Edexcel International A Level Chemistry: Organic synthesis (YCH11) – Revision Notes

These notes condense Topic 20: Organic Synthesis (outcomes 20.1 to 20.5, with Core Practical 16) as set out in Issue 1 (September 2017) of Pearson Edexcel’s International Advanced Subsidiary/Advanced Level in Chemistry specification, YCH11. It sits in Unit 5 (Transition Metals and Organic Nitrogen Chemistry), an International A2 (IA2) unit. The organic synthesis study guide explains each idea in full, with worked examples.

Also useful: the practice questions, the checklist, a free diagnostic, the course hub, and the earlier notes on mass spectra and IR.

Key definitions

• Empirical formula: the elements in a compound in their lowest whole-number ratio.
• Molecular formula: the full count of each kind of atom in a molecule; a whole-number multiple of the empirical formula.
• Molecular ion (M⁺): the ion of the whole molecule; its m/z gives Mr.
• Grignard reagent: an organomagnesium halide, RMgX, made from a halogenoalkane and magnesium in dry ether.
• Reflux: heating with a vertical condenser so vapour returns to the flask.
• Steam distillation: distilling a volatile, water-immiscible compound with steam, below 100 °C.
• Recrystallisation: purifying a solid by dissolving it in minimum hot solvent and letting it crystallise on cooling.

20.1 Structure from data: method in steps

1. Combustion: n(C) = n(CO₂); n(H) = 2 × n(H₂O); mass O = sample − C − H.
2. Percentage composition: divide each % by Ar, then divide by the smallest.
3. Scale the empirical formula to Mr from M⁺.
4. Tests and IR give functional groups. NMR gives the skeleton.
5. Check the structure fits every piece of data.

Evidence and what it tells you

Evidence	Points to
Effervescence with Na₂CO₃(aq)	COOH
Orange precipitate with 2,4-DNPH	C=O in aldehyde or ketone
Silver mirror (Tollens’) or red-brown precipitate (Fehling’s)	aldehyde
Pale yellow precipitate with iodine and alkali	CH₃CO– or CH₃CH(OH)–
Orange to green with acidified dichromate(VI)	primary or secondary alcohol, or aldehyde
Steamy fumes with PCl₅	OH (alcohol or acid)
IR very broad band about 2500–3300 cm⁻¹	O–H of a carboxylic acid
IR broad band about 3200–3550 cm⁻¹	O–H of an alcohol
IR strong band about 1700–1750 cm⁻¹	C=O
¹³C NMR: number of peaks	number of carbon environments
¹H NMR peak areas	ratio of H atoms in each environment
¹H NMR splitting	n + 1 peaks, where n = H atoms on adjacent carbons

Use the chemical shift values the question supplies. Typical regions: alkyl CH about δ 0.9–1.7; CH next to C=O about δ 2.0–2.6; CH on C–O about δ 3.3–4.2; aromatic H about δ 6.5–8.0; CHO about δ 9–10; COOH about δ 10–12.

20.2 Grignard reagents

Make: RX + Mg → RMgX, in dry ether. Use: add the second reactant in dry ether, then hydrolyse with dilute acid.

Reagent with RMgX	Product	Alcohol class
CO₂	RCOOH	(acid, not alcohol)
HCHO	RCH₂OH	primary
R′CHO	RCH(OH)R′	secondary
R′COR″	RC(OH)R′R″	tertiary

Why it works: the C bonded to Mg is δ−, so it attacks the δ+ carbon of C=O, making a new C–C bond.

Why dry: water reacts with RMgX to give the alkane RH, destroying the reagent.

Working backwards to a Grignard pair

1. Find the carbon carrying OH (or COOH).
2. Break one C–C bond to that carbon.
3. One fragment becomes RMgX; the other becomes the C=O compound (or CO₂).
4. Check the carbon count adds up.

20.3 Problem solving

Predicting properties (20.3 i)

Name every group. Predict each group’s reactions and its effect on boiling temperature and solubility (hydrogen bonding from O–H or N–H). Give the reason with each prediction.

Route planning (20.3 ii)

1. Compare start and target: carbon count, functional groups, positions.
2. Work backwards one step at a time.
3. Give reagents and conditions for every step, and name each intermediate.
4. Up to four steps; the shorter route usually wins on overall yield.
5. Use any reaction the question gives you exactly as described.

Conversions often needed

Change	Reagents and conditions
alcohol → halogenoalkane	PCl₅, or KBr with 50% H₂SO₄, or red P with I₂
halogenoalkane → alcohol	NaOH(aq), heat under reflux
halogenoalkane → nitrile (+1 C)	KCN in ethanol, heat under reflux
nitrile → amine	LiAlH₄ in dry ether
halogenoalkane → amine	excess ammonia in ethanol, heat in a sealed tube
1° alcohol → aldehyde	K₂Cr₂O₇/H₂SO₄, distil as it forms
1° alcohol → acid; 2° → ketone	K₂Cr₂O₇/H₂SO₄, heat under reflux
C=O → alcohol	LiAlH₄ in dry ether
acid → acyl chloride	PCl₅
benzene → nitrobenzene → phenylamine	conc. HNO₃/conc. H₂SO₄ below 55 °C; then Sn/conc. HCl, then NaOH

Practical choices and hazards (20.3 iii, iv)

Pick the technique for the job. Then pair each hazard with a matching control: flammable → no flames, water bath or electric heater; corrosive → gloves and eye protection; toxic vapour → fume cupboard; water-reactive → dry apparatus.

20.4 Core Practical 16: aspirin

Equation: C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH

Stage	Purpose
Excess ethanoic anhydride, acid catalyst, warm in a water bath	Make the reaction go to completion at a useful rate
Add cold water	Destroy excess anhydride
Cool in ice; filter under reduced pressure	Crystallise and collect crude product
Recrystallise from minimum hot solvent	Remove soluble impurities
Dry; weigh; melting temperature	Yield and purity

Pure aspirin melts sharply at about 135–136 °C. A lower, broad range shows impurity (often unreacted 2-hydroxybenzoic acid) or water.

20.5 Techniques at a glance

Technique	Removes or achieves	Key detail
Reflux	long heating without losing volatile substances	vertical condenser, anti-bumping granules
Washing with water	water-soluble impurities	separating funnel
Washing with Na₂CO₃(aq)	acid impurities	release CO₂ pressure
Solvent extraction	moves product into a second solvent	several small portions beat one large
Recrystallisation	soluble and insoluble impurities from a solid	minimum hot solvent, hot filtration, cool, cold wash
Drying	water from an organic liquid	anhydrous MgSO₄ or CaCl₂ until clear
Distillation	separates by boiling temperature	bulb level with side arm
Steam distillation	volatile, water-immiscible product that would decompose	distils below 100 °C
Melting temperature	purity of a solid	sharp and matches data if pure
Boiling temperature	identity and purity of a liquid	steady reading during distillation

Must-know distinctions

• Reflux vs distillation: reflux keeps everything in the flask; distillation removes the volatile part.
• Washing vs extraction: washing removes impurities from the product layer; extraction moves the product out of the mixture.
• Grignard + methanal vs other aldehydes: methanal gives a primary alcohol; other aldehydes give secondary alcohols.
• Empirical vs molecular formula: you need Mr to go from one to the other.

Quick self-test

1. A compound contains C 48.6%, H 8.1% and O 43.2% by mass. M⁺ is at m/z = 74. Find the molecular formula.
2. Complete combustion of 0.430 g of a compound of C, H and O gives 1.100 g of CO₂ and 0.450 g of H₂O. Find the empirical formula.
3. Name the Grignard reagent made from iodomethane.
4. Name the product when propylmagnesium bromide reacts with CO₂, then dilute acid.
5. Name the product when methylmagnesium iodide reacts with propanal, then dilute acid.
6. Which carbonyl compound gives a primary alcohol with a Grignard reagent?
7. Why must the ether be dry?
8. Calculate the mass of 2-hydroxybenzoic acid needed to make 2.70 g of aspirin at 100% yield (Mr 138.0 and 180.0).
9. Why is cold water added after the aspirin reaction?
10. Which technique purifies a volatile, water-immiscible liquid that decomposes near its boiling temperature?
11. A proton NMR peak is a triplet. How many H atoms are on adjacent carbons?

Answers

1. Moles: 4.05 : 8.1 : 2.7 = 1.5 : 3 : 1 = 3 : 6 : 2. C₃H₆O₂ has Mr 74.0, so C₃H₆O₂.
2. n(C) = 0.0250 mol (0.300 g); n(H) = 0.0500 mol (0.050 g); O = 0.080 g = 0.0050 mol. Ratio 5 : 10 : 1, C₅H₁₀O.
3. Methylmagnesium iodide, CH₃MgI.
4. Butanoic acid.
5. Butan-2-ol.
6. Methanal.
7. Water reacts with the Grignard reagent, giving an alkane.
8. 2.70 ÷ 180.0 = 0.0150 mol; 0.0150 × 138.0 = 2.07 g.
9. To destroy the excess ethanoic anhydride.
10. Steam distillation.
11. Two.

Where marks are usually lost

• Leaving out oxygen by difference in combustion data.
• Rounding mole ratios too early, so 1.5 becomes 2.
• Writing “Grignard in water” or leaving out “dry ether”.
• Giving the acid step before the addition step in a Grignard reaction.
• Reagents without conditions (reflux, distil, sealed tube, temperature).
• Naming a reagent, such as “dichromate”, without the acid.
• Saying a melting temperature is “higher” when impure; it is lower and over a range.
• Controls that do not answer the stated hazard.
• A recrystallisation description with no “minimum volume of hot solvent”.

Official syllabus

Source: Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry, specification XCH11/YCH11, Issue 1, September 2017 (Pearson Education Limited). These notes cover Topic 20: Organic Synthesis, outcomes 20.1 to 20.5, including Core Practical 16, in Unit 5.