20.13: Additional Problems

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Visualizing Chemistry

PROBLEM 20-17 Give IUPAC names for the following carboxylic acids (reddish brown = Br):

(a) (b) A ball-and-stick model of a four-carbon chain with a carboxyl end. There is a double bond on C 2 and a methyl on C 3.

(c) The ball-and-stick model represents a cyclopentadiene ring with a carboxylic acid group attached to C 1. The double bonds are located on the first and third carbon atoms. (d) A ball-and-stick model of a three-carbon chain with a carboxyl end. There is a methyl on C 2 and a cyclopentane on C 3.

PROBLEM 20-18 Would you expect the following carboxylic acids to be more acidic or less acidic than benzoic acid? Explain. (Reddish brown = Br.)

(a) (b) A ball-and-stick model of a benzene ring with a carboxyl group and a dimethylamine group para to the carboxyl.

PROBLEM 20-19 The following carboxylic acid can’t be prepared from an alkyl halide by either the nitrile hydrolysis route or the Grignard carboxylation route. Explain.

A ball-and-stick model of a four-carbon chain with a carboxyl end. There are two methyl groups on C 2 and a hydroxy group on C 4.

PROBLEM 20-20 Electrostatic potential maps of anisole and thioanisole are shown. Which do you think is the stronger acid, p-methoxybenzoic acid or p-(methylthio)benzoic acid? Explain.

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Mechanism Problems

PROBLEM 20-21 Predict the product(s) and write the mechanism of each of the following reactions:

(a) conversion=

(b) Conversion of 2-bromobutane to unknown products depicted by a question mark by reaction first with magnesium, then with carbon dioxide, and finally with hydronium.

Problem 20-22 Predict the product(s) and write the mechanism of each of the following reactions:

(a) Conversion of cyclopentanecarboxamide to unknown products depicted by a question mark by reaction with thionyl chloride.

(b) Conversion of benzamide to unknown products depicted by a question mark by reaction with thionyl chloride.

Problem 20-23 Predict the product(s) and write the mechanism of each of the following reactions:

(a) Conversion of 2-methylbutanenitrile to unknown products depicted by a question mark by reaction with sodium hydroxide and water. (b)

(b) Conversion of 3-methylbenzonitrile to unknown products depicted by a question mark by reaction with sodium hydroxide and water.

Problem 20-24 Predict the product(s) and write the complete mechanism of each of the following reactions:

(a) 2-methoxybenzonitrile reacts with methylmagnesium bromide in the presence of ether followed by acidic hydrolysis to give unknown products depicted by a question mark.

(b) Cyclopropanecarbonitrile reacts with phenylmagnesium bromide in the presence of ether followed by acidic hydrolysis to give unknown products depicted by a question mark.

Problem 20-25 Acid-catalyzed hydrolysis of a nitrile to give a carboxylic acid occurs by initial protonation of the nitrogen atom, followed by nucleophilic addition of water. Review the mechanism of base-catalyzed nitrile hydrolysis in Section 20.7 and then predict the products for the following reactions. Write the steps involved in the acid-catalyzed reaction, using curved arrows.

(a) Conversion of 3,3-dimethylbutane nitrile to unknown products depicted by a question mark by reaction with hydrochloric acid and water.

(b) Conversion of 4-methylbenzonitrile to unknown products depicted by a question mark by reaction with hydrochloric acid and water.

Problem 20-26 Nitriles can be converted directly to esters by the Pinner reaction, which first produces an iminoester salt that is isolated and then treated with water to give the final product. Propose a mechanism for the Pinner reaction using curved arrows to show the flow of electrons at each step.

Conversion of benzonitrile to methyl benzoate. An imine ester intermediate is formed (reacting with hydrochloric acid in methanol) in which carbon is double-bonded to nitrogen, carrying a positive charge.

Problem 20-27 Naturally occurring compounds called <i">cyanogenic glycosides, such as lotaustralin, release hydrogen cyanide, HCN, when treated with aqueous acid. The reaction occurs by hydrolysis of the acetal linkage to form a cyanohydrin, which then expels HCN and gives a carbonyl compound.

