Chapter 14 Carboxylic Acids and Derivatives

Question 1

Benzoic acids are the

Answer 1

• simplest aromatic carboxylic acids with the molecular formula of C₆H₅COOH
• Benzoic acids and their derivatives are often used as reagents in the synthesis of esters
• The compounds can be produced from the oxidation of alkylbenzenes

Question 2

Oxidation of alkylbenzenes

Answer 2

• The alkyl side-chain in alkylbenzenes, such as methylbenzene, can be oxidised to a carboxylic acid
• The alkylbenzene is heated under reflux with a solution of hot alkaline KMnO₄ (this is the oxidising agent)
  
  • The purple colour of the Mn⁷⁺ ions disappears as they are reduced to Mn⁴⁺ ions
  • A brown precipitate of MnO₂ is formed
• The mixture is then acidified with dilute acid (such as hydrochloric acid) to protonate the organic product form and produce a benzoic acid

Question 3

Reactions of Carboxylic Acids to Produce Acyl Chlorides

Answer 3

• Acyl chlorides are compounds with the functional group -COCl
• They look similar in structure to carboxylic acids but have a Cl atom instead of an -OH group attached to the carbonyl (C=O)
• Acyl chlorides are more reactive than their corresponding carboxylic acids and are therefore often used as starting materials in the production of organic compounds such as esters

Question 4

Production Acyl Chlorides can be prepared from the reaction of carboxylic acids with:

Answer 4

• Solid phosphorus(V) chloride (PCl₅)
• Liquid phosphorus(III) chloride (PCl₃) and heat
• Liquid sulfur dichloride oxide (SOCl₂)

Question 5

• For example, the acyl chloride ethanoyl chloride can be formed from ethanoic acid in the above reactions

Answer 5

Question 6

Carboxylic acids can be formed from the

Answer 6

• oxidation of primary alcohols
• The primary alcohols are firstly oxidised to aldehydes and then further oxidised to carboxylic acids
• Some carboxylic acids can get even further oxidised

Question 7

Methanoic acid is a

Answer 7

• strong reducing agent and gets further oxidised to carbon dioxide (CO₂)
• The oxidation of methanoic acid can occur by:
  
  • Warming methanoic acid with mild oxidising agents such as Fehling’s or Tollens’ reagent
    
    • In a Fehling’s solution, the Cu²⁺ ion is reduced to Cu⁺ ion which precipitates as red Cu₂O
    • With Tollens’ reagent, the Ag⁺ is reduced to Ag
  • Using stronger oxidising agents such as acidified KMnO₄ or acidified K₂Cr₂O₇
    
    • The purple KMnO₄ solution turns colourless as Mn⁷⁺ ions are reduced to Mn²⁺ ions
    • The orange K₂Cr₂O₇ solution turns green as the Cr⁶⁺ ions are reduced to Cr³⁺ ions

Question 8

Ethanedioic acid

Answer 8

• Another carboxylic acid that can get further oxidised is ethanedioic acid
• A strong oxidising agent such as warm acidified KMnO₄ is required for the oxidation of ethanedioic acid to carbon dioxide

Question 9

Carboxylic acids are compounds with

Answer 9

• a -COOH functional group
• They can act as acids and lose a proton (H⁺ ion) in an aqueous solution to form carboxylate salts and water
• However, carboxylic acids are only weak acids as the position of equilibrium lies well over to the left-hand side

Question 10

The pK_a values of carboxylic acids, phenols, and alcohols suggest that carboxylic acids are

Answer 10

stronger acids than alcohols and phenols

• The pK_a is a measure of the relative strength of a species as an acid
• The smaller the pK_a value, the stronger the acid

Question 11

Relative acidity of ethanol, phenol & carboxylic acids table

Answer 11

• This order of relative acidities can be explained by looking at the strength of the O-H bond and the stability of the conjugate bases of the acids

Question 12

Strength of O-H bond

Answer 12

• In carboxylic acids, the electrons in the O-H bond are drawn towards the C-O bond
• The electrons in the C-O bond are drawn towards the C=O bond
• Overall, the O-H bond is weakened due to the carbonyl (C=O) group removing electron density from it and drawing it towards itself
• Carboxylic acids can therefore more easily lose a proton compared to phenols and alcohols which lack this electron-withdrawing carbonyl group

Question 13

Stability of carboxylate ions

Answer 13

• The conjugate base of carboxylic acids is the carboxylate ion
• The charge density on the oxygen atom is spread out over the carboxylate ion
• This is because the charge is delocalised on an electronegative carbonyl oxygen atom
• As a result, the electrons on the oxygen atom are less available for bond formation with an H⁺ ion to reform the undissociated acid molecule with -COOH group
• The position of the dissociation equilibrium lies more to the right compared to alcohols and phenols

