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OCR A-level Chemistry > Chapter 26 - Carbonyls and carboxylic acids > Flashcards

Chapter 26 - Carbonyls and carboxylic acids Flashcards

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1
Q

What is the carbonyl group?

A

C=O

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2
Q

What does oxidation of an aldehyde form?

A

Carboxylic acid

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3
Q

What are the reagents and conditions of oxidation of an aldehyde?

A
  • Reflux
  • aldehyde + pottasium dichromate (VI) ions, K2Cr2O7 + H2SO4
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4
Q

Do ketones undergo oxidation?

A

No

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5
Q

What effect does C=O bond have on the reactivity of carbonyls compared to the C=C bond of alkenes?

A

C=O bond is polar, therefore aldehydes and ketones can react with some nucleophiles. Nucleophiles attacks the slightly positive carbon atom resulting in addition across the C=O bond (nucleophillic addition)

Non polar C=C bond in alkenes recats with electrophiles by electrophillic substitution.

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6
Q

How is a reducing agen represented in an equation?

A

[H]

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7
Q

What is the reducing agent in reduction of ketones and aldehydes to alcohols

A

(aldehyde/ketone warmed with) NaBH4 in aqueous solution

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8
Q

What do aldehydes reduce to?

A

Primary alcohols

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9
Q

Write the equation for the reduction of butanal

A
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10
Q

Write the equation for reduction of propanone

A
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11
Q

What happens when HCN is reacted with aldehydes or ketones?

A

Adds across C=O bond, forming a hydroxynitrile (contains -OH and C=-N funstional groups)

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12
Q

How is the HCN provided in this reaction? Why can’t HCN just be added?

A

NaCN and H2SO4 are used to provide the hydrogen cyanide.

HCN is a colourless, extremely poisonous liquid that boils slightly above room temp, so it can’t be used safely in an open lab

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13
Q

What kind of reactions are the reduction of aldehydes/ketones and reaction with HCN?

A

nucleophillic addition

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14
Q

Write the mechanism for the reduction of aldehydes/ketones with NaBH4

A

NaBH4 can be considered as containing the hydride ion, :H-, which acts as a nucleophile

  1. ) The lone pair of electrons from the hydride ion, :H- is attracted and donated to the delta+ carbon atom in the aldehyde or ketone C=O double bond
  2. ) A dative covalent bond is formed between the hydride ion and the carbon atom of the C=O double bond
  3. ) The π-bond in the C=O double bond breaks by heterolytic fission, forming a negatively charged intermediate
  4. ) The oxygen atom of the intermediate donates a lone pair of electrons to a hydrogen atom in a water molecule. The intermediate has then been pronated to form an alcohol.
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15
Q

Write the mechanism for the reaction of aldehydes/ketones with HCN (NaCN/H+)

A

cyanide ion, :CN- attacks the electron-deficient carbon atom in the aldehyde or ketone.

  1. ) The lone pair of electrons from the cyanide ion, :CN- is attracted and donated to the delta+ carbon atom in the aldehyde or ketone C=O double bond. A dative covalent bond forms.
  2. ) The π-bond in the C=O double bond breaks by heterolytic fission, forming a negatively charged intermediate.
  3. ) The intermediate is pronated by donating a lone pair of electrons to a hydrogen ion, to form the product
  4. ) Product is a hydroxynitrile.
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16
Q

What is used to detect a carbonyl fuctional group?

A

2,4-dinitrophenylhydrazine (2,4-DNP) also known as Brady’s reagent

yellow or orange precipitate of 2,4-dinitrophenylhydrazone produced in presence of carbonyl group

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17
Q

Describe the procedure of testing for the carbonyl group using 2,4-DNP (brady’s reagent)

A
  1. ) Add 5cm3 of 2,4-DNP to a clean test tube. This is in excess
  2. ) Using a dropping pipette, add 3 drops of the unknown compound. Leave to stand.
  3. ) If no crystals form, add a few drops of sulphuric acid.
  4. ) A yellow/orange precipitate indicates the presence of an aldehyde/ketone.
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18
Q

Which reagent ca distinguish between an aldehyde and ketone?

A

Tollens’ reagent

(a solution of silver nitrate in aqueous ammonia)

in presence of an aldehyde group, a silver mirror is produced.

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19
Q

Describe the procedure of testing for an aldehyde group using tollens’ reagent

A

Tollens’ reagent made up by:
1.) In a clean test tube, add 3cm depth of aqueous silver nitrate, AgNO3(aq).

  1. ) Add aqueous sodium hydroxide to the silver nitrate until a brown precipitate of silver oxide, Ag2O is formed.
  2. ) Add dilute ammonia solution until the brown precipitate just dissolves to form a clear colourless solution - tollens’ reagent

carrying out test for an aldehyde:

  1. ) Pour 2cm depth of the unknown solution into a clean test tube
  2. ) Add an equal volume of freshly prepared Tollens’ reagent
  3. ) Leave the test tube to stand in a beaker of warm water at about 50ºC for about 10 to 15 minutes and then observe whether any silver mirror is formed. If so then the unknown solution contains an aldehyde.If the unknown solution is a ketone then no reaction is observed
20
Q

Give the oxidation and reduction reactions that take place in a reaction between an aldehyde and silver ions

A

reduction:

Ag+(aq) + e- → Ag(s)

oxidation:

R-CHO + [O] → R-COOH

silver mirroe formed from reduction of silver

21
Q

Describe the procedure of identifying an aldehyde or ketone by melting point

A

2,4-dinitrophenylhydrazone precipitate formed in the 2,4-DNP test can be analysed by:

  1. ) The impure yellow/orange solid is filtered to seperate the solid precipitate from the solution.
  2. ) The solid is then recrystallised to produce a pure sample of crystals
  3. ) Melting point of the purified 2,4-dinitrophenylhydrazone is measured an recorded.
  4. ) melting point compared to a database to identify original carbonyl compound
22
Q

Why are carboxylic acids (with up to 4 carbons) soluble in water?

