John McMurry, Professor Emeritus

9 • Additional Problems

Visualizing Chemistry

Problem 9-14

Name the following alkynes, and predict the products of their reaction with (1) H₂ in the presence of a Lindlar catalyst and (2) H₃O⁺ in the presence of HgSO₄:

(a)

A ball and stick model of a C8 alkyne. Carbon and hydrogen are denoted using gray and white spheres, respectively.

(b)

A ball and stick model of a C10 alkyne. Carbon and hydrogen are denoted using gray and white spheres, respectively.

Problem 9-15

From what alkyne might each of the following substances have been made? (Green = Cl.)

(a)

A ball and stick model of a C6 alkyne. Carbon, oxygen, and hydrogen are denoted using gray, red, and white spheres, respectively.

(b)

A ball and stick model of a substituted cyclohexanxe. Carbon and hydrogen are denoted using gray and white spheres, respectively. Two chlorine groups are denoted using green spheres.

Problem 9-16

How would you prepare the following substances, starting from any compounds having four carbons or fewer?

(a)

A ball and stick model of a cyclopropane that is further linked to a three-carbon chain. Carbon, hydrogen, and oxygen are denoted using gray, white, and red spheres, respectively.

(b)

A ball and stick model of a C6 chain with a double bond and carbonyl. Carbon, hydrogen, and oxygen are denoted using gray, white, and red spheres, respectively.

Problem 9-17

The following cycloalkyne is too unstable to exist. Explain.

A ball and stick model of a five-membered chain with a triple bond. Carbon and hydrogen are denoted using gray and white spheres, respectively.

Mechanism Problems

Problem 9-18

Assuming that halogens add to alkynes in the same manner as they add to alkenes, propose a mechanism for and predict the product(s) of the reaction of phenylpropyne with Br₂.

Problem 9-19

Assuming that strong acids add to alkynes in the same manner as they add to alkenes, propose a mechanism for each of the following reactions.

(a)

A C5 terminal alkyne reacts with two moles of hydrogen chloride to form 2,2-dichloropentane.

(b)

A C4 internal alkyne reacts with two moles of hydrogen bromide to form 2,2-dibromobutane.

(c)

A phenyl alkyne reacts with two moles of hydrogen chloride to form a benzene ring with a carbon connected to a dichloroethyl group

Problem 9-20

The mercury-catalyzed hydration of alkynes involves the formation of an organomercury enol intermediate. Draw the electron-pushing mechanism to show how each of the following intermediates is formed.

(a)

A phenyl alkyne reacts with water, sulfuric acid, and mercuric sulfate to form a benzene ring connected to an ionic organomercury enol sulfate intermediate.

(b)

A C3 alkyne reacts with water, sulfuric acid, and mercuric sulfate to form a C3 ionic organomercury enol sulfate intermediate.

(c)

A C4 terminal alkyne reacts with water, sulfuric acid, and mercuric sulfate to form a C4 ionic organomercury enol sulfate intermediate.

Problem 9-21

The final step in the hydration of an alkyne under acidic conditions is the tautomerization of an enol intermediate to give the corresponding ketone. The mechanism involves a protonation followed by a deprotonation. Show the mechanism for each of the following tautomerizations.

(a)

A benzene ring connected to an OH-substituted alkene reacts with hydronium ions to form a benzene ring with a C O C H 3 group.

(b)

A C3 OH-substituted alkene reacts with hydronium ions to form acetone.

(c)

A C5 OH-substituted alkene reacts with hydronium ion to form a C5 methyl ketone.

Problem 9-22

Predict the product(s) and show the complete electron-pushing mechanism for each of the following dissolving metal reductions.

(a)

(b)

The figure shows a C5 terminal alkyne reacting with lithium and deuterated ammonia.

(c)

The figure shows a phenyl-substituted C3 internal alkyne reacting with lithium and deuterated ammonia.

Problem 9-23

Identify the mechanisms for the following reactions as polar, radical, or both.

