Laboratório de Química Orgânica · Classic laboratory preparations
Preparações de Grignard, Wittig, Diels–Alder e redox
Uma lição resolvida gratuita e 15 problemas de prática em que você mesmo desenha cada seta curva.
Travou em preparações de Grignard, Wittig, Diels–Alder e redox? Escaneie o problema.
Tire uma foto da reação da sua lista ou do livro e desenhe o mecanismo com cada seta corrigida. 1 scan grátis agora, sem cadastro; mais 1 com uma conta gratuita.
Depois, Scan Standard: /mês
O que você vai aprender
- Draw Grignard additions to esters, ketones and carbon dioxide, and explain why water must be excluded and how many equivalents are needed.
- Draw Wittig and Diels–Alder preparations, including ylide formation, concerted cycloadditions and the stereochemistry they fix.
- Explain the camphor–isoborneol redox pair and an interfacial polyamide synthesis as hydride delivery, hypochlorite oxidation and acyl substitution.
- Calculate percent yields and diagnose side products from what is observed at the bench.
Lição gratuita
O texto da lição resolvida. No Arrow Lab, cada etapa vem com as estruturas e as setas desenhadas.
Ler a aula gratuita (5 partes)
Dry glassware or no product
A Grignard reagent is made by inserting magnesium into the C–Br bond of bromobenzene in dry ether. The C–Mg bond is so polarised toward carbon that phenylmagnesium bromide behaves like a phenyl carbanion: a powerful nucleophile and an even stronger base. Water, alcohols and the moisture film on glassware protonate it at once, giving benzene and Mg(OH)Br, and every molecule lost this way can no longer add to the carbonyl compound. That is why the glassware is oven-dried, the ether is anhydrous and a drying tube protects the condenser. The reagent must meet the carbonyl compound before it meets any O–H or N–H proton.
Two equivalents on an ester
Methyl benzoate takes two phenyl groups. The first PhMgBr adds to the ester carbonyl to give a tetrahedral alkoxide that still carries a methoxy leaving group, and it collapses to benzophenone and methoxide. Benzophenone is more electrophilic than the ester, because no alkoxy oxygen donates into its carbonyl, so it captures a second PhMgBr at once. The final alkoxide has no leaving group and waits for the aqueous ammonium chloride workup, which protonates it to triphenylmethanol. The stoichiometry follows from the mechanism: at least two equivalents of Grignard reagent per ester, plus a margin for reagent lost to moisture.
Endo adducts from cyclopentadiene
Cyclopentadiene is locked in the s-cis conformation, so it reacts with maleic anhydride within minutes at room temperature. Because it also dimerises at room temperature, the lab first cracks dicyclopentadiene by a retro-Diels–Alder reaction and distils the monomer into a cold receiver. The cycloaddition is concerted: three curved arrows run round a ring of six atoms and both new σ bonds form on one face of the dienophile, so groups cis on the alkene stay cis in the ring. The kinetic product is the endo adduct, with the anhydride ring tucked under the new C=C rather than the CH2 bridge, because the carbonyl π system overlaps the diene in the transition state. The drawing omits wedges; the lab product is cis-norbornene-5,6-endo-dicarboxylic anhydride.
A redox pair you can smell
Camphor (left) and isoborneol (right) differ by two hydrogens, and the lab runs the pair in both directions. Sodium borohydride in methanol delivers hydride to the camphor carbonyl carbon. The gem-dimethyl bridge blocks the exo face, so hydride arrives from the endo face and the new OH ends exo: isoborneol, with only a little borneol. Going back, sodium hypochlorite in acetic acid generates HOCl, which turns isoborneol into an alkyl hypochlorite; loss of HCl across the carbinol C–H and the O–Cl bond re-forms the C=O. IR follows each direction: the strong C=O band of camphor near 1740 cm⁻¹ is replaced by a broad O–H band near 3400 cm⁻¹, and vice versa.