(a) Show the mechanism of the acetal hydrolysis and the structure of the cyanohydrin that results.

(b) Propose a mechanism for the loss of HCN, and show the structure of the carbonyl compound that forms.

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Problem 20-28 2-Bromo-6,6-dimethylcyclohexanone gives 2,2-dimethylcyclopentanecarboxylic acid on treatment with aqueous NaOH followed by acidification, a process called the Favorskii reaction. Propose a mechanism.

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Problem 20-29 Naturally occurring compounds called terpenoids, which we’ll discuss in Section 27.5, are biosynthesized by a pathway that involves loss of CO₂ from 3-phosphomevalonate 5-diphosphate to yield isopentenyl diphosphate. Use curved arrows to show the mechanism of this reaction.

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Problem 20-30 In the Ritter reaction, an alkene reacts with a nitrile in the presence of strong aqueous sulfuric acid to yield an amide. Propose a mechanism.

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Naming Carboxylic Acids and Nitriles

Problem 20-31 Give the IUPAC names for the following compounds:

(a) (b) A three-carbon chain in which the first carbon is part of a carboxyl group. There are two methyl groups on the adjacent carbon. (c) A benzene ring with a carboxyl substituent. Meta to the carboxyl is a cyano substituent. (d) A ten-carbon ring with a carboxyl substituent. There is a double bond on the alpha carbon, with trans orientation.

(e) A three-carbon chain in which the first carbon is part of a nitrile group. There are two methyl groups on the adjacent carbon. (f) A six-carbon chain with a substituent on the third carbon. The substituent has the condensed formula C H 2 C O 2 H. (g) A five-carbon chain in which the first carbon is part of a carboxyl group. There are bromine groups on the furthest and next-to-furthest carbons from the carboxyl. (h) A cyclopentene with a cyano substituent one carbon from the end of the double bond.

Problem 20-32 Draw structures corresponding to the following IUPAC names:

(a) cis-1,2-Cyclohexanedicarboxylic acid (b) Heptanedioic acid (c) 2-Hexen-4-ynoic acid (d) 4-Ethyl-2-propyloctanoic acid (e) 3-Chlorophthalic acid (f) Triphenylacetic acid (g) 2-Cyclobutenecarbonitrile (h) m-Benzoylbenzonitrile

Problem 20-33 Draw and name the following compounds:

(a) The eight carboxylic acids with the formula C₆H₁₂O₂

(b) Three nitriles with the formula C₅H₇N

Problem 20-34 Pregabalin, marketed as Lyrica, is an anticonvulsant drug that is also effective in treating chronic pain. The IUPAC name of pregabalin is (S)-3-(aminomethyl)-5-methylhexanoic acid. (An aminomethyl group is –CH₂NH₂.) Draw the structure of pregabalin.

Problem 20-35 Isocitric acid, an intermediate in the citric acid cycle of food metabolism, has the systematic name (2R,3S)-3-carboxy-2-hydroxypentanedioic acid. Draw the structure.

Acidity of Carboxylic Acids

Problem 20-36 Order the compounds in each of the following sets with respect to increasing acidity:

(a) Acetic acid, oxalic acid, formic acid

(b) p-Bromobenzoic acid, p-nitrobenzoic acid, 2,4-dinitrobenzoic acid

Problem 20-37 Arrange the compounds in each of the following sets in order of increasing basicity:

(a) Magnesium acetate, magnesium hydroxide, methylmagnesium bromide

(b) Sodium benzoate, sodium p-nitrobenzoate, sodium acetylide

Problem 20-38 Calculate the pK_a’s of the following acids:

(a) Lactic acid, K_a = 8.4 × 10^–4

(b) Acrylic acid, K_a = 5.6 × 10^–6

Problem 20-39 Calculate the K_a’s of the following acids:

(a) Citric acid, pK_a = 3.14

(b) Tartaric acid, pK_a = 2.98

Problem 20-40 Thioglycolic acid, HSCH₂CO₂H, a substance used in depilatory agents (hair removers) has pK_a = 3.42. What is the percent dissociation of thioglycolic acid in a buffer solution at pH = 3.0?