(a)

A C6 internal alkyne reacts with lithium and ammonia to form a trans alkene.

(b)

A C6 internal alkyne reacts with bromine to form a trans dibromoalkene

(c)

A C6 internal alkyne reacts with two moles of hydrogen bromide to form a dibromo substituted alkane

Problem 9-24

Predict the product and provide the complete electron-pushing mechanism for the following two-step synthetic processes.

(a)

A benzene ring connected to a C2 alkyne reacts with sodium amide and methyl iodide.

(b)

A C5 keto substituted alkyne reacts with sodium amide and ethyl iodide.

(c)

Problem 9-25

Reaction of acetone with D₃O⁺ yields hexadeuterioacetone. That is, all the hydrogens in acetone are exchanged for deuterium. Review the mechanism of mercuric-ion-catalyzed alkyne hydration, and then propose a mechanism for this deuterium incorporation.

The figure shows the reaction of acetone with tri-deuterium oxide to form hexadeuterioacetone.

Naming Alkynes

Problem 9-26

Give IUPAC names for the following compounds:

(a)

A C8 internal alkyne with two methyl groups at C2 and a triple bond at C3.

(b)

(c)

A C9 enyne with a triple bond at C3, double bond at C5 , and two methyl groups at C2 and C5.

(d)

A C8 dialkyne with triple bonds at C1 , C5, and two methyl groups at C4.

(e)

(f)

A C13 alkyne with a methyl group at C2, a triple bond at C4, and two ethyl groups at C3 and C6.

Problem 9-27

Draw structures corresponding to the following names:

(a)

3,3-Dimethyl-4-octyne

(b)

3-Ethyl-5-methyl-1,6,8-decatriyne

(c)

2,2,5,5-Tetramethyl-3-hexyne

(d)

3,4-Dimethylcyclodecyne

(e)

3,5-Heptadien-1-yne

(f)

3-Chloro-4,4-dimethyl-1-nonen-6-yne

(g)

3-sec-Butyl-1-heptyne

(h)

5-tert-Butyl-2-methyl-3-octyne

Problem 9-28

The following two hydrocarbons have been isolated from various plants in the sunflower family. Name them according to IUPAC rules.

(a)

CH₃CH

\text{=}

CHC

\text{≡}

CC

\text{≡}

CCH

\text{=}

CHCH

\text{=}

CHCH

\text{=}

CH₂ (all trans)

(b)

CH₃C

\text{≡}

CC

\text{≡}

CC

\text{≡}

CC

\text{≡}

CC

\text{≡}

CCH

\text{=}

CH₂

Reactions of Alkynes

Problem 9-29

Terminal alkynes react with Br₂ and water to yield bromo ketones. For example:

A phenyl alkyne reacts with bromine and water to form a phenyl bromo methyl ketone.

Propose a mechanism for the reaction. To what reaction of alkenes is the process analogous?

Problem 9-30

Predict the products of the following reactions:

A terminal enyne reacts with hydrogen and palladium catalyst to form an unknown product A. The reactant also reacts with hydrogen and Lindlar catalyst to form an unknown product B.

Problem 9-31

Predict the products from reaction of 1-hexyne with the following reagents:

(a)

1 equiv HBr

(b)

1 equiv Cl₂

(c)

H₂, Lindlar catalyst

(d)

NaNH₂ in NH₃, then CH₃Br

(e)

H₂O, H₂SO₄, HgSO₄

(f)

2 equiv HCl

Problem 9-32

Predict the products from reaction of 5-decyne with the following reagents:

(a)

H₂, Lindlar catalyst

(b)

Li in NH₃

(c)

1 equiv Br₂

(d)

BH₃ in THF, then H₂O₂, OH⁻

(e)

H₂O, H₂SO₄, HgSO₄

(f)

Excess H₂, Pd/C catalyst

Problem 9-33

Predict the products from reaction of 2-hexyne with the following reagents:

(a)

2 equiv Br₂

(b)

1 equiv HBr

(c)

Excess HBr

(d)

Li in NH₃

(e)

H₂O, H₂SO₄, HgSO₄

Problem 9-34

Propose structures for hydrocarbons that give the following products on oxidative cleavage by KMnO₄ or O₃:

(a)

(b)

The figure shows acetic acid and benzoic acid, the products of oxidative cleavage.