A

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

23
Q

Why are carboxylic acids only soluble with up to 4 carbon atoms?

A

As the number of atoms increases, the solubility decreases as the non-polar carbon chain has a greater effect on the overall polarity of the molecule.

24
Q

Are carboxylic acids strong or weak acids?

A
  • Weak acids

when dissolved in water, carnoxylic acids partially dissociate:

HCOOH(aq) ⇔ H+(aq) + HCOO-(aq)

25
Q

What do carboxylic acids form when reacted in redox reactions with metals and in neutralisation reactions with bases?

A

Carboxylate salts

Carboxylate ion in salt named by replacing -ic acid ending with -ate

26
Q

What is formed in the redox reaction between aqueous solutions of carboxylic acids and metals? State what you would observe

A

Carboxylate salt and hydrogen gas

Wouls observe the metal dissapearing and effervescence, as hydrogen gas is evolved

27
Q

Write an equation for propanoic acid and magnesium?

A

2CH3CH2COOH(aq) + Mg(s) → (CH3CH2COO-)2Mg2+(aq) + H2(g)

28
Q

What is formed in a neutralisation reaction between carboxylic acids with metal oxides?

A

Salt and water

29
Q

Write the equation for the reaction of aqueous ethanoic acid with calcium oxide

A

2CH3COOH(aq) + CaO(s) → (CH3COO-)2Ca2+(aq) + H2O

30
Q

What is formed in the neutralisation reaction between carboxylic acids and alkalis?

A

Salt and water.

May not see any reaction as two solutions react together to form an aqueous solution of the salt.

31
Q

Write the overall and ionic equation for the reaction between ethanoic acids and sodium hydroxide?

A

overall: CH3COOH(aq) +NaOH(aq) → CH3COO-Na+ + H2O
ionic: H+(aq) + OH-(aq) → H2O(l)

32
Q

What is formed in the neutralisation reaction between a carboxylic acid and a carbonate?

A

salt, water and carbon dioxide

gas evolved and if carboxylic acid is in excess, a solid carbonate would disappear

33
Q

Write the equation for a neutralisation reaction between ethanoic acid and aqueous sodium carbonate

A

2CH3COOH(aq) + Na2CO3(aq) → 2CH3COO-Na+(aq) + H2O(l) + CO2(g)

34
Q

How can you distinguish between a carboxylic acid and a phenol?

A

Phenols are not acidic enough to react with carbonates, but carboxylic acids are - they neutralise carbonates e.g. sodium carbonate to form CO2, water and a salt

35
Q

What is a derivative of a carboxylic acid?

A

Compound that can be hydrolysed to form the parent carboxylic acid.

36
Q

How do you name a acyl chloride?

A
  • Named after the parent carboxylic acid from which it’s derived.
  • Replace the -oic acid suffix and replace with -oyl chloride
37
Q

How are acid anhydrides formed?

A

Removal of water from 2 carboxylic acid molecules

38
Q

What is formed in acid hydrolysis of an ester (heated under reflux with hot aqueous acid)?

A
  • Carboxylic acid and an alcohol
  • reverse of esterification. Ester broken down by water, with the acid acting as a catalyst
39
Q

What does alkaline hydrolysis of an ester form (heated under reflux with hot aqueous hydroxide ions)?

A

salt (carboxylate ion) and an alcohol

(if sodium hydroxide is alkali used, the salt sodium ethanoate is formed)

40
Q

Is acid hydrolysis or alkaline hydrolysis of esters irreversible?

A

Alkaline hydrolysis of esters is irreversible

41
Q

How are acyl chlorides prepared?

A

Reacting carboxylic acid with thinoyl chloride (SOCl2). Produces HCl and SO2, which are evolved as gases, leaving just the acyl chloride

42
Q

What is formed in the reaction of acyl chlorides with alcohols?

A

Ester

43
Q

What is formed in a reaction between an acyl chloride and a phenol?

A

Phenyl ester and HCl

44
Q

What is formed when an acyl chloride is reacted with water?

A

Carboxylic acid and HCl fumes in a violent reaction

45
Q

What is formed when ammonia reacts with acyl chlorides?

A

primary amide - nitrogen attched to one carbon

lone pair on ammonia donated electron deficient species

46
Q

What is formed when a primary amine reacts with an acyl chloride?

A

Secondary amide - nitrogen attached to 2 carbons

47
Q

What are some properties of acid anhydrides compared to acyl chlorides?

A
  • React in a similar way to acyl chlorides with alcohols, phenols, water, ammonia and amines
  • Less reactive than acyl chlorides
  • Useful for some lab preperations where acyl chlorides may be too reactive

OCR A-level Chemistry (30 decks)

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