Organic Chemistry 2 · Nitrogen and biological molecules
Amine basicity and electronic structure: practice problems
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- An amine meets an acid
- Recover a neutral amine
- 4-(Aminomethyl)pyridine gives (pyridin-4-yl)methanaminium
- 4-Methoxyaniline and HCl give 4-methoxyanilinium
- 4-Methylaniline and HCl give 4-methylanilinium chloride
- 4-Nitroanilinium loses a proton to water to give 4-nitroaniline
- Acetamide protonates on oxygen in sulfuric acid
- N,N,N′-Trimethylacetamidine and HCl give the amidinium ion
- Aniline and HCl give anilinium chloride
Draw the mechanism 25
- An amine meets an acidDraw paired electron movements for protonation without losing the counterion.
- Recover a neutral amineDraw deprotonation of ammonium and restore its lone pair.
- 4-(Aminomethyl)pyridine gives (pyridin-4-yl)methanaminiumDraw hydronium protonating the more basic of the two nitrogens in 4-(aminomethyl)pyridine.
- 4-Methoxyaniline and HCl give 4-methoxyaniliniumDraw hydronium protonating the amino nitrogen of 4-methoxyaniline rather than its ether oxygen.
- 4-Methylaniline and HCl give 4-methylanilinium chlorideDraw the nitrogen lone pair of 4-methylaniline taking the proton from HCl to form the anilinium salt.
- 4-Nitroanilinium loses a proton to water to give 4-nitroanilineDraw water removing a proton from 4-nitroanilinium, then push the freed nitrogen lone pair through the ring into the nitro group.
- Acetamide protonates on oxygen in sulfuric acidDraw the carbonyl oxygen of acetamide taking a proton from sulfuric acid, then the resonance step that delocalises the charge onto nitrogen.
- N,N,N′-Trimethylacetamidine and HCl give the amidinium ionDraw the imine nitrogen of the amidine taking the proton, then the resonance step that shares the charge with the amino nitrogen.
- Aniline and HCl give anilinium chlorideDraw the aniline nitrogen lone pair taking the proton from HCl.
- DABCO and benzyl bromide give N-benzyl-DABCO bromideDraw one bridgehead nitrogen of DABCO displacing bromide from benzyl bromide.
- 4-(Dimethylamino)pyridine and HCl give its pyridinium saltDraw the ring nitrogen of DMAP taking the proton, then push the dimethylamino lone pair through the ring onto the protonated nitrogen.
- Hydrazine and one equivalent of HCl give hydraziniumDraw hydronium protonating one nitrogen of hydrazine and explain why the second stays free.
- Imidazole and HCl give imidazolium chlorideDraw the pyridine-like nitrogen of imidazole taking the proton, then the resonance step that makes both N–H nitrogens equivalent.
- Hydroxide frees methylamine from methylammonium chlorideDraw hydroxide removing an N–H proton from methylammonium.
- N-Methylmorpholine and H2O2 give N-methylmorpholine N-oxideDraw the tertiary nitrogen of N-methylmorpholine attacking hydrogen peroxide, then the proton transfer that gives the N-oxide.
- N,N-Dimethylaniline and HCl give N,N-dimethylaniliniumDraw the nitrogen of N,N-dimethylaniline, not a ring carbon, taking the proton from HCl.
- (S)-Nicotine and two equivalents of HCl give the dicationDraw both protonations of (S)-nicotine in order of basicity, pyrrolidine nitrogen first.
- Proton sponge and HCl give protonated proton spongeDraw one dimethylamino nitrogen of proton sponge taking the proton from HCl.
- Pyridine and mCPBA give pyridine N-oxideDraw the concerted oxygen transfer from mCPBA to the pyridine nitrogen lone pair.
- Pyridine and methyl iodide give N-methylpyridinium iodideDraw the pyridine nitrogen lone pair displacing iodide from methyl iodide in one SN2 step.
- Pyridine and HCl give pyridinium chlorideDraw the pyridine nitrogen lone pair taking the proton from HCl.
- Pyrrole protonates on carbon to give 2H-pyrroliumDraw the nitrogen lone pair pushing through the ring so that C2 of pyrrole takes the proton from sulfuric acid.
- Enamine C-protonation gives 1-cyclohexylidenepyrrolidiniumDraw the nitrogen lone pair pushing the enamine π bond so that the β-carbon takes the proton from HCl.
- Tetramethylguanidine and HCl give tetramethylguanidiniumDraw the imine nitrogen of tetramethylguanidine taking the proton from HCl.
- Urea protonates on oxygen to give O-protonated ureaDraw the carbonyl oxygen of urea, not an amino nitrogen, taking the proton from sulfuric acid.
Predict the product 1
- Basic nitrogen of imidazoleIdentify the basic nitrogen of imidazole and explain its pKaH.
Classify 9
- Classify the nitrogen environmentIdentify why a carbonyl-adjacent nitrogen is not an ordinary amine.
- 1°, 2° or 3°?Classify an amine as primary, secondary or tertiary by counting carbons bonded to nitrogen.
- What DBU is used forIdentify the typical role of DBU in synthesis.
- Alcohol versus amine priorityName a compound with both OH and NH2 using suffix priority.
- Find 4-methylanilineMatch the name 4-methylaniline (p-toluidine) to its structure.
- Name an N-substituted amineName a secondary amine using the N- locant for the smaller substituent.
- Name a quaternary ammonium saltName a quaternary ammonium salt.
- Chiral ammonium ionsDecide when a nitrogen stereocentre is configurationally stable.
- Name the salt of methylamine and HClName a simple alkylammonium salt.
Compare 9
- Compare conjugate-acid stabilityRelate lone-pair delocalization to aniline basicity.
- Ammonia versus methylamine basicityCompare the basicity of ammonia and methylamine using the effect of an alkyl group on the conjugate acid.
- Basicity of 4-methoxy, 4-chloro and 4-nitroanilinesRank para-substituted anilines by basicity from the electronic effect of the substituent.
- Boiling points of ethanol, ethylamine and propaneCompare boiling points of an alcohol, an amine and an alkane of similar mass.
- 4-Nitroaniline vs 3-nitroaniline basicityExplain why a para nitro group lowers aniline basicity more than a meta nitro group.
- Rank pyrrolidine, piperidine and Et3N as nucleophilesRank amines by nucleophilicity using steric hindrance at nitrogen.
- Alkyl amine, aniline, amideRank an alkyl amine, an aniline and an amide by basicity.
- Guanidine as the strongest neutral nitrogen basePick the strongest base among guanidine, methylamine, pyridine and acetamide and justify it by conjugate-acid resonance.
- Piperidine, pyridine, pyrroleRank three nitrogen heterocycles by basicity using hybridization and aromaticity.
Calculate 4
- Compute amine speciationConvert a pH–pKa difference into a protonated fraction.
- Percent protonated amine at pH 7.4Use the Henderson–Hasselbalch equation to find the fraction of an amine protonated at physiological pH.
- pH at which methylamine is half protonatedFind the pH at which an amine and its ammonium ion are present in equal concentrations.
- pKb of cyclohexylamine from its pKaHConvert the pKa of an ammonium ion into the pKb of its amine.
Find the error 1
- Audit an aromatic-base argumentDistinguish pyridine-like and pyrrole-like lone pairs.
Interpret the evidence 3
- Interpret an extraction switchConnect protonation state with acid–base partitioning.
- Methylamine basicity in gas phase vs waterInterpret why the basicity order of methylamines differs between the gas phase and water.
- Geometry at an amine nitrogenDescribe the hybridization and geometry of an alkyl amine nitrogen.
Work backward 1
- Infer a basicity differenceRecover a conjugate-acid pKa from a proton-transfer equilibrium.
Plan a synthesis 2
- Choose an informative comparisonDesign a solvent-controlled basicity comparison.
- Separate an amine from a neutral compoundPlan an acid–base extraction that separates an amine from a neutral organic compound.
Explain why 8
- Separate rate from equilibriumExplain why a hindered base may be a poor substitution nucleophile.
- Why aniline is a weak baseExplain why aniline is a much weaker base than an alkyl amine.
- Why DMAP is more basic than pyridineExplain how a para dimethylamino group raises the basicity of a pyridine ring nitrogen.
- Why amine drugs are sold as hydrochloridesExplain why amine drugs are formulated as hydrochloride salts.
- Why amine enantiomers can't be resolvedExplain why an amine with three different groups on nitrogen cannot be resolved into enantiomers.
- Why triethylamine is a poor nucleophileExplain why triethylamine acts as a base rather than a nucleophile.
- Why trimethylamine boils lower than propylamineExplain why tertiary amines boil lower than primary or secondary amine isomers.
- Why an amide nitrogen is not basicExplain why the nitrogen of an amide is not a basic site, unlike the nitrogen of an alkyl amine.
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