Aldehydes, Ketones and Carboxylic Acids • Topic 3 of 3

Carboxylic Acids

Carboxylic acids contain the carboxyl group –COOH (a carbonyl fused with a hydroxyl). The general formula of monocarboxylic acids is CnH2n+1COOH. The simplest is methanoic acid (HCOOH, formic acid) and the next is ethanoic acid (CH3COOH, acetic acid).

Nomenclature & structure

IUPAC names use the suffix -oic acid, numbering the carboxyl carbon as C-1 (CH3CH2COOH is propanoic acid; HOOC–COOH is ethanedioic/oxalic acid). The carboxyl carbon is sp2; both C–O bonds are intermediate between single and double because of resonance, so the group is planar. Carboxylic acids form strong intermolecular H-bonds (often as dimers), giving boiling points higher than alcohols of similar mass.

Methods of preparation

(a) Oxidation of 1° alcohols or aldehydes with KMnO4/K2Cr2O7 (e.g. CH3CH2OH → CH3COOH). (b) From nitriles and amides by acidic or basic hydrolysis (RCN + H2O/H+ → RCOOH). (c) From Grignard reagents + CO2: RMgX + CO2 → RCOOMgX, then H3O+ → RCOOH (adds one carbon). (d) From acyl halides/esters/anhydrides by hydrolysis. Oxidation of alkylbenzenes (KMnO4) gives benzoic acid.

Acidity

Carboxylic acids are far more acidic than alcohols and phenols. On losing H+ they give the carboxylate ion, in which the negative charge is delocalised equally over two equivalent oxygens by resonance — a far greater stabilisation than the phenoxide ion enjoys. The strength is tuned by substituents: electron-withdrawing groups (e.g. –Cl, –NO2) stabilise the anion and increase acidity (so trichloroacetic acid > dichloroacetic acid > chloroacetic acid > acetic acid), while electron-donating groups (alkyl, +I) decrease it. The closer the EWG is to –COOH and the more such groups present, the stronger the acid. A lower $pK_a$ means a stronger acid.

Reactions

(a) Salt formation: with NaOH, Na2CO3 or NaHCO3 (effervescence of CO2 with bicarbonate distinguishes acids from phenols). (b) Formation of derivatives: with SOCl2/PCl5 → acid chloride; with an alcohol/H+ → ester (esterification); on heating with another acid molecule (loss of water, P4O10) → anhydride; with NH3 then heat → amide. (c) Reduction by LiAlH4 (or B2H6) gives a primary alcohol. (d) Decarboxylation: heating the sodium salt with soda-lime (NaOH/CaO) removes –COOH as CO2 to give an alkane (CH3COONa → CH4). (e) Hell–Volhard–Zelinsky (HVZ): acids with an α-H react with Cl2/Br2 + red P to give α-halo acids. (f) Ring substitution: in benzoic acid the –COOH is deactivating and meta-directing, so halogenation/nitration occurs mainly at the m-position.

Distinction tests for carbonyl compounds and carboxylic acids
Reagent / TestAliphatic aldehydeAromatic aldehydeMethyl ketoneCarboxylic acid
Tollens (ammoniacal AgNO3)Silver mirrorSilver mirrorNo reactionNo reaction
Fehling solutionRed Cu2O pptNo reactionNo reactionNo reaction
Iodoform (I2/NaOH)Only ethanal (+)No reactionYellow CHI3 (+)No reaction
NaHCO3No reactionNo reactionNo reactionCO2 effervescence
Solved Examples
1
Worked Example
Arrange in increasing acid strength: CH3COOH, ClCH2COOH, Cl2CHCOOH, Cl3CCOOH.
Solution
  1. Chlorine is electron-withdrawing (–I); it stabilises the carboxylate anion.
  2. More Cl atoms → greater stabilisation → stronger acid.
  3. So strength rises with the number of Cl atoms.

