Laboratório de Química Orgânica · Classic laboratory preparations
Alcenos, haletos de alquila, arenos e corantes no laboratório
Uma lição resolvida gratuita e 15 problemas de prática em que você mesmo desenha cada seta curva.
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O que você vai aprender
- Explain how distillation, phase separation and choice of acid steer dehydrations and alkyl halide preparations toward one product.
- Choose between SN1 and SN2 for an alcohol-to-halide conversion from the structure of the alcohol.
- Predict the regiochemistry of electrophilic aromatic substitution on activated and deactivated rings used in lab.
- Follow an amine through diazotisation and azo coupling to a dye, and explain the dye’s colour change with pH.
Lição gratuita
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Ler a aula gratuita (5 partes)
Distil the alkene out
Heating cyclohexanol with phosphoric acid gives cyclohexene by E1: the acid protonates the hydroxyl, water leaves to form a secondary cation, and water removes a neighbouring hydrogen. Every step is reversible, and in a closed flask the acid would also add water back to the alkene. The lab fix is physical. Cyclohexene boils at 83 °C while cyclohexanol boils at 161 °C, so the alkene distils out with water as soon as it forms. Removing a product shifts the equilibrium toward more alkene. Phosphoric acid is preferred to sulfuric acid because it is not an oxidant and chars less.
SN1 or SN2 by substrate
Both preparations start by protonating an alcohol so that water can leave, but the next event depends on the carbon. tert-Butyl alcohol shaken with concentrated HCl in a separatory funnel ionises to the tert-butyl cation, which chloride captures (SN1); the reaction is fast at room temperature because the tertiary cation is well stabilised. Butan-1-ol cannot form a primary cation, so with NaBr and sulfuric acid under reflux, bromide displaces water from the backside in one step (SN2). The substrate, not the halide, decides the mechanism, and with it the conditions: mild for the tertiary alcohol, hot for the primary.
The nitronium ion meets a deactivated ring
Mixed nitric and sulfuric acid generates the nitronium ion, a strong electrophile. Methyl benzoate carries an ester that withdraws electron density from the ring, so it reacts more slowly than benzene and needs the ice bath only to control heat and over-nitration. The drawing shows the arenium ion from attack meta to the ester: the positive charge sits on carbons ortho and para to the new C–N bond, and never on the carbon bonded to the ester. Ortho or para attack would put the charge right next to the electron-poor carbonyl, so methyl 3-nitrobenzoate is the main product.
From diazonium to dye
Methyl orange is made in two lab operations. First, sulfanilic acid is dissolved in carbonate, mixed with sodium nitrite and poured onto ice and hydrochloric acid: nitrous acid nitrosates the amino group, and proton transfers and loss of water give the arenediazonium ion, kept below 5 °C so it does not lose N2. Second, the diazonium ion, a weak electrophile, couples with N,N-dimethylaniline at the carbon para to the dimethylamino group, whose lone pair activates the ring. The N=N bridge conjugates the two rings, so the product absorbs visible light; protonation in acid changes that conjugation and the colour.
What to carry forward
In the lab, mechanism decides the conditions: remove a volatile product to drive an equilibrium, match SN1 or SN2 to the alcohol, let ring substituents steer electrophiles, and keep diazonium ions cold until they couple.
Problemas de prática (15)
Praticar é grátis com uma conta.
Desenhe o mecanismo 10
Alkenes and alkyl halides
- SN2 conversion of butan-1-ol to 1-bromobutaneDraw the conversion of butan-1-ol into 1-bromobutane with NaBr and sulfuric acid: protonation of the hydroxyl, then one-step backside displacement of water by bromide.
- E1 dehydration of cyclohexanol to cyclohexeneDraw the E1 dehydration of cyclohexanol in phosphoric acid: protonation of the hydroxyl, loss of water to the cyclohexyl cation, and removal of a neighbouring ring hydrogen by water to give cyclohexene.
- Zaitsev dehydration of 2-methylcyclohexanolDraw the E1 dehydration of 2-methylcyclohexanol in phosphoric acid and choose which neighbouring hydrogen water removes so that 1-methylcyclohexene, the more substituted alkene, forms.
- Anti bromination of trans-stilbeneDraw the addition of bromine to trans-stilbene through the bromonium ion and its backside opening by bromide, and explain why the meso dibromide forms.
- SN1 conversion of tert-butyl alcohol to tert-butyl chlorideDraw the SN1 conversion of 2-methylpropan-2-ol into 2-chloro-2-methylpropane with concentrated HCl: protonation by hydronium, loss of water to the tert-butyl cation, and capture by chloride.
Electrophilic aromatic substitution
- Para bromination of acetanilideDraw the bromination of acetanilide in acetic acid: attack by the ring carbon para to the acetamido group on bromine, the amide-nitrogen contributor of the arenium ion, and removal of the ring proton by bromide to give 4-bromoacetanilide.
- Meta nitration of methyl benzoateDraw the nitration of methyl benzoate in mixed acid: formation of the nitronium ion, attack by the ring carbon meta to the ester, and deprotonation of the arenium ion by hydrogen sulfate to give methyl 3-nitrobenzoate.
Diazonium and SNAr
- Azo coupling to methyl orangeDraw the coupling of 4-diazoniobenzenesulfonate with N,N-dimethylaniline: attack by the carbon para to the dimethylamino group on the terminal diazonium nitrogen, then removal of the ring proton by acetate to give the methyl orange anion.
- SNAr of 1-chloro-2,4-dinitrobenzene with piperidineDraw the addition–elimination substitution of chloride in 1-chloro-2,4-dinitrobenzene by piperidine: addition to form the Meisenheimer complex, its para-nitro contributor, removal of the N–H proton by a second piperidine, and loss of chloride.
- Diazotisation of sulfanilic acidDraw the diazotisation of the sulfanilate amino group in cold aqueous acid: protonation of nitrous acid, N-nitrosation with loss of water, proton transfers to the diazohydroxide, and loss of water to give 4-diazoniobenzenesulfonate.
Preveja o produto 1
- Major alkene from 2-methylcyclohexanolPredict the major alkene from acid-catalysed dehydration of 2-methylcyclohexanol.
Comparar 1
- Why methyl benzoate goes meta and anisole goes ortho/paraCompare the arenium ions from para attack on methyl benzoate and on anisole.
Encontre o erro 1
- A failed bromine test on the distillateDiagnose why a dehydration distillate does not decolourise bromine.
Interprete as evidências 1
- Methyl orange turns red in acidInterpret the colour change of methyl orange below pH 3.1 in terms of protonation and conjugation.
Explique por quê 1
- Why distil cyclohexene as it forms?Explain how distilling the alkene during an acid-catalysed dehydration raises the yield.
Travou em alcenos, haletos de alquila, arenos e corantes no laboratório? 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