Chapter 18 - Carboxylic Acids & Derivatives

Question 1

Why do carboxylic acids have higher boiling points than its corresponding alcohols? (2)

Answer 1

1) hydrogen bonding between carboxylic acid molecules is stronger than that between alcohol molecules. This is due to the presence of electron withdrawing C=O group that causes the d+ on the H atom to be intensified, and the O-H bond to be more polarised in the carboxylic acid.
2) Carboxylic acids can dimerise (forming a dimer, a chemical structure formed by 2 similar sub units) through hydrogen bonding, effectively doubling the number of electrons and thus the size of electron cloud in the dimer. This results in stronger dispersion forces between dimers.

Question 2

Why can carboxylic acids dissolve in non-hydrogen-bonding solvents?

Answer 2

Carboxylic acids can form hydrogen-bonded dimers in such solvents, especially if they have longer carbon chains. This is because dispersion forces can be formed between solvent molecules and the hydrocarbon chain of the dimers.

Question 3

How do carboxylic acids dissolve in water and how does the solubility of carboxylic acids change in water (as the carbon chain grows longer)?

Answer 3

In water, the carboxylic acid does not dimerise, but instead form hydrogen bonding with the water molecules.
Solubility in water decreases as the hydrocarbon chain grows longer. The longer the non-polar hydrocarbon chain, the stronger the dispersion forces between the carboxylic acid molecules. Energy released from the hydrogen bonding between the -COOH group and water molecules is less than the energy needed to overcome these dispersion forces and the existing hydrogen bonding between molecules.

Question 4

How can carboxylic acids be prepared? (7)

Answer 4

1) oxidative cleavage of alkenes
2) side chain oxidation of alkylbenzenes
3) acid and alkaline hydrolysis of nitriles
4) oxidation of primary alcohols
5) oxidation of aldehydes
6) hydrolysis of acyl chlorides
7) acid and alkaline hydrolysis of esters/amides

Question 5

What are the 4 types of reactions that carboxylic acids can undergo?

Answer 5

1) acid-metal/acid-base reaction (where O-H bond is broken to form salts)
2) nucleophilic substitution (replacement of -OH group)
3) reduction of the COOH group
4) oxidation (only methanoic and ethanedioic acid)

Question 6

Why are carboxylic acids unable to undergo nucleophilic addition?

Answer 6

The delocalisation of electrons from the O-H group into the C=O group makes the carboxyl carbon less electron deficient and less readily attacked by nucleophiles.

Question 7

Why are carboxylic acids stronger acids than phenols or aliphatic alcohols? (2)

Answer 7

1) The negative charge on the carboxylate ion (the conjugate base) is delocalised equally over two highly electronegative oxygen atoms. With a dispersed negative charge, the carboxylate anion is greatly stabilised. It is thus less likely to accept a proton to reform the acid.
2) although the negative charge on the phenoxide ion is also delocalised, it is less than that on the carboxylate ion as the carbon atoms in the benzene ring do not bear the negative charge as well as the two electronegative O atoms in the carboxylate ion. Hence the carboxylate ions are more stabilised and makes it more stable.

Question 8

What are 4 factors of different substituents that can affect the acidity of carboxylic acids?

Answer 8

1) electron-donating groups: intensify the negative charge on the carboxylate anion, destabilise the anion and decrease acidity
2) electron-withdrawing groups: disperse the negative charge on the carboxylate anion, stabilise the anion and increase acidity
3) number of electron-withdrawing groups: increasing the number of groups will increase acidity
4) proximity of electron-withdrawing groups to the -COOH group: the nearer the electron-withdrawing groups to the carboxyl groups, the greater the inductive effect, and the stronger the acid. (However, the electron withdrawing effect becomes ineffective when it is acting through more than 4 atoms)

Question 9

Why can carboxylic acids and its derivatives undergo nucleophilic (acyl) substitution reactions? (2)

Answer 9

1) the C=O carbon has a partial positive charge due to the electronegative O
2) the C=O carbon has a planar geometry and is relatively unhindered, which makes it susceptible toward nucleophilic attack.

Question 10

What are the 2 nucleophilic substitution reactions carboxylic acids can undergo？

Answer 10

1) formation of acyl chloride (-OH group replaced with Cl)

2) condensation to form esters (C-O bond of the -COOH group is broken)

Question 11

What are the reagents, conditions and observations for the formation of acyl chlorides through carboxylic acids?

Answer 11

Reagents and conditions: PCl5/PCl3/SOCl2, room temperature (warm for SOCl2)
Observations: white fumes of HCl produced, H3PO3 for PCl3

Question 12

What are the reagents and conditions needed for the reduction of carboxylic acids into primary alcohols?

Answer 12

LiAlH4, dry ether, room temperature. LiAlH4 must be used in anhydrous conditions

Question 13

What are the reagents and conditions needed for the reduction of carboxylic acids into ketones?

