp so sodium cyanide and sulfuric acid is used to provide the hydrogen cyanide

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11
Q

What does the reaction between aldehydes/ketones and hydrogen cyanide produce?

A

Hydroxynitrile

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12
Q

Why do ketones not undergo oxidation reactions?

A

Due to a lack of reactivity.

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13
Q

What solution is used to detect the presence of a carbonyl functional group in both aldehydes and ketones?

A

2,4-DNP (2,4-dinitrophenylhydrazine)

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14
Q

What is 2,4-DNP also known as?

A

Brady’s reagent

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15
Q

In the presence of a carbonyl group what precipitate is formed?

A

In the presence of a carbonyl group, a yellow or orange precipitate called a 2,4-dinitrophenylhydrazone is produced.

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16
Q

Describe the steps in testing for the carbonyl group in aldehydes and ketones?

A

1) Add 5cm depth of a solution of 2,4-dinitrophenylhydrazine to a clean test tube. This is in excess.
2) Using a dropping pipette, add three drops of the unknown compound. Leave to stand.
3) If no crystals form, add a few drops of sulfuric acid
4) A yellow/orange precipitate indicates the presence of an aldehyde or ketone.

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17
Q

What is Tollen’s reagent?

A
  • Tollen’s reagent is a solution of silver nitrate in aqueous ammonia.
  • In the presence of an aldehyde group, a silver mirror is produced.
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18
Q

How would you form Tollen’s reagent?

A

1) In a clean test tube, add 3cm depth of aqueous silver nitrate, AgNO3.
2) Add aqueous sodium hydroxide to the silver nitrate until a brown precipitate of silver oxide, Ag2O, is formed.
3) Add dilute ammonia solution until the brown precipitate just dissolves to form a clear colourless solution. This is Tollen’s reagent.

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19
Q

Why are Ag+ ions reduced in the reaction?

A

Ag+ ions are reduced to silver as the aldehyde is oxidised to a carboxylic acid.

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20
Q

How can you identify an aldehyde by melting point?

A

1) The impure yellow/orange solid is filtered to seperate the solid precipitate from the solution.
2) The solid is then crystallised to produce a pure sample of crystals.
3) The melting point of the purified 2,4-dinitrophenylhydrazone is measured and recorded.
4) The melting point is compared to a database or data table of melting points to identify the original carbonyl compound.

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21
Q

What is a carboxylic acid?

A

A carboxylic acid is an organic acid which contains the carboxyl group.
It contains both a carbonyl group and a hydroxyl group.

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22
Q

Common uses of carboxylic acids

A
  • MEDICINE: Aspirin, made from salicylic acid.
  • Fruit juices
  • Vinegar
  • Rhubarb leaves
23
Q

Why can carboxylic acids form hydrogen bonds with water?

A

The C=O and O-H bonds in carboxylic acids are polar allowing carboxylic acids to form hydrogen bonds with water molecules.

24
Q

Up to how many carbon atoms is carboxylic acid soluble in water? Why is this?

A
  • Carboxylic acids with up to four carbon atoms are soluble in water.
  • As the number of carbon atoms increases, the solubility decreases as the non-polar carbon chain has a greater effect on the overall polarity of the molecule.
25
Q

Why are carboxylic acids classified as weak acids?

A
  • Carboxylic acids partially dissociate when dissolved in water, which is what weak acids do.
26
Q

What is produced when carboxylic acids react with metals in a redox reaction?

A

Hydrogen gas and the carboxylate salt.

27
Q

What would you observe in the redox reaction of carboxylic acids with metals?

A
  • In this reaction you would observe the metal disappearing and effervescence as hydrogen gas is evolved.
28
Q

What is produced in the reaction between carboxylic acids and metal oxides?

A

A salt and water.

2CH3COOH (aq) + Cao -> (CH3COO-)2 Ca2+ (aq) + H2O (l)

29
Q

What is produced in the reaction between carboxylic acids and alkalis?

