Organic Chemistry Questions & Answers

A named reaction is an organic transformation named after its discoverer(s). These names provide a shorthand for describing complex synthetic sequences. Examples include Robinson Annulation (Robinson, 1935), Diels-Alder reaction (Diels & Alder, 1928), and Wittig reaction (Wittig, 1954).

Robinson Annulation is a tandem reaction combining Michael addition and intramolecular aldol condensation to build a new six-membered carbocyclic ring bearing an α,β-unsaturated ketone (cyclohex-2-enone). Published by Sir Robert Robinson in 1935, it is fundamental to steroid and terpenoid synthesis.

The two starting materials are: (1) a ketone with an α-hydrogen (the Michael donor, e.g. cyclohexanone) and (2) methyl vinyl ketone, MVK (the Michael acceptor, CH₂=CH–CO–CH₃, but-3-en-2-one). Full answer with mechanism →

Michael Addition is the 1,4-conjugate addition of a nucleophile (Michael donor: enolate, malonate, etc.) to the β-carbon of an α,β-unsaturated carbonyl compound (Michael acceptor). It forms a new C–C bond without ring closure. Named after Arthur Michael (1887).

Aldol Condensation is a reaction between two carbonyl compounds (aldehyde or ketone) in which the α-carbon of one attacks the carbonyl carbon of the other to give a β-hydroxy carbonyl compound (aldol product), which then undergoes dehydration under basic or acidic conditions to give an α,β-unsaturated carbonyl compound.

Claisen Condensation is the reaction of two ester molecules (or an ester and a ketone) in the presence of a strong base (NaOEt) to give a β-keto ester. The α-carbon of one ester attacks the carbonyl of the other, with elimination of an alkoxide. Dieckmann condensation is the intramolecular version.

The Diels–Alder reaction is a [4+2] cycloaddition between a conjugated diene (4π electrons) and a dienophile (2π electrons, usually an α,β-unsaturated carbonyl) to give a cyclohexene ring. It is a pericyclic reaction that proceeds in a single concerted step, often with high stereospecificity. Awarded the Nobel Prize in 1950.

A Grignard reagent (RMgX) is an organomagnesium halide formed by reacting an organic halide (RX) with magnesium metal in anhydrous ether. The carbon–Mg bond is highly polarised, making the carbon strongly nucleophilic. Grignard reagents react with aldehydes and ketones to give alcohols. Discovered by Victor Grignard (Nobel Prize, 1912).

The Wittig reaction converts an aldehyde or ketone to an alkene using a phosphorus ylide (phosphonium ylide, R₂C=PPh₃). The reaction proceeds through a four-membered oxaphosphetane intermediate. It offers excellent control over alkene geometry. Georg Wittig received the Nobel Prize in 1979.

Friedel–Crafts reactions are electrophilic aromatic substitution reactions catalysed by a Lewis acid (AlCl₃). Alkylation: ArH + RX → ArR. Acylation: ArH + RCOCl → ArCOR. Acylation is preferred synthetically because it avoids polyalkylation and carbocation rearrangements.

The Cannizzaro reaction is a disproportionation of an aldehyde without an α-hydrogen (e.g. benzaldehyde, formaldehyde) in concentrated NaOH. One molecule is oxidised to the carboxylate and the other is reduced to the alcohol. The hydride is transferred directly between the two aldehyde molecules.

The Sandmeyer reaction converts aryl diazonium salts (ArN₂⁺) to aryl halides (ArX, where X = Cl, Br, CN, SCN) using cuprous salts (CuCl, CuBr, CuCN). It is widely used to introduce halogens regiospecifically onto aromatic rings. The diazonium salt is formed from ArNH₂ by treatment with NaNO₂/HCl at 0–5°C.

The Beckmann rearrangement converts a ketone oxime to an amide under acidic conditions (H₂SO₄, PCl₅, BF₃). For cyclic oximes the product is a lactam (ring expansion by one atom: cyclohexa-none oxime → ε-caprolactam, used in Nylon-6 production). The anti-substituent migrates to the nitrogen.

Addition: Two reactants combine to form one product with no atoms lost. Common with alkenes (electrophilic addition), carbonyls (nucleophilic addition) and conjugated systems (1,4-addition). Substitution: One atom or group replaces another. Common with alkyl halides (SN1/SN2) and aromatic compounds (electrophilic or nucleophilic aromatic substitution).

An enolate is the anion formed when a base removes an α-hydrogen from a carbonyl compound. It is resonance-stabilised between a carbanion form (charge on C) and an alkoxide form (charge on O). Enolates are key nucleophiles in aldol condensations, Michael additions, Claisen condensations, and Robinson Annulation.

Conjugate addition (1,4-addition) is the nucleophilic addition of a nucleophile to the β-carbon (C-4 position) of an α,β-unsaturated carbonyl. This is contrasted with 1,2-addition (direct addition to the carbonyl carbon). Soft nucleophiles (enolates, thiols) prefer 1,4-addition (thermodynamic); hard nucleophiles (Grignard, organolithium) prefer 1,2-addition.

A 1,5-diketone is a compound with two ketone groups separated by five carbons in the chain. It is the key intermediate in Robinson Annulation, formed after the Michael addition of an enolate to MVK. The 1,5-spacing positions the two carbonyls perfectly for intramolecular aldol ring closure to give a six-membered ring.

Cyclohex-2-enone is a six-membered carbocyclic ring bearing a ketone at C-1 and a C=C double bond between C-2 and C-3. It is an α,β-unsaturated ketone (enone) and is the product of Robinson Annulation. It is also an important Michael acceptor and is widely found in natural product frameworks.

For simple substrates: KOH or NaOEt in ethanol at room temperature or gentle warming. For regioselective enolate formation: LDA (lithium diisopropylamide) or KHMDS in THF at −78°C. Amine-catalysed variants (piperidine/AcOH) use enamine intermediates instead of enolates.

The Wieland–Miescher ketone is a bicyclic diketone synthesised by Robinson Annulation of 2-methyl-1,3-cyclohexanedione with methyl vinyl ketone (MVK). It is a key chiral building block used in the total synthesis of steroids, sesquiterpenes, and many polycyclic natural products. It can be made in enantiopure form using asymmetric amine catalysis.