Answer 13

NaBH4 in methanol

Question 14

What are the reagents, conditions and observations for the oxidation of carboxylic acids?

Answer 14

Reagents and conditions: KMnO4, dilute H2SO4, heat under reflux
Observations: Purple solution decolourises, effervescence of CO2 observed.

Question 15

What are carboxylic acid derivatives?

Answer 15

They are compounds in which the -OH group is replaced by a halogen, -OR’ or -NH2.

Question 16

Why do esters and acyl chlorides (carboxylic acid derivatives) have lower boiling points than their corresponding carboxylic acids?

Answer 16

Their intermolecular pd-pd attractions are weaker than the hydrogen bonding for carboxylic acids, and require less energy to break. They are unable to form intermolecular hydrogen bonding as they lack a hydrogen atom that is directly attached to electronegative O.

Question 17

What is the solubility of esters in water compared to carboxylic acids and alcohols?

Answer 17

Esters are much less soluble in water and many esters are immiscible with water.

Question 18

How does acyl chloride smell like?

Answer 18

They are colourless but have a sharp and irritating odour.

Question 19

Compare the relative reactivity of the 4 types of carboxylic acid derivatives (including carboxylic acid itself) towards nucleophilic substitution. Explain the trend. (2)

Answer 19

In descending order of reactivity,
Acyl chloride > carboxylic acid > ester > amide

1) electron donating/withdrawing: a halogen substituent withdraws electrons through inductive effect from the C=O group, thus increasing the partial positive charge on the carbon atom. This makes acyl chlorides more readily attacked by nucleophiles. However, an amino group or an alkoxy group (-OR’) donates electrons to the carbonyl carbon through resonance effect. This decreases the partial positive charge on the carbon atom, and makes amides and esters less readily attacked by nucleophiles.
2) leaving group: the weaker the base, the better the leaving group. Halide anions are better leaving groups compared to the hydroxide ion, as halides are weaker bases.

Question 20

What are the 3 nucleophilic substitution reactions acyl chlorides undergo?

Answer 20

1) hydrolysis in water to form carboxylic acids
2) condensation to form amides
3) condensation to form esters by reacting with alcohol (topic 6 hydroxy compounds)

Question 21

What are the reagents and conditions needed and the observations for the hydrolysis of acyl chlorides?

Answer 21

Reagents and conditions: water, room temperature

Observations: rapid reaction forming white fumes of HCl

Question 22

What are the reagents and conditions needed for the condensation of acyl chlorides to form amides?

Answer 22

Ammonia/primary amines/secondary amines in excess, room temperature

Question 23

Why are excess ammonia/amines added during condensation of acyl chlorides to form amides?

Answer 23

To ensure complete reaction, as some ammonia or amines reacts with acidic HCl to form an ammonium salt (acid base)

Question 24

What is the relative rate of hydrolysis of acyl, alkyl and aryl chlorides respectively?

Answer 24

In descending order of reactivity,

Acyl chloride > alkyl chloride > aryl chloride

Question 25

What are the different hydrolysis conditions needed for acyl, alkyl and aryl chlorides respectively?

Answer 25

Acyl: H2O, room temperature
Alkyl: aqueous NaOH/KOH, heat under reflux
Aryl: not possible under ordinary lab conditions; only possible under very h igh temperature and pressure with aqueous alkali

Question 26

Explain the trend of relative acidity of acyl, alkyl and aryl chlorides. (3)

Answer 26

1) Electronic factor: The electron deficiency level of the carbon on C-Cl depends on the number of electronegative atoms it is attached to. Acyl has 2 (Cl and O), alkyl has 1 (Cl) and the C on aryl is the least electron deficient due to the delocalisation of lone pair of electrons of Cl into the benzene ring, imparting partial double bond character into the C-Cl bond and thus strengthening the bond. The relative difficulty of cleaving the C-Cl bond thus increases (acyl < Alkyl < aryl) and so does its ability to attract nucleophiles.
2) steric factor: acyl is planar, alkyl is tetrahedral and aryl has a bulky aromatic ring. Steric hindrance increases.
3) electron rich benzene ring repels nucleophiles.

Question 27

How can esters be prepared? (2)

Answer 27

1) condensation reaction between carboxylic acid and alcohol

2) condensation reaction between acyl chloride and alcohol

Question 28

What are the 2 reactions esters can undergo?

Answer 28

1) acid hydrolysis
2) base hydrolysis (saponification)
Both forms carboxylic acid

Question 29

What are the reagents and conditions needed for esters to undergo acid hydrolysis?

Answer 29

Dilute sulfuric acid and heat under reflux

Question 30

What are the reagents and conditions needed for esters to undergo base hydrolysis?

Answer 30

Dilute NaOH, heat under reflux, followed by acidification with dilute H2SO4 at rtp