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Chemistry · Unit 18

Carboxylic Acids: MDCAT Notes and MCQs

Nomenclature & structure

  • Functional group –COOH (carboxyl = carbonyl + hydroxyl). IUPAC suffix -oic acid: HCOOH methanoic (formic) acid — from ants; CH3COOH ethanoic (acetic) acid — vinegar (4–5%); C6H5COOH benzoic acid.
  • Carboxyl carbon is sp2. Strong H-bonding → exist as dimers; high b.p.

Preparation

  • Oxidation of 1° alcohols / aldehydes (K2Cr2O7/H2SO4, KMnO4).
  • Hydrolysis of nitriles: RCN + 2H2O (H+) → RCOOH + NH4+ (adds one carbon via RX + KCN).
  • Grignard reagent + CO2, then acid hydrolysis → RCOOH.
  • Oxidation of alkylbenzenes → benzoic acid.

Reactivity / acidity

  • Acidic because the carboxylate ion is resonance-stabilized (charge shared by two equivalent O atoms). Acidity: carboxylic acid > phenol > water > alcohol. Liberate CO2 from NaHCO3 (brisk effervescence).
  • Electron-withdrawing groups increase acidity: CCl3COOH > CHCl2COOH > CH2ClCOOH > CH3COOH. Electron-donating alkyl groups decrease acidity: HCOOH > CH3COOH.
  • Reduction with LiAlH4 → 1° alcohol. Formic acid also acts as a reducing agent (gives Tollens' test).

Acid derivatives (RCO–Z)

DerivativeMade from acid by
Acyl halide RCOClSOCl2 (best; by-products SO2 + HCl are gases), PCl5, PCl3
Acid anhydride (RCO)2OHeating with P2O5 (dehydration), or RCOCl + RCOONa
Ester RCOOR'Alcohol + conc. H2SO4 (Fischer esterification)
Amide RCONH2Ammonia → ammonium salt → heat

Reactivity of derivatives (towards nucleophilic acyl substitution): acyl chloride > anhydride > ester > amide. A more reactive derivative can be converted into a less reactive one: RCOCl + R'OH → ester; RCOCl + NH3 → amide; anhydride + alcohol → ester; all hydrolyse back to the acid (acyl chlorides fastest — fume in moist air). Ester hydrolysis with NaOH = saponification (soap).

PMDC learning outcomes for this unit
  • Carboxylic acids & derivatives: 18.1 Describe nomenclature, structure and preparation of carboxylic acid.
  • Carboxylic acids & derivatives: 18.2 Discuss reactivity of carboxylic acid.
  • Carboxylic acids & derivatives: 18.3 Describe the chemistry of carboxylic acid by conversion to carboxylic acid derivatives: acyl halides, acid anhydrides, esters and reactions involving inter-conversion of these.

Carboxylic Acids MCQs with answers

Practise and track your score →
Q1
The IUPAC name of CH₃COOH is:
AEthanoic acid
BMethanoic acid
CPropanoic acid
DAcetic aldehyde
Show answer
Answer: A (Ethanoic acid). Two-carbon carboxylic acid = ethanoic acid.
Q2
Carboxylic acids are more acidic than phenols because:
AThey are liquids
BCarboxylate ion is resonance-stabilized over two oxygen atoms
CThey have higher molar mass
DThey have no H-bonding
Show answer
Answer: B (Carboxylate ion is resonance-stabilized over two oxygen atoms). Charge is shared equally by two electronegative oxygens.
Q3
Which reagent distinguishes acetic acid from phenol?
ANaHCO₃
BNa
CWater
DNaOH
Show answer
Answer: A (NaHCO₃). Carboxylic acids liberate CO2 from NaHCO3; phenol does not.
Q4
The strongest acid among the following is:
ACH₃COOH
BCl₂CHCOOH
CClCH₂COOH
DCl₃CCOOH
Show answer
Answer: D (Cl₃CCOOH). Three electron-withdrawing Cl atoms stabilize the anion most.
Q5
The best reagent to convert a carboxylic acid to an acyl chloride is:
ACl₂
BNaCl
CHCl
DSOCl₂
Show answer
Answer: D (SOCl₂). Thionyl chloride gives gaseous by-products (SO2, HCl), so product is easily purified.
Q6
The most reactive carboxylic acid derivative is:
AEster
BAcyl chloride
CAcid anhydride
DAmide
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Answer: B (Acyl chloride). Cl⁻ is the best leaving group: acyl chloride > anhydride > ester > amide.
Q7
Alkaline hydrolysis of esters is called:
AHydrogenation
BSaponification
CFermentation
DEsterification
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Answer: B (Saponification). Saponification gives the carboxylate salt (soap) and alcohol.
Q8
Hydrolysis of CH₃CN gives:
AEthanal
BEthanol
CMethanoic acid
DEthanoic acid
Show answer
Answer: D (Ethanoic acid). Nitrile hydrolysis gives the carboxylic acid with the same number of carbons (CH3COOH).
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