Carbonyls, carboxylic acids and esters

How are alcohols classified as primary, secondary or tertiary?

• Primary (1°): –OH group attached to a carbon bonded to one alkyl group (e.g. ethanol).

• Secondary (2°): –OH group attached to a carbon bonded to two alkyl groups (e.g. propan-2-ol).

• Tertiary (3°): –OH group attached to a carbon bonded to three alkyl groups (e.g. 2-methylpropan-2-ol).

How is a primary alcohol oxidised? Include reagents, conditions, and products.

• Reagent: Acidified potassium dichromate (K₂Cr₂O₇ / H₂SO₄).

• Partial oxidation: Distil to form an aldehyde (R–CH₂OH → R–CHO).

• Further oxidation: Reflux to form a carboxylic acid (R–CHO → R–COOH).

• Observation: Orange → Green (Cr⁶⁺ reduced to Cr³⁺).

How is a secondary alcohol oxidised? Include reagents, conditions, and products.

• Reagent: Acidified potassium dichromate (K₂Cr₂O₇ / H₂SO₄).

• Condition: Reflux.

• Product: Ketone (R₁–CHOH–R₂ → R₁–CO–R₂).

• Observation: Orange → Green.

• Note: Tertiary alcohols do not oxidise

Why do aldehydes with 3 or more carbons have isomers that are ketones?

Because the same molecular formula can give structures with the carbonyl group at the end (aldehyde) or within the chain (ketone).
Example: Propanal (CH₃CH₂CHO) and propanone (CH₃COCH₃)

Describe the mechanism of nucleophilic addition of NaBH₄ to a carbonyl compound.

1. Nucleophile: Hydride ion (H⁻) from NaBH₄.

2. H⁻ attacks δ⁺ carbon in C=O, breaking the π bond.

3. O⁻ intermediate forms, then protonated by H₂O or methanol → alcohol.
   Product: Aldehyde → primary alcohol; Ketone → secondary alcohol.

Describe the mechanism of nucleophilic addition of HCN to a carbonyl compound.

1. CN⁻ (from NaCN) attacks δ⁺ carbon of C=O.

2. O⁻ intermediate forms, then protonated by H⁺ (from H₂SO₄).
   Product: Hydroxynitrile (contains –OH and –CN).
   Reaction type: Nucleophilic addition.

How is 2,4-dinitrophenylhydrazine (2,4-DNP) used to detect carbonyls?

• Reagent: 2,4-DNP (Brady’s reagent).

• Observation: Orange/yellow precipitate = presence of carbonyl (aldehyde or ketone).

• Identification: Recrystallise derivative and measure melting point, then compare to data table to identify the carbonyl compound.

How is Tollens’ reagent used to detect an aldehyde?

• Reagent: Tollens’ reagent = ammoniacal silver nitrate ([Ag(NH₃)₂]⁺).

• Procedure: Warm gently with sample.

• Observation:

  • Aldehyde → silver mirror (Ag⁺ reduced to Ag(s)); aldehyde oxidised to carboxylic acid.

  • Ketone → no visible change.
    Equation: RCHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → RCOO⁻ + 2Ag(s) + 4NH₃ + 2H₂O.

How do you name carboxylic acids and draw their displayed and skeletal formulae?

• Suffix: –oic acid

• Functional group: –COOH

• Example 1: Ethanoic acid (CH₃COOH)

  • Displayed: H–C(=O)–OH

  • Skeletal: two-carbon chain ending with COOH

• Example 2: Butanoic acid (CH₃CH₂CH₂COOH)

Explain the solubility of carboxylic acids and show hydrogen bonding to water.

• Carboxylic acids can hydrogen bond with water molecules via their C=O and O–H groups.

• Small-chain acids (up to 4 carbons) are highly soluble.

• Solubility decreases as hydrocarbon chain length increases.
  Diagram description:
  O–H···O and C=O···H hydrogen bonds form between the acid and H₂O molecules.

How do carboxylic acids react with metals, bases, and carbonates?

1. With metals:
   2RCOOH+2Na→2RCOONa+H22RCOOH+2Na→2RCOONa+H2​
   → effervescence (hydrogen gas).

2. With metal oxides / hydroxides:
   2RCOOH+CaO→(RCOO)2Ca+H2O2RCOOH+CaO→(RCOO)2​Ca+H2​O
   RCOOH+NaOH→RCOONa+H2ORCOOH+NaOH→RCOONa+H2​O

3. With carbonates:
   2RCOOH+Na2CO3→2RCOONa+H2O+CO22RCOOH+Na2​CO3​→2RCOONa+H2​O+CO2​
   → fizzing (carbon dioxide released).

How are esters formed? Give three examples

1. Carboxylic acid + alcohol (acid catalyst):
   RCOOH+R′OH⇌RCOOR′+H2ORCOOH+R′OH⇌RCOOR′+H2​O
   → Use H₂SO₄ (reflux).
   Example: Ethanoic acid + ethanol → ethyl ethanoate.

2. Acid anhydride + alcohol:
   (RCO)2O+R′OH→RCOOR′+RCOOH(RCO)2​O+R′OH→RCOOR′+RCOOH.

3. Acyl chloride + alcohol:
   RCOCl+R′OH→RCOOR′+HClRCOCl+R′OH→RCOOR′+HCl.

What are the two types of ester hydrolysis?

1. Acid hydrolysis (reversible):
   RCOOR′+H2O⇌RCOOH+R′OHRCOOR′+H2​O⇌RCOOH+R′OH
   Conditions: Hot aqueous acid (HCl or H₂SO₄), reflux.

2. Alkaline hydrolysis (irreversible):
   RCOOR′+NaOH→RCOONa+R′OHRCOOR′+NaOH→RCOONa+R′OH
   Conditions: Hot aqueous alkali, reflux.

How do you name esters?

• Format: [alkyl] [alkanoate]

  • Alkyl = from alcohol

  • Alkanoate = from carboxylic acid

• Example: ethanol + ethanoic acid → ethyl ethanoate.

How are acyl chlorides formed from carboxylic acids?

• Reagent: Thionyl chloride (SOCl₂)

• Reaction:
  RCOOH+SOCl2→RCOCl+SO2+HClRCOOH+SOCl2​→RCOCl+SO2​+HCl

• Observation: White fumes of HCl; toxic gases SO₂ and HCl evolved.

What are the main reactions of acyl chlorides?

1. With alcohols: → Ester + HCl
   RCOCl+R′OH→RCOOR′+HClRCOCl+R′OH→RCOOR′+HCl

2. With water: → Carboxylic acid + HCl
   RCOCl+H2O→RCOOH+HClRCOCl+H2​O→RCOOH+HCl

3. With ammonia: → Primary amide + HCl
   RCOCl+NH3→RCONH2+HClRCOCl+NH3​→RCONH2​+HCl

4. With amines: → Secondary amide + HCl
   RCOCl+R′NH2→RCONHR′+HClRCOCl+R′NH2​→RCONHR′+HCl

How do you name acyl chlorides?

• Replace “–oic acid” with “–oyl chloride”.

• Examples:

  • Methanoic acid → methanoyl chloride

  • Ethanoic acid → ethanoyl chloride