Carboxylic acids

Question 1

Physical properties of carboxylic acids and derivatives

Answer 1

Structure: Simple molecular structure
1. Higher BP than alkenes (hydrogen bond between carboxylic acid molecules > idid)
2. BP increases down the group
3. Higher BP than alcohols with same alkyl chain
( Alcohols have electron withdrawing carbonyl group –> -OH group more polarised –> carboxylic acid forms stronger intermolecular hydrogen bonds than alcohols –> more energy needed )
4. Higher BP than acyl chlorides and esters
(Hydrogen bond vs pdpd)
5. Soluble in water
6. Insoluble in organic solvents

Question 2

Structure and bonding of acyl chlorides and esters

Answer 2

For both:
Bonding: pdpd
Structure: simple molecular structure

Question 3

Synthesis of carboxylic acid

Answer 3

1. oxidative cleavage of alkenes
2. oxidation of primary alcohols
3. oxidation of aldehydes
4. acidic hydrolysis of nitriles
5. hydrolysis of acyl chlorides
6. hydrolysis of esters
7. hydrolysis of amides
8. side-chain oxidation to give benzoic acid

Question 4

oxidative cleavage of alkenes

Answer 4

Hot KMnO4, H2SO4(aq)

Question 5

oxidation of aldehyde

Answer 5

K2Cr2O7/KMnO4, H2SO4(aq), heat under reflux

Question 6

oxidation of primary alcohol

Answer 6

K2Cr2O7/KMnO4,H2SO4(aq), heat under reflux

Question 7

Side-chain oxidation to give benzoic acid

Answer 7

KMnO4, H2SO4(aq), heat under reflux
–> white ppt of benzoic acid

Question 8

Acidic hydrolysis of nitriles

Answer 8

HCl/H2SO4(aq), heat under reflux

Question 9

Hydrolysis of amides
(Synthesis of carboxylic acids)

Answer 9

H2SO4(aq), heat under reflux

Question 10

Hydrolysis of acyl chlorides

Answer 10

H2O(l), rtp (milder conditions because acyl chlorides are very reactive)

Question 11

Hydrolysis of esters

Answer 11

HCl/H2SO4(aq), heat under reflux

Question 12

Reactions of carboxylic acids

Answer 12

1. condensation to form esters
2. nucleophilic substitution to form acyl chlorides
3. redox (acid-metal)
4. Acid-base
5. Acid-carbonate
6. oxidation of ethanedioic acid
7. oxidation of methanoic acid
8. reduction to form alcohols

Question 13

Condensation to form esters
(carboxylic acid)

Answer 13

alcohol, concentrated H2SO4, heat under reflux

Question 14

Nucleophilic substitution to form acyl chloride
(carboxylic acid)

Answer 14

PCl5, room temperature OR
PCl3, heat under reflux OR
SOCl2, heat under reflux

Question 15

Redox (acid-metal)
(carboxylic acid)

Answer 15

Na(s) ,produce H2 gas
OH –> O-Na+

Question 16

Acid-base
(carboxylic acid)

Answer 16

NaOH(aq), produce H2O
OH–>O-Na+

Question 16

Acid-carbonate
(carboxylic acid)

Answer 16

Na2CO3(aq), produce CO2 and H2O
OH–>O-Na+

Question 16

Oxidation of methanoic acid
(carboxylic acid)

Answer 16

K2Cr2O7/KMnO4, H2SO4(aq), heat under reflux
–> must have H on C atom thus oxidation occurs

Question 17

Oxidation of ethanedioic acid
(carboxylic acid)

Answer 17

*special case for strong oxidation
KMnO4, H2SO4(aq), heat under reflux

Question 18

Reduction of alcohols
(carboxylic acid)

Answer 18

LiAlH4 in dry ether

Question 19

Synthesis of acyl chlorides + reagents

Answer 19

Nucleophilic substitution (carboxylic acid)
- PCl5, room temperature OR
PCl3, heat under reflux
SOCl2, heat under reflux

Question 20

Reactions of acyl chlorides

Answer 20

1. Hydrolysis of acyl chlorides (synthesis of carboxylic acids)
   - H2O(l), rtp
2. Condensation of acyl chlorides to form amides
3. condensation of acyl chlorides to form esters

Question 21

Condensation of acyl chlorides to form amides

Answer 21

NH3/amine, rtp

Question 22

Condensation of acyl chlorides to form esters

Answer 22

alcohol+acyl chlorides, rtp

acyl chlorides + phenol:
1. NaOH(aq)
2. add acyl chloride, rtp

Question 23

Why does carboxylic acids not react with phenols?

Answer 23

- Phenols are weak nucleophiles because the LP of electrons on O delocalises into benzene ring - electrons on O less available for donation to the carbon deficient C in carboxylic acid

Question 24

Why acyl chlorides react with phenols?

Answer 24

Carbonyl carbon in acyl chloride is highly electron deficient

Question 25

Synthesis of esters

Answer 25

1. condensation of alcohols and carboxylic acid 2. condensation of acyl chlorides and alcohols 3. condensation of acyl chlorides and phenols

Question 26

Reactions of esters

Answer 26

1. acidic hydrolysis HCl/H2SO4(aq), heat under reflux 2. alkaline hydrolysis NaOH(aq), heat under reflux 3. reduction to form CH2OH and alcohol LiAlH4 in dry ether

Question 27

Why acyl chlorides most reactive?

Answer 27

- Cl is electron-withdrawing via the inductive effect (due to EN difference) - OH, -OR, and -NH2 are all electron-donating via the resonance effect (due to delocalisation of electrons) - C in Cl most electron-deficient, thus most susceptible to nucleophilic attack

Question 28

Why amides less reactive than esters

Answer 28

- N is less EN than O - LP of electrons on N delocalises across C=O to a larger extent - C in amides less electron deficient than C in esters - attracts nucleophile less readily than esters

Question 29

Distinguishing tests for carboxylic acids

Answer 29

acid-carbonate reaction Na2CO3(aq) effervescence of CO2 gas which forms white ppt in Ca(OH)2.

Question 30

why are carboxylic acids acidic?

Answer 30

- electron-withdrawing COOH group - disperses negative charge - more stable conjugate base - more acidic