Chapter 26 - Carbonyls and carboxylic acids

Question 1

What is the carbonyl group?

Answer 1

C=O

Question 2

What does oxidation of an aldehyde form?

Answer 2

Carboxylic acid

Question 3

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

Answer 3

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

Question 4

Do ketones undergo oxidation?

Answer 4

No

Question 5

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

Answer 5

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.

Question 6

How is a reducing agen represented in an equation?

Answer 6

[H]

Question 7

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

Answer 7

(aldehyde/ketone warmed with) NaBH₄ in aqueous solution

Question 8

What do aldehydes reduce to?

Answer 8

Primary alcohols

Question 9

Write the equation for the reduction of butanal

Answer 9

Question 10

Write the equation for reduction of propanone

Answer 10

Question 11

What happens when HCN is reacted with aldehydes or ketones?

Answer 11

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

Question 12

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

Answer 12

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

Question 13

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

Answer 13

nucleophillic addition

Question 14

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

Answer 14

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.

Question 15

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

Answer 15

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.

Question 16

What is used to detect a carbonyl fuctional group?

Answer 16

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

Question 17

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

Answer 17

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.

Question 18

Which reagent ca distinguish between an aldehyde and ketone?

Answer 18

Tollens’ reagent

(a solution of silver nitrate in aqueous ammonia)

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

Question 19

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

Answer 19

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

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 - 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

Question 20

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

Answer 20

reduction:

Ag⁺(aq) + e^- → Ag(s)

oxidation:

R-CHO + [O] → R-COOH

silver mirroe formed from reduction of silver

Question 21

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

Answer 21

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

Question 22

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

Answer 22

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

Question 23

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

Answer 23

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.

Question 24

Are carboxylic acids strong or weak acids?

Answer 24

• Weak acids

when dissolved in water, carnoxylic acids partially dissociate:

HCOOH(aq) ⇔ H⁺(aq) + HCOO^-(aq)

Question 25

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

Answer 25

Carboxylate salts

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

Question 26

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

Answer 26

Carboxylate salt and hydrogen gas

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

Question 27

Write an equation for propanoic acid and magnesium?

Answer 27

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

Question 28

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

Answer 28

Salt and water

Question 29

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

Answer 29

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

Question 30

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

Answer 30

Salt and water.

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

Question 31

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

Answer 31

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

Question 32

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

Answer 32

salt, water and carbon dioxide

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

Question 33

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

Answer 33

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

Question 34

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

Answer 34

Phenols are not acidic enough to react with carbonates, but carboxylic acids are - they neutralise carbonates e.g. sodium carbonate to form CO_2, water and a salt

Question 35

What is a derivative of a carboxylic acid?

Answer 35

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

Question 36

How do you name a acyl chloride?

Answer 36

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

Question 37

How are acid anhydrides formed?

Answer 37

Removal of water from 2 carboxylic acid molecules

Question 38

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

Answer 38

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

Question 39

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

Answer 39

salt (carboxylate ion) and an alcohol

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

Question 40

Is acid hydrolysis or alkaline hydrolysis of esters irreversible?

Answer 40

Alkaline hydrolysis of esters is irreversible

Question 41

How are acyl chlorides prepared?

Answer 41

Reacting carboxylic acid with thinoyl chloride (SOCl₂). Produces HCl and SO_2, which are evolved as gases, leaving just the acyl chloride

Question 42

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

Answer 42

Ester

Question 43

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

Answer 43

Phenyl ester and HCl

Question 44

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

Answer 44

Carboxylic acid and HCl fumes in a violent reaction

Question 45

What is formed when ammonia reacts with acyl chlorides?

Answer 45

primary amide - nitrogen attched to one carbon

lone pair on ammonia donated electron deficient species

Question 46

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

Answer 46

Secondary amide - nitrogen attached to 2 carbons

Question 47

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

Answer 47

• 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