What to carry forward
Every preparation here is a textbook mechanism with bench constraints attached: keep protons away from Grignard reagents and count equivalents from the mechanism, generate reactive partners such as ylides, cyclopentadiene and HOCl just before use, and let concerted geometry and steric shielding decide the stereochemistry.
Problemas de prática (15)
Praticar é grátis com uma conta.
Desenhe o mecanismo 10
Grignard preparations
- Cyclohexylmagnesium bromide adds to acetoneDraw cyclohexylmagnesium bromide adding to the carbonyl carbon of acetone and protonation of the resulting alkoxide by hydronium, giving 2-cyclohexylpropan-2-ol.
- Carboxylation of phenylmagnesium bromide with dry iceDraw phenylmagnesium bromide adding to carbon dioxide to give a benzoate, then protonation of the benzoate by hydronium in the acidic workup, giving benzoic acid.
- Triphenylmethanol from methyl benzoate and two equivalents of PhMgBrDraw the first Grignard addition to methyl benzoate, loss of methoxide to give benzophenone, the second addition, and protonation of the alkoxide by ammonium, giving triphenylmethanol.
Wittig and Diels–Alder
- Diels–Alder reaction of anthracene with maleic anhydrideDraw the three arrows of the concerted [4+2] cycloaddition in which maleic anhydride bridges C9 and C10 of anthracene, giving 9,10-dihydroanthracene-9,10-α,β-succinic anhydride.
- Cracking dicyclopentadiene by a retro-Diels–Alder reactionDraw the three arrows of the concerted retro-Diels–Alder reaction that splits dicyclopentadiene into two molecules of cyclopenta-1,3-diene.
- Diels–Alder reaction of cyclopentadiene with dimethyl fumarateDraw the three arrows of the concerted cycloaddition of cyclopentadiene with dimethyl fumarate and decide how the two ester groups are related in dimethyl bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate.
- Solvent-free Wittig synthesis of 4-bromostilbeneDraw phosphate removing the benzylic proton of the phosphonium salt, the ylide and 4-bromobenzaldehyde closing the oxaphosphetane, and its fragmentation to 4-bromostilbene and triphenylphosphine oxide.
Redox and polymers
- NaBH4 reduction of camphor to isoborneolDraw hydride transfer from borohydride to the camphor carbonyl carbon and protonation of the alkoxide by methanol, and decide which face of the ketone the hydride approaches.
- Hypochlorite (bleach) oxidation of isoborneol to camphorDraw acetic acid activating hypochlorous acid, chlorine transfer to the isoborneol oxygen to form the alkyl hypochlorite, and the E2-like elimination of HCl that gives camphor.
- One amide link of nylon 6,10 from sebacoyl chloride and hexane-1,6-diamineDraw hexane-1,6-diamine adding to one acid chloride carbonyl of sebacoyl chloride, loss of chloride from the tetrahedral intermediate, and removal of the N–H proton by hydroxide to give the first amide link.
Preveja o produto 1
- Endo when cold, exo when hotPredict how temperature changes the endo/exo outcome of a reversible Diels–Alder reaction.
Calcular 1
- Percent yield of triphenylmethanolCalculate theoretical and percent yield for the triphenylmethanol preparation.
Encontre o erro 1
- Trace a non-polar by-productDiagnose where biphenyl in a Grignard preparation comes from.
Planeje uma síntese 1
- How much Grignard reagent?Plan the Grignard stoichiometry for an ester from its mechanism.
Explique por quê 1
- Why the glassware must be dryExplain how traces of water destroy a Grignard reagent.
Travou em preparações de Grignard, Wittig, Diels–Alder e redox? Escaneie o problema.
Tire uma foto da reação da sua lista ou do livro e desenhe o mecanismo com cada seta corrigida. 1 scan grátis agora, sem cadastro; mais 1 com uma conta gratuita.
Depois, Scan Standard: /mês