6.1.2 Carbonyl and Carboxylic Acids

Question 1

Aldehyde → Carboxylic acid

Answer 1

Mechanism: oxidation
Reagents: K2Cr2O7/H2SO4
Conditions: reflux
RCHO + [O] → RCOOH

Question 2

Aldehyde → Primary Alcohol

Answer 2

Mechanism: nucleophilic addition
Reagents: NaBH4
Conditions: heat
RCHO + [H] → RCOH

Question 3

Ketone → Secondary Alcohol

Answer 3

Mechanism: nucleophilic addition
Reagents: NaBH4
Conditions: heat
RC(O)R’ + [H] → RC(OH)R’

Question 4

How to detect the presence of a carbonyl group in an organic compound?

Answer 4

2,4 - DNP (in a solution of CH3OH and H2SO4) = Brady’s Reagent
Add equal volume to test tube with organic compound
Forms yellow/orange ppt = aldehyde / ketone

Question 5

How to identify a carbonyl compound from the melting point of a derivative?

Answer 5

1) use ppt from adding Brady’s reagent and filter the solids
2) recrystallise the filtrate to produce a pure sample of crystals
3) measure the melting point and compare to a dataset

Question 6

How to distinguish between aldehydes and ketones?

Answer 6

Use equal volume of Tollen’s Reagent (AgNO3 + NaOH → Ag2O, Ag2O + NH3 (aq) → Tollen’s Reagent)
Silver mirror / ppt forms = aldehyde
Ag+ + e- → Ag(s) (reduction)
RCHO + [O] → RCOOH (oxidation)

Question 7

Explain the solubility of carboxylic acids

Answer 7

Weak acids - partially dissociate
HCOOH ⇌ H+ + HCOO-

C=O and O-H bonds are polar so Carboxylic Acids form H bonds with water
RCOO- = carboxylate ion

Question 8

Carboxylic acid → Carboxylate salt

Answer 8

Mechanism: redox
Reagents: metals
2RCOOH(aq) + Mg(s) → (RCOO-)2Mg+2(aq) + H2(g)

Question 9

Carboxylic acid → Carboxylate salt

Answer 9

Mechanism: neutralisation
Reagents: carbonates
2RCOOH(aq) + Na2CO3(aq) → 2RCOO-Na+(aq) + CO2(g) + H2O(l)

Question 10

Carboxylic acid → Carboxylate salt

Answer 10

Mechanism: neutralisation
Reagent: metal oxide
2RCOOH(aq) + CaO(s) → (RCOO-)2Ca+2(aq) + H2O(l)

Question 11

Carboxylic acid → Carboxylate salt

Answer 11

Mechanism: neutralisation
Reagents: alkalis
RCOOH(aq) + NaOH(aq) → RCOO-Na+(aq) + H2O(l)

Question 12

Carboxylic acid → Ester

Answer 12

Mechanism: esterification
Reagents: alcohol, acid catalyst e.g. con. H2SO4
Conditions: heat
RCOOH + R’OH → RC(O)OR’ + H2O

Question 13

Acid anhydride → Ester + Carboxylic acid

Answer 13

Mechanism: esterification
Reagents: alcohols
RC(O)OC(O)OR’ + R”OH → R”C(O)OR’ + RCOOH

Question 14

Ester → Carboxylic acid + Alcohol

Answer 14

Mechanism: acid hydrolysis
Reagents: hot aqueous acid
Conditions: reflux
RC(O)OR’ + H2O ⇌ RCOOH + R’OH

Question 15

Ester → Carboxylate salt + Alcohol

Answer 15

Mechanism: alkaline hydrolysis (saponification)
Reagents: hot aqueous hydroxide ions
Conditions: reflux
RC(O)OR’ + NaOH → RCOO-Na+ + R’OH

Question 16

Carboxylic acid → Acyl chloride

Answer 16

Mechanism: nucleophilic substitution
Reagents: SOCl2
RCOOH + SOCl2 → RCOCl + SO2(g)+ HCl(g)

Question 17

Acyl chloride → Ester

Answer 17

Mechanism: nucleophilic addition-elimination
Reagents: Alcohol
RCOCl + R’OH → RC(O)OR’ + HCl

Question 18

Acyl chloride → Carboxylic acid

Answer 18

Mechanism: nucleophilic addition-elimination
Reagents: H2O
RCOCl + H2O → RCOOH + HCl

Question 19

Acyl chloride → Primary Amide

Answer 19

Mechanism: nucleophilic addition-elimination
Reagents: Ammonia
RCOCl + 2NH3 → RC(O)NH2 + NH4Cl

Question 20

Acyl chloride → Secondary Amide

Answer 20

Mechanism: nucleophilic addition-elimination
Reagents: Primary amine
RCOCl + 2R’NH2 → RC(O)NHR’ + R’NH3+Cl-