(c)

(d)

Acetal aldehyde, a C5 carboxylic acid with a C2 carbonyl, and carbon dioxide, the products of oxidative cleavage.

(e)

C7 carboxylic acid with carbonyl at C2, aldehyde at C7, and carbon dioxide, the products of oxidative cleavage.

Problem 9-35

Identify the reagents a–c in the following scheme:

A C5 terminal alkyne reacts with reagent a to add an ethyl group at C1, forming a cis alkene via reagent b and a substituted cyclopropane ring with reagent c.

Organic Synthesis

Problem 9-36

How would you carry out the following multistep conversions? More than one step may be needed in some instances.

Figure shows eight reactions of a terminal alkyne. Each arrow points in different directions to give either an aldehyde, alkane, alkene, epoxide, ketone, alkyne, alcohol, or an alkyl halide.

Problem 9-37

How would you carry out the following reactions?

(a)

A C4 alkyne chain reacts in the presence of an unknown reagent represented as a question mark to form methyl ethyl ketone.

(b)

(c)

A phenyl alkyne reacts with an unknown reagent represented as a question mark forming a phenyl alkyne with a terminal methyl group.

(d)

A phenyl alkyne reacts with an unknown reagent represented as a question mark forming a cis-phenyl alkene with a terminal methyl group.

(e)

(f)

Problem 9-38

Each of the following syntheses requires more than one step. How would you carry them out?

(a)

(b)

A C6 alkyne reacts in the presence of an unknown reagent indicated as a question mark to form an alkene.

Problem 9-39

How would you carry out the following multistep transformation?

1-Hexyne reacts in the presence of an unknown reagent indicated by a question mark to form a C8 compound having a cyclopropane ring.

Problem 9-40

How would you carry out the following multistep conversions?

Styrene reacts in the presence of an unknown reagent indicated by a question mark to form 2-phenyl acetaldehyde.

Styrene reacts in the presence of an unknown reagent indicated by a question mark to form prop-1-en-1ylbenzene.

Problem 9-41

Synthesize the following compounds using 1-butyne as the only source of carbon, along with any inorganic reagents you need. More than one step may be needed.

(a)

1,1,2,2-Tetrachlorobutane

(b)

1,1-Dichloro-2-ethylcyclopropane

Problem 9-42

How would you synthesize the following compounds from acetylene and any alkyl halides with four or fewer carbons? More than one step may be needed.

(a)

(b)

(c)

(d)

The figure shows an C8 carbon chain with a carbonyl group at C4.

(e)

Problem 9-43

How would you carry out the following reactions to introduce deuterium into organic molecules?

(a)

A C6 alkyne reacts in the presence of an unknown reagent denoted as a question mark to form a cis-deuterated alkene.

(b)

A C6 alkyne reacts in the presence of an unknown reagent denoted as a question mark to form a trans-deuterated alkene.

(c)

(d)

Ethynylbenzene reacts with an unknown reagent indicated as a question mark to form a deuterium-substituted alkene.

Problem 9-44

How would you prepare cyclodecyne starting from acetylene and any required alkyl halide?

Problem 9-45

The sex attractant given off by the common housefly is an alkene named muscalure. Propose a synthesis of muscalure starting from acetylene and any alkyl halides needed. What is the IUPAC name for muscalure?

General Problems

Problem 9-46

A hydrocarbon of unknown structure has the formula C₈H₁₀. On catalytic hydrogenation over the Lindlar catalyst, 1 equivalent of H₂ is absorbed. On hydrogenation over a palladium catalyst, 3 equivalents of H₂ are absorbed.

(a)

How many degrees of unsaturation are present in the unknