Organic Chemistry 2 · Carboxyl and acyl chemistry
Carboxylic acids and acid–base control: practice problems
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- Follow a proton transfer
- Return an anion to its acid form
- Sodium acetate and iodomethane give methyl acetate
- Acetic acid destroys methylmagnesium bromide
- Acetic acid and methylamine give methylammonium acetate
- Barton decarboxylation of a cyclohexanecarboxylic ester
- Hydrochloric acid turns sodium benzoate back into benzoic acid
- Benzoic acid and benzyl bromide give benzyl benzoate
- Acetic acid and sodium bicarbonate give acetate
Draw the mechanism 28
- Follow a proton transferMove both electron pairs in carboxylic acid deprotonation.
- Return an anion to its acid formAccount for hydronium and water during carboxylate protonation.
- Sodium acetate and iodomethane give methyl acetateDraw the acetate oxygen attacking iodomethane in an SN2 step that expels iodide.
- Acetic acid destroys methylmagnesium bromideDraw the C–Mg bond of methylmagnesium bromide removing the carboxyl proton of acetic acid.
- Acetic acid and methylamine give methylammonium acetateDraw the methylamine nitrogen taking the carboxyl proton of acetic acid.
- Barton decarboxylation of a cyclohexanecarboxylic esterDraw the tributyltin radical chain that turns the Barton ester of cyclohexanecarboxylic acid into cyclohexane and CO2.
- Hydrochloric acid turns sodium benzoate back into benzoic acidDraw a benzoate oxygen taking a proton from hydronium to regenerate benzoic acid.
- Benzoic acid and benzyl bromide give benzyl benzoateDraw carbonate deprotonating benzoic acid and the benzoate oxygen displacing bromide from benzyl bromide.
- Acetic acid and sodium bicarbonate give acetateDraw bicarbonate removing the O–H proton of acetic acid to give acetate and carbonic acid.
- Acetate from acetic acid and its two resonance contributorsDraw hydroxide deprotonating acetic acid, then push the carboxylate charge onto the other oxygen to show the second contributor.
- Cesium acetate and (R)-2-bromobutane: SN2 with inversionDraw acetate attacking (R)-2-bromobutane from the back side and give the configuration of the butan-2-yl acetate formed.
- Chloroacetic acid and bicarbonate give chloroacetateDraw bicarbonate removing the carboxyl proton of chloroacetic acid, then the resonance that spreads the charge of chloroacetate.
- 2-Chlorobutanoic acid protonates sodium acetateDraw acetate removing the carboxyl proton of 2-chlorobutanoic acid and estimate the equilibrium constant from the two pKa values.
- Cyanoacetic acid and triethylamine form an ammonium saltDraw triethylamine removing the carboxyl proton of cyanoacetic acid to give triethylammonium cyanoacetate.
- Acetic acid and diazomethane give methyl acetateDraw the diazomethane carbon taking the carboxyl proton, then the acetate oxygen displacing N2 from the methyldiazonium ion.
- Dichloroacetic acid and hydroxide give dichloroacetateDraw hydroxide removing the carboxyl proton of dichloroacetic acid and the resonance that delocalizes the charge of dichloroacetate.
- Hunsdiecker reaction of silver cyclohexanecarboxylateDraw the Hunsdiecker reaction of silver cyclohexanecarboxylate: acyl hypobromite formation, O–Br homolysis, loss of CO2 and capture of bromine to give bromocyclohexane.
- Hunsdiecker reaction of silver 3-phenylpropanoateDraw the Hunsdiecker reaction of silver 3-phenylpropanoate: acyl hypobromite formation, O–Br homolysis, loss of CO2 and capture of bromine to give (2-bromoethyl)benzene.
- 4-Hydroxybenzoic acid and bicarbonate give 4-hydroxybenzoateDraw bicarbonate removing the carboxyl proton, not the phenol proton, of 4-hydroxybenzoic acid.
- Krapcho decarboxylation of ethyl 2-oxocyclohexanecarboxylateDraw chloride dealkylating ethyl 2-oxocyclohexanecarboxylate, loss of CO2 to the enolate, and protonation by water to give cyclohexanone.
- 4-Methoxybenzoic acid and hydroxide give 4-methoxybenzoateDraw hydroxide removing the carboxyl proton of 4-methoxybenzoic acid and the resonance of the 4-methoxybenzoate anion.
- 4-Nitrobenzoic acid and bicarbonate give 4-nitrobenzoateDraw bicarbonate removing the carboxyl proton of 4-nitrobenzoic acid and the resonance of 4-nitrobenzoate.
- 4-Nitrobenzoic acid protonates sodium benzoateDraw benzoate removing the carboxyl proton of 4-nitrobenzoic acid and decide which side of the equilibrium is favoured.
- First ionisation of oxalic acid in waterDraw water removing one carboxyl proton of oxalic acid to give hydrogen oxalate and hydronium.
- Silver benzoate and iodoethane give ethyl benzoateDraw the benzoate oxygen attacking iodoethane while silver ion captures the leaving iodide as silver iodide.
- Sodium acetate and allyl bromide give allyl acetateDraw the acetate oxygen attacking the CH2Br carbon of allyl bromide in one SN2 step that expels bromide.
- Trichloroacetic acid ionizes in water to trichloroacetateDraw water removing the carboxyl proton of trichloroacetic acid to give trichloroacetate and hydronium, then the carboxylate resonance.
- Trifluoroacetic acid and hydroxide give trifluoroacetateDraw hydroxide removing the carboxyl proton of trifluoroacetic acid.
Predict the product 6
- Benzoic acid with sodium borohydridePredict that sodium borohydride deprotonates a carboxylic acid instead of reducing it.
- Propanoic acid with sodium hydroxidePredict the salt and water formed when a carboxylic acid is neutralized by hydroxide.
- Heating succinic acid gives a cyclic anhydridePredict the five-membered cyclic anhydride formed when a 1,4-dicarboxylic acid is heated.
- Hot KMnO4 cleavage of (E)-hex-3-enePredict the carboxylic acid formed when hot permanganate cleaves a symmetric disubstituted alkene.
- Haloform reaction of acetophenonePredict the carboxylic acid and by-product formed when a methyl ketone undergoes the iodoform (haloform) reaction.
- Benzyl bromide to phenylacetic acid via the nitrilePredict the carboxylic acid formed by cyanide substitution of a benzylic bromide followed by acidic nitrile hydrolysis.
Classify 7
- Classify two forms of an acidDistinguish reversible salt formation from covalent ester formation.
- Which acid decarboxylates on gentle heating?Identify the carboxylic acid that loses CO2 on heating from the presence of a carbonyl group beta to the carboxyl group.
- Name the acidMatch the name 3-methylbutanoic acid to its structure.
- Common names of acidsMatch the common names of the C1–C4 carboxylic acids.
- Name a dicarboxylic acidName HOOC(CH2)4COOH by IUPAC and common names.
- Acid on a ringName a carboxylic acid attached to a cyclopentane ring.
- Name the saltName the sodium salt of propanoic acid.
Compare 9
- Separate distance from atom countPredict the distance dependence of an electron-withdrawing substituent.
- Rank para-substituted benzoic acids by acidityRank para-methoxy, para-chloro and para-nitro benzoic acids against benzoic acid using electron-donating and electron-withdrawing effects.
- Rank oxalic, malonic, succinic and glutaric acid by pKa1Rank dicarboxylic acids by first pKa from the distance between the two carboxyl groups.
- Distance effectRank chlorobutanoic acids by how far Cl is from the carboxyl group.
- F versus Cl versus BrRank haloacetic acids by halogen electronegativity.
- Formic acid versus acetic acidDecide whether formic or acetic acid is stronger and explain the effect of the methyl group on the carboxylate.
- Rank the haloacetic acidsRank acetic and haloacetic acids by acidity using number and electronegativity of halogens.
- Water solubility trendCompare the water solubility of acetic acid, decanoic acid and sodium decanoate.
- Rank acetic acid, phenol and ethanol by acidityRank acetic acid, phenol and ethanol by acidity from the resonance stabilization of their conjugate bases.
Calculate 6
- Quantify a proton-transfer preferenceConvert a supplied pKa difference into a proton-transfer equilibrium constant.
- Estimate a protonation fractionTranslate a supplied pH–pKa difference into a fraction rather than a ratio.
- K for acetic acid and phenoxide from pKa valuesCalculate the equilibrium constant of a proton transfer between a carboxylic acid and phenoxide from the difference in pKa values.
- Percent of acetic acid ionized at pH 7.0Calculate the percentage of a carboxylic acid present as its carboxylate at a given pH with the Henderson–Hasselbalch equation.
- pKa from the half-equivalence pH of a titrationRead the pKa of a carboxylic acid from the pH at the half-equivalence point of its titration with hydroxide.
- Carboxylate-to-acid ratio of acetic acid at pH 3Calculate the carboxylate-to-acid ratio of a carboxylic acid at a pH below its pKa.
Find the error 2
- Grignard reagent compatibility — carboxylic-acid quenchRecognize acid–base consumption of a carbon nucleophile by a free carboxylic acid.
- Spot an invalid contributorReject resonance drawings that change atoms or violate second-row carbon valence.
Interpret the evidence 3
- Read a carboxylate drawingDistinguish a resonance contributor from a localized physical ion.
- Equal C–O bond lengths in acetateInterpret the equal C–O bond lengths of acetate as evidence of a resonance-delocalized carboxylate.
- Ortho effect: 2-methyl versus 4-methylbenzoic acidInterpret the unusually high acidity of 2-methylbenzoic acid as steric inhibition of conjugation between the ring and the carboxyl group.
Plan a synthesis 6
- Recover an acid from a neutral mixtureDesign a reversible acid–base separation with explicit partition assumptions.
- Separate benzoic acid from 4-methylphenolChoose a base that extracts a carboxylic acid but not a phenol, by comparing the pKa values of the acids and the base's conjugate acid.
- Separate benzoic acid from acetophenonePlan an acid–base extraction that separates a carboxylic acid from a neutral ketone and recovers both.
- Reduce butanoic acid to butan-1-olChoose a reagent strong enough to reduce a carboxylic acid to a primary alcohol.
- Pivalic acid from tert-butyl bromide: pick the routeChoose between the Grignard–CO2 and nitrile routes to a carboxylic acid when the alkyl halide is tertiary.
- Pentanoic acid from 1-bromobutane via a GrignardChoose the reagent sequence that converts an alkyl bromide into a carboxylic acid with one more carbon.
Explain why 4
- Benzoic acid versus acetic acid acidityExplain why benzoic acid is more acidic than acetic acid from the hybridization of the carbon bonded to the carboxyl group.
- Why oxalic acid has a larger pKa2 than pKa1Explain why the second ionization of a dicarboxylic acid is weaker than the first.
- Why carboxylic acids boil highExplain the high boiling points of carboxylic acids.
- Why acetic acid is far more acidic than ethanolExplain the 11-unit pKa gap between acetic acid and ethanol from the stability of their conjugate bases.
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