Question 14

Stability of alkoxide ions

Answer 14

• The conjugate base of alcohols is the alkoxide ion
• The alkyl group in the ion is an electron-donating group that donates electron density to the oxygen atom
• As a result, the electron density on the oxygen atom is more readily available for bond formation with an H⁺ ion
• Alkoxide ions also lack the ability to delocalise the charge density on the entire ion
• The conjugate bases of alcohols are therefore less stable than the alcohols themselves and are more likely to reform the alcohol
• This means that alcohols are weaker acids compared to carboxylic acids and phenols
• The position of the dissociation equilibrium lies more to the left

Question 15

Stability of phenoxide ions

Answer 15

• In the phenoxide ion (which is the conjugate base of phenol) the charge density on the oxygen atom is spread out over the entire ion
  
  • This delocalisation of electrons stabilises the phenoxide ion
• As a result, the electrons on the oxygen atom are less available for bond formation with a proton (H⁺ ion)
• The conjugate base of phenols is therefore more stable than phenol
• However, since the delocalisation of charge density is on carbon atoms and not on electronegative oxygen atoms like in the carboxylate ion, phenoxide ions are less stable than carboxylate ions
• Therefore, phenols are weaker acids relative to carboxylic acids
• The position of the dissociation equilibrium lies more to the right compared to alcohols and more to the left compared to carboxylic acids

Question 16

Electron-withdrawing groups bonded to the carbon attached to the -COOH group make the carboxylic acids

Answer 16

• stronger acids
• This is because the O-H bond in the undissociated acid molecule is even further weakened as the electron-withdrawing group draws even more electron density away from this bond
• Furthermore, the electron-withdrawing groups extend the delocalisation of the negative charge on the -COO^– group of the carboxylate ion
• The -COO^– group is now even more stabilised and is less likely to bond with an H⁺ ion
• Chlorine-substituted carboxylic acids are examples of carboxylic acids with electron-withdrawing groups

Question 17

pK_avalues of ethanoic acid and chlorine-substituted derivatives table

Answer 17

Question 18

The pK_a values of ethanoic acid and chloro-substituted derivatives show that the

Answer 18

more electron-withdrawing groups there are on the carbon attached to the -COOH group, the stronger the acid

Question 19

Trichloroethanoic acid is the

Answer 19

strongest acid as:

• The O-H bond in CCl₃COOH is the weakest since there are three very strong electronegative Cl atoms withdrawing electron density from the -COOH group
• When the O-H is broken to form the carboxylate (-COO^–) ion, the charge density is further spread out by the three electron-withdrawing Cl atoms
• The carboxylate ion is so stabilised that it is less attracted to H⁺ ions

Question 20

Ethanoic acid is the

Answer 20

• weakest acid as:
  
  • It contains an electron-donating methyl group which strengthens the O-H bond
  • The methyl group donates negative charge towards the -COO^– group which becomes more likely to accept an H⁺ ion

Question 21

Esters are

Answer 21

• organic compounds with an -COR functional group
• They have characteristic smells and are used in perfumes, cosmetics and as solvents
• Esters can be prepared from the condensation reaction between alcohols and carboxylic acids
  
  • This is also called an esterification reaction

Question 22

A more effective way of preparing esters is from the

Answer 22

• condensation reaction between alcohols and acyl chlorides instead
• Unlike the reactions with carboxylic acids, acyl chlorides are more reactive (so the reactions happen faster) and their reactions go to completion (so no equilibrium mixture is formed and the yield of the ester is maximum)
• Examples of esterification reactions include:
  
  • Formation of ethyl ethanoate from ethanol and ethanoyl chloride
  • Formation of phenyl benzoate from phenol and benzoyl chloride

Question 23

Formation of esters from the reaction of alcohols with acyl chlorides

Answer 23

Question 24

Acyl chlorides are compounds that contain

Answer 24

• an -COCl functional group and can be prepared from the reaction of carboxylic acids with:
  
  • Solid phosphorus(V) chloride (PCl₅)
  • Liquid phosphorus(III) chloride (PCl₃) and heat
  • Liquid sulfur dichloride oxide (SOCl₂)

Question 25

Propanoyl chloride can this way be prepared from propanoic acid using the reactions above

Answer 25

Question 26

Reactions of Acyl Chlorides

Answer 26

• are reactive organic compounds that undergo many reactions such as addition-elimination reactions
• In addition-elimination reactions, the addition of a small molecule across the C=O bond takes place followed by elimination of a small molecule
• Examples of these addition-elimination reactions include:
  
  • Hydrolysis
  • Reaction with alcohols and phenols to form esters
  • Reaction with ammonia and amines to form amides

Question 27

The hydrolysis of acyl chlorides results in the

Answer 27

• formation of a carboxylic acid and HCl molecule
• This is an addition-elimination reaction
  
