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Flashcards · 9620 · A Level Topic 32

Organic synthesis (A Level) flashcards

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Topic 32: Organic synthesis

Why do chemists aim for processes with no solvent?
No energy or raw materials to make, transport and remove the solvent (often by distillation), no solvent waste, and no flammable or toxic vapour to leak.
Why do chemists aim for non-hazardous starting materials?
Less cost for storage, transport, protective equipment and emergency plans, and lower risk to workers, neighbours and the environment. Example of a hazardous one: KCN (very toxic; acid releases HCN).
Why is a synthesis with fewer steps and a high atom economy preferred?
Every step loses some product and needs its own reagents, energy, separation and waste disposal. High atom economy means most reactant atoms end up in the product, so less waste.
A route has three steps with yields of 92%, 88% and 61%. Overall yield?
0.92 × 0.88 × 0.61 = 0.494, so 49.4%. Multiply the step yields as decimals.
The four 9620 reactions (other than polymerisation) that form a new C–C bond
Halogenoalkane + KCN (nitrile); aldehyde or ketone + KCN then dilute acid (hydroxynitrile); Friedel–Crafts acylation; Friedel–Crafts alkylation.
Halogenoalkane with NaOH(aq) vs with KOH in ethanol
Aqueous, warm: substitution, giving an alcohol. Ethanolic, hot: elimination, giving an alkene.
Two steps to make phenylamine from benzene
1. Concentrated HNO₃ and concentrated H₂SO₄ → nitrobenzene. 2. Reduce (e.g. tin and concentrated HCl, reflux, then NaOH) → phenylamine.
Converting methylbenzene into phenylmethanol, C₆H₅CH₂OH
1. Cl₂ in UV light → (chloromethyl)benzene (side-chain substitution). 2. Warm NaOH(aq) → phenylmethanol.
Why does benzene + Cl₂ (UV) then NaOH(aq) not give an alcohol?
In UV light chlorine adds to benzene (C₆H₆Cl₆), not chlorobenzene. And a chlorine bonded directly to a benzene ring is not substituted by hydroxide.
Reagent and conditions: secondary alcohol from a ketone, and ketone from a secondary alcohol
Ketone → alcohol: NaBH₄ in aqueous solution. Alcohol → ketone: acidified potassium dichromate(VI), heat under reflux.

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