Problem 20-41 In humans, the final product of purine degradation from DNA is uric acid, pK_a = 5.61, which is excreted in the urine. What is the percent dissociation of uric acid in urine at a typical pH = 6.0? Why do you think uric acid is acidic even though it does not have a CO₂H group?

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Problem 20-42 Some pK_a data for simple dibasic acids are shown. How can you account for the fact that the difference between the first and second ionization constants decreases with increasing distance between the carboxyl groups?

Name/th>	Structure	pK₁	pK₂
Oxalic	HO₂CCO₂H	1.2	4.2
Succinic	HO₂CCH₂CH₂CO₂H	4.2	5.6
Adipic	HO₂C(CH₂)₄CO₂H	4.4	5.4

Reactions of Carboxylic Acids and Nitriles

Problem 20-43 How could you convert butanoic acid into the following compounds? Write each step showing the reagents needed.

(a) 1-Butanol (b) 1-Bromobutane (c) Pentanoic acid (d) 1-Butene (e) Octane

Problem 20-44 How could you convert each of the following compounds into butanoic acid? Write each step showing all reagents.

(a) 1-Butanol (b) 1-Bromobutane (c) 1-Butene (d) 1-Bromopropane (e) 4-Octene

Problem 20-45 How could you convert butanenitrile into the following compounds? Write each step showing the reagents needed.

(a) 1-Butanol (b) Butylamine (c) 2-Methyl-3-hexanone

Problem 20-46 How would you prepare the following compounds from benzene? More than one step is needed in each case.

(a) m-Chlorobenzoic acid (b) p-Bromobenzoic acid (c) Phenylacetic acid, C₆H₅CH₂CO₂H

Problem 20-47 Predict the product of the reaction of p-methylbenzoic acid with each of the following:

(a) LiAlH₄, then H₃O⁺

(b) N-Bromosuccinimide in CCl₄

(d) KMnO₄, H₃O⁺

Problem 20-48 Using ¹³CO₂ as your only source of labeled carbon, along with any other compounds needed, how would you synthesize the following compounds?

(a) CH₃CH₂¹³CO₂H (b) PRODCH₃¹³CH₂CO₂H

Problem 20-49 How would you carry out the following transformations?

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Problem 20-50 Which method—Grignard carboxylation or nitrile hydrolysis—would you use for each of the following reactions? Explain.

(a) 2-(bromomethyl)phenol=

(b) 2-bromobutane separated from 3-methylbutanoic acid by a reaction arrow.

(d)

Problem 20-51 1,6-Hexanediamine, a starting material needed for making nylon, can be made from 1,3-butadiene. How would you accomplish the synthesis?

Problem 20-52 3-Methyl-2-hexenoic acid (mixture of E and Z isomers) has been identified as the substance responsible for the odor of human sweat. Synthesize the compound from starting materials having five or fewer carbons.

Spectroscopy

Problem 20-53 Propose a structure for a compound C₆H₁₂O₂ that dissolves in dilute NaOH and shows the following ¹H NMR spectrum: 1.08 δ (9 H, singlet), 2.2 δ (2 H, singlet), and 11.2 δ (1 H, singlet).

Problem 20-54 What spectroscopic method could you use to distinguish among the following three isomeric acids? Tell what characteristic features you would expect for each acid.

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Problem 20-55 How would you use NMR (either ¹³C or ¹H) to distinguish between the following pairs of isomers?

(a) the=

(b)

(c)

(d) The condensed formula of 4-methylpent-3-enoic acid, and the chemical structure of cyclopentanecarboxylic acid.

Problem 20-56 Compound A, C₄H₈O₃, has infrared absorptions at 1710 and 2500 to 3100 cm^–1 and has the ¹H NMR spectrum shown. Propose a structure for A.

The proton spectrum of C 4 H 8 O 3 shows peaks at 1.26 (triplet), 3.64 (quartet), 4.14 (singlet), and 11.12 (singlet).

General Problems

Problem 20-57 A chemist in need of 2,2-dimethylpentanoic acid decided to synthesize some by reaction of 2-chloro-2-methylpentane with NaCN, followed by hydrolysis of the product. After the reaction sequence was carried out, however, none of the desired product could be found. What do you suppose went wrong?

Problem 20-58 Show how you might prepare the anti-inflammatory agent ibuprofen starting from isobutylbenzene. More than one step is needed.

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Problem 20-59 The following synthetic schemes all have at least one flaw in them. What is wrong with each?

(a) conversion=

(b) Conversion of 2-phenylacetic acid to ethylbenzene through reaction first with lithium aluminum hydride, then hydronium.

(c) Conversion of 4-chloro-2-methylbutan-2-ol to 4-hydroxy-4-methylpentanoic acid using sodium cyanide followed by acid hydrolysis.

Problem 20-60 p-Aminobenzoic acid (PABA) was once widely used as a sunscreen agent. Propose a synthesis of PABA starting from toluene.

Problem 20-61 Propose a synthesis of the anti-inflammatory drug fenclorac from phenylcyclohexane.

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Problem 20-62 The pK_a’s of five p-substituted benzoic acids (YC₆H₄CO₂H) are listed below. Rank the corresponding substituted benzenes (YC₆H₅) in order of their increasing reactivity toward electrophilic aromatic substitution. If benzoic acid has pK_a = 4.19, which of the substituents are activators and which are deactivators?

Substituent Y
–Si(CH₃)₃	4.27
–CH═CHC≡N	4.03
–HgCH₃	4.10
–OSO₂CH₃	3.84
–PCl₂	3.59

Problem 20-63 How would you carry out the following transformations? More than one step is needed in each case.

(a) benzene=

(b) Cyclohexanone separated from 1-phenylcyclohexanecarboxylic acid by a reaction arrow. 1-phenylcyclohexanecarboxylic acid has a cyclohexane ring single bonded to a phenyl group and a carboxyl group at C 1.

Problem 20-64 The following pK_a values have been measured. Explain why a hydroxyl group in the para position decreases the acidity while a hydroxyl group in the meta position increases the acidity.

The acid strengths of 4-hydroxybenzoic acid (p K a equals 4.48), benzoic acid (p K a equals 4.19), and 3-hydroxybenzoic acid (p K a equals 4.07) are compared.

Problem 20-65 Identify the missing reagents a–f in the following scheme:

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Problem 20-66 Propose a structure for a compound, C₄H₇N, that has the following IR and ¹H NMR spectra:

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Problem 20-67 The two ¹H NMR spectra shown here belong to crotonic acid (trans-CH₃CH $\text{=$ $\text{=$ ₂H) and methacrylic acid [H₂C $\text{=}$ C(CH₃)CO₂H]. Which spectrum corresponds to which acid? Explain.

(a) The proton spectrum shows peaks at 1.91 (doublet), 5.83 (doublet), 7.10 (multiplet), and 12.21 (singlet).

(b) The proton spectrum shows peaks at 1.93 (singlet), 5.66 (singlet), 6.25 (singlet), and 12.24 (singlet).

Problem 20-68 The ¹H and ¹³C NMR spectra below belong to a compound with formula C₆H₁₀O₂. Propose a structure for this compound.

(a) The proton spectrum shows peaks at 1.05 (triplet), 1.80 (singlet), 2.25 (multiplet), 6.85 (triplet), and 12.5 (singlet).

(b) The carbon-13 spectra show peaks at 11.79, 12.89, 22.24, 126.62, 146.71, and 174.19.

Problem 20-69 Propose structures for carboxylic acids that show the following peaks in their ¹³C NMR spectra. Assume that the kinds of carbons (1°, 2°, 3°, or 4°) have been assigned by DEPT-NMR.

(a) C₇H₁₂O₂: 25.5 δ (2°), 25.9 δ (2°), 29.0 δ (2°), 43.1 δ (3°), 183.0 δ (4°)

(b) C₈H₈O₂: 21.4 δ (1°), 128.3 δ (4°), 129.0 δ (3°), 129.7 δ (3°), 143.1 δ (4°), 168.2 δ (4°)

Problem 20-70 Carboxylic acids having a second carbonyl group two atoms away lose CO₂ (decarboxylate) through an intermediate enolate ion when treated with base. Write the mechanism of this decarboxylation reaction using curved arrows to show the electron flow in each step.

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