  • A water molecule adds across the C=O bond
  • A hydrochloric acid (HCl) molecule is eliminated

Question 28

• An example is the hydrolysis of propanoyl chloride to form propanoic acid and HCl

Answer 28

Question 29

Formation of esters

Answer 29

• Acyl chlorides can react with alcohols and phenols to form esters
  
  • The reaction with phenols requires heat and a base
• Esters can also be formed from the reaction of carboxylic acids with phenol and alcohols however, this is a slower reaction as carboxylic acids are less reactive and the reaction does not go to completion (so less product is formed)
• Acyl chlorides are therefore more useful in the synthesis of esters
• The esterification of acyl chlorides is also an addition-elimination reaction
  
  • The alcohol or phenol adds across the C=O bond
  • A HCl molecule is eliminated

Question 30

Acyl chlorides undergo esterification with alcohols and phenols to form esters

Answer 30

Question 31

Acyl chlorides can form amides from their

Answer 31

• condensation reaction with amines and ammonia
• The nitrogen atom in ammonia and amines has a lone pair of electrons which can be used to attack the carbonyl carbon atom in the acyl chlorides
• The product is a non-substituted amide (when reacted with ammonia) or substituted amide (when reacted with primary and secondary amines)

Question 32

Acyl chlorides undergo condensation reactions with ammonia and amines to form amides

Answer 32

Question 33

Acyl chlorides undergo addition-elimination reactions such as hydrolysis, esterification reactions to form esters, and condensation reactions to form

Answer 33

• amides
• The general mechanism of these addition-elimination reactions involve two steps:
  
  • Step 1 – Addition of a nucleophile across the C=O bond
  • Step 2 – Elimination of a small molecule such as HCl or H₂O

Question 34

In the hydrolysis of acyl chlorides, the water molecule acts as a

Answer 34

nucleophile

• The lone pair on the oxygen atoms carry out an initial attack on the carbonyl carbon
• This is followed by the elimination of a hydrochloric acid (HCl) molecule

Question 35

In the esterification reaction of acyl chlorides, the alcohols or phenols act as a

Answer 35

• nucleophile
  
  • The lone pair on the oxygen atoms carry out an initial attack on the carbonyl carbon
  • This is again followed by the elimination of an HCl molecule
• With phenols, the reaction requires heat to proceed and needs to be carried out in the presence of a base
• The base deprotonates the phenol to form a phenoxide ion which is a better nucleophile than the phenol molecule
  
  • The phenoxide ion carries out an initial attack on the carbonyl carbon
  • A small molecule of NaCl is eliminated

Question 36

The nitrogen atom in ammonia and primary/secondary amines act as a

Answer 36

• nucleophile
  
  • The lone pair on the nitrogen atoms carry out an initial attack on the carbonyl carbon
  • This is followed by the elimination of an HCl molecule
• Both reactions of acyl chlorides with ammonia and amines are vigorous however there are also differences
  
  • With ammonia – The product is a non-substituted amide and white fumes of HCl are formed
  • With amines – The product is a substituted amide and the HCl formed reacts with the unreacted amine to form a white organic ammonium salt

Question 37

Hydrolysis is the

Answer 37

• breakdown of a compound using water
• The ease of hydrolysis for different organic compounds may differ
• For example, the ease of hydrolysis, starting with the compounds most readily broken down, is: acyl chloride > alkyl chloride > aryl chloride
• This trend can be explained by looking at the strength of the C-Cl

Question 38

Strength of C-Cl bond in acyl chlorides

Answer 38

• Acyl chlorides are hydrolysed most readily at room temperature
• This is because the carbon bonded to the chlorine atom is also attached to an oxygen atom
• There are two strong electronegative atoms pulling electrons away from the carbonyl carbon, leaving it very δ⁺
• The C-Cl bond is therefore weakened and nucleophilic attack of the carbonyl carbon is much more rapid

Question 39

Strength of C-Cl bond in alkyl chlorides

Answer 39

• The carbonyl carbon in alkyl chlorides is only attached to one electronegative atom which pulls electrons away from it
• This carbon atom is therefore not very δ⁺ and the C-Cl bond is stronger than the C-Cl bond in acyl chlorides
• The hydrolysis of alkyl chlorides, therefore, requires a strong alkali (such as OH^–) to be refluxed with it
• An OH^– ion will hydrolyse the alkyl chloride as it a stronger nucleophile than H₂O

Question 40

Strength of C-Cl bond in aryl chlorides

Answer 40

• In aryl chlorides, the carbon atom bonded to the chlorine atom is part of the delocalised π bonding system of the benzene ring
• One of the lone pairs of electrons of the Cl atom overlaps with this delocalised system
• The C-Cl bond, therefore, has some double-bond character causing it to become stronger
• As a result, the C-Cl bond is difficult to break and hydrolysis will not occur