A

A salt and water

E.g ethanoic acid + sodium hydroxide -> sodium ethanoate + water

30
Q

What is produced in the reaction between carboxylic acids and carbonates?

A

Salt, water and carbon dioxide.

31
Q

How can you test for the carboxyl group?

A
  • The neutralisation reaction of carboxylic acids with carbonates e.g sodium carbonate
  • This is because carboxylic acids are the only common organic compounds sufficiently acidic to react with carbonates.
  • This is especially useful for distinguishing carboxylic acids from phenols which aren’t acidic enough to react with carbonates.
32
Q

What is a derivative of a carboxylic acid?

A

A derivative of a carboxylic acid is a compound that can be hydrolysed to form the parent carboxylic acid.

33
Q

State the derivatives of carboxylic acids.

A
  • Ester
  • Acyl chloride
  • Acid anhydride
  • Amide
34
Q

How can we name an ester?

A

To name an ester remove the -oic acid suffix from the parent carboxylic acid and replace with -oate.
- The alkyl chain is added as the first word.
E.g ethanoic acid -> methyl ethanoate

35
Q

How can we name acyl chlorides?

A
  • To name an acyl chloride, remove the -oic acid suffix from the parent carboxylic acid and replace with -oyl chloride.
  • E.g propanoic acid -> propanoyl chloride
36
Q

How is an acid anhydride formed?

A
  • An acid anhydride is formed by the removal of water from two carboxylic acid molecules
37
Q

What is esterification?

A
  • Esterification is the reaction of an alcohol with a carboxylic acid to form an ester.
38
Q

What are the conditions needed for esterification?

Reaction between alcohol and carboxylic acid

A
  • The alcohol is warmed with a small amount of concentrated sulfuric acid
39
Q

What can esters be hydrolysed by?

A

Esters can be hydrolysed by aqueous acid or alkali.

40
Q

What is hydrolysis?

A

Hydrolysis is the chemical breakdown of a compound in the presence of water or in aqueous solution

41
Q

How is acid hydrolysis carried out?

A
  • The ester is heated under reflux with dilute aqueous acid.

- The ester is broken down by water, with the acid acting as a catalyst.

42
Q

What is saponification?

A

Alkaline hydrolysis. It is IRREVERSIBLE

43
Q

How is alkaline hydrolysis carried out?

A

The ester is heated under reflux with aqueous hydroxide ions.

44
Q

How are acyl chlorides prepared?

A
  • Acyl chlorides can be prepared directly from carboxylic acids by reaction with thionyl chloride, SOCl2
45
Q

What products are formed during preparation of acyl chloride?

A

SO2, HCl

- Both are evolved as gases, leaving just the acyl chloride

46
Q

Where should acyl chlorides be prepared?

A

In a fume cupboard as the products are harmful.

47
Q

What does the reaction between acyl chlorides and alcohols form?

A

An ester and hydrogen chloride.

48
Q

What does the reaction between acyl chloride and phenol form?

A

Phenyl ester and hydrogen chloride

49
Q

what does the reaction between acyl chloride and water form?

A

carboxylic acid and hydrogen chloride

50
Q

what observation is seen when water is added to acyl chloride?

A

a violent reaction takes place with the evolution of dense steamy hydrogen chloride fumes.

51
Q

What does the reaction between acyl chloride and ammonia form?

A

PRIMARY Amides and - chloride

The Nitrogen is attached to ONE CARBON ATOM

52
Q

How does ammonia/amines act as nucleophiles?

A

They donate the lone pair of electrons on the nitrogen atom to an electron-deficient species.
This reaction with acyl chloride forms amine..

53
Q

How is a SECONDARY AMIDE form?

A

The primary amine reacts with the acyl chloride in the same way as ammonia to form a secondary amide.
- The nitrogen atom is attached to TWO carbon atoms.

A-Level Chemistry (27 decks)

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