OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2025: Question 21

✅ Correct Answer

An atom or group of atoms responsible for the characteristic reactions (or chemical properties) of a compound.

🧠 Exam Technique

Keep definitions textbook-exact. Examiners look specifically for:

• "Atom or group of atoms" (or part of an organic molecule)
• "Characteristic reactions" or "chemical properties"

❌ Common Errors

Referring to physical properties (such as boiling points or solubility). A functional group dictates chemical reactivity, not solely physical characteristics.

✅ Correct Answer

• Reagent: 2,4-dinitrophenylhydrazine (or 2,4-DNP / Brady's reagent) [1]
• Observation: Orange / red / yellow precipitate (or solid/crystals) [1]

💡 Key Knowledge

All three molecules (pyruvic acid, methylglyoxal, and acetol) contain a C=O ketone group (and methylglyoxal also contains an aldehyde group). 2,4-DNP condenses with both aldehydes and ketones to form insoluble hydrazone derivatives.

🧠 Exam Technique

The observation mark is awarded completely independently of the reagent mark, but you must specify a precipitate (or solid/crystals). Stating merely "turns orange" scores 0.

💡 Chemical Logic

• Tollens' reagent: Tests selectively for aldehydes (oxidised to carboxylic acid while Ag⁺ is reduced to Ag). Methylglyoxal reacts; pure pyruvic acid does not.
• Na₂CO₃(aq): Acid-base test. Only pyruvic acid possesses a -COOH group acidic enough to liberate CO₂(g).
• Acidified Dichromate: Oxidises aldehydes (-CHO → -COOH) and primary alcohols (-CH₂OH → -CHO / -COOH). Ketones and carboxylic acids cannot be further oxidised.

❌ Common Errors

• Writing "CO₂ gas formed" instead of an observable change ("effervescence" or "fizzing").
• Leaving boxes blank instead of explicitly stating "no change".
• Stating dichromate turns "blue" instead of green (Cr³⁺).

✅ Detailed Step-by-Step Mechanism

1. Mark 1: Curly arrow from the lone pair on :H⁻ (hydride ion) to the carbonyl C atom (δ+).
2. Mark 2: Correct permanent dipole (δ+ on C, δ- on O) on the ketone C=O, AND curly arrow starting from the C=O double bond onto the oxygen atom (O).
3. Mark 3 (Intermediate): Formation of tetrahedral intermediate H₃C-C(O:⁻)(COOH)-H , with a curly arrow from a lone pair on :O⁻ to the H atom of H₂O (or H⁺).
4. Mark 4 (Final Product): Correct organic structure: 2-hydroxypropanoic acid (lactic acid), H₃C-CH(OH)-COOH .

🧠 Curly Arrow Precision Guide

Examiners check arrow geometry strictly:

• The first arrow must start from the lone pair on :H⁻ (or traced back to the negative charge) and terminate on the carbonyl carbon atom.
• The C=O breaking arrow must originate within the C=O double bond line and terminate cleanly on the oxygen atom.
• The intermediate oxygen must possess a formal full negative charge (:O⁻). Writing δ- on the intermediate oxygen is penalised!
• The protonation arrow starts from the lone pair on :O⁻ and points to the H atom of H₂O (or H⁺).

✅ 1. Formation of Acyl Chloride

Reagent: SOCl₂ (thionyl chloride)
(also accept PCl₃, PCl₅, or COCl₂)

Acyl Chloride Structure:
CH₃-C(=O)-C(=O)Cl (Pyruvoyl chloride)

2 Marks total (1 for reagent, 1 for structure)

✅ 2. Compound P (Amide Synthesis)

Reaction of acyl chloride with excess NH₃ replaces -Cl with -NH₂:

Structure:
CH₃-C(=O)-CONH₂ (2-oxopropanamide)

1 Mark (Do NOT write -CONH₃⁺)

✅ 3. Reaction with NaOH(aq)

Neutralisation of the carboxylic acid group:

Structure:
CH₃-C(=O)-COO⁻ Na⁺ (Sodium pyruvate)

1 Mark

✅ 4. Reaction with CH₃OH / H₂SO₄

Acid-catalysed esterification of the -COOH group:

Structure:
CH₃-C(=O)-COOCH₃ (Methyl pyruvate)

1 Mark

✅ 5. Synthesis of Compound Q (Bottom Pathway)

Compound Q contains a 1,2-hydroxyamine group ( -CH₂NH₂ ), formed by catalytic hydrogenation (H₂/Ni) of a nitrile group (-C≡N):

Reagent: HCN OR NaCN / H⁺ (Acid required) [1]

Cyanohydrin Intermediate:
CH₃-C(OH)(CN)-COOH [1]

2 Marks total (1 for reagent, 1 for intermediate)

🧠 Retrosynthetic Clue for Compound Q

Inspect the product Q: H₂N-CH₂-C(OH)(CH₃)-COOH . Notice the -CH₂NH₂ group and the -OH on the same carbon where the ketone carbonyl used to be. Adding HCN across the ketone C=O yields a hydroxynitrile with an extra carbon. Reduction of -C≡N using H₂/Ni directly yields primary amine -CH₂-NH₂ .

❌ Flowchart Red Flags

• Writing just NaCN without an acid ( H⁺ or H₂SO₄ ) for cyanohydrin formation. The acid is mandatory for the mark!
• Inverting connectivity on the nitrile or hydroxyl groups: write -CN or -C≡N (never -NC ) and HO-C (never C-OH with bond to H).
• Hydrolysing the ketone during esterification: only the carboxylic acid forms the ester.