Answer: CH3COOH < ClCH2COOH < Cl2CHCOOH < Cl3CCOOH.

2
Worked Example
How can ethanoic acid be prepared from methyl magnesium bromide (CH3MgBr)?
Solution
  1. A Grignard reagent adds to CO2 (dry ice).
  2. CH3MgBr + CO2 → CH3COOMgBr.
  3. Acidic work-up (H3O+) gives CH3COOH.

Answer: CH3MgBr + CO2 → CH3COOMgBr ⟶(H3O+) CH3COOH (ethanoic acid).

3
Worked Example
Why is a carboxylic acid more acidic than the corresponding alcohol and phenol?
Solution
  1. Acidity depends on the stability of the anion left after losing H+.
  2. The carboxylate ion has its negative charge delocalised equally over two equivalent O atoms (strong resonance).
  3. An alkoxide has no such resonance; phenoxide has resonance into the ring but the charge ends on less electronegative carbons, so it is less stabilised.

Answer: The carboxylate anion is the most resonance-stabilised, so the carboxylic acid is the strongest of the three.

4
Worked Example
Identify the reagent that gives an acid chloride from a carboxylic acid, and write the reaction with ethanoic acid.
Solution
  1. SOCl2 (thionyl chloride) is preferred because the by-products (SO2, HCl) are gases.
  2. CH3COOH + SOCl2 → CH3COCl + SO2 + HCl.
  3. (PCl5 or PCl3 also work.)

Answer: SOCl2; CH3COOH → CH3COCl (acetyl chloride).

5
Worked Example
What is the product of heating sodium acetate with soda-lime? Name the reaction.
Solution
  1. Soda-lime (NaOH + CaO) on heating removes the –COOH group as CO2.
  2. CH3COONa + NaOH ⟶(CaO, Δ) CH4 + Na2CO3.
  3. This is decarboxylation.

Answer: Methane (CH4) by decarboxylation.

6
Worked Example
How would you distinguish ethanoic acid from phenol using a simple test?
Solution
  1. Carboxylic acids are stronger than carbonic acid; phenol is weaker.
  2. Add aqueous NaHCO3 to each.
  3. Ethanoic acid liberates CO2 (brisk effervescence); phenol gives no reaction.

Answer: NaHCO3 test — ethanoic acid gives CO2 effervescence, phenol does not.

Key Points

  • The carboxyl group –COOH has an sp2 carbon and equivalent resonance-shared C–O bonds; acids exist as H-bonded dimers (high b.p.).
  • Preparation: oxidation of 1° alcohols/aldehydes, hydrolysis of nitriles/amides/esters, and Grignard + CO2 (adds one carbon).
  • Acidity comes from the resonance-stabilised carboxylate; electron-withdrawing groups (–Cl, –NO2) raise acidity (lower $pK_a$), electron-donating groups lower it.
  • Reactions: salts (NaHCO3 gives CO2, unlike phenol), acid chlorides (SOCl2), esters (ROH/H+), anhydrides, amides; reduction (LiAlH4) to 1° alcohols.
  • Decarboxylation (soda-lime) gives alkanes; HVZ gives α-halo acids; in benzoic acid –COOH is deactivating and meta-directing.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.Which is the strongest acid?
Explanation: Fluorine is the most electronegative halogen, so its –I effect stabilises the carboxylate most, giving the strongest acid.
Q2.Grignard reagent + CO2 followed by acid hydrolysis gives a:
Explanation: RMgX adds to CO2 to give RCOOMgX, which on H3O+ gives RCOOH.
Q3.Which reagent best converts a carboxylic acid into an acid chloride?
Explanation: SOCl2 gives the acid chloride with gaseous by-products (SO2, HCl) that escape, leaving a pure product.
Q4.The carboxyl group on a benzene ring directs an incoming electrophile mainly to the:
Explanation: –COOH is deactivating and meta-directing, so substitution occurs chiefly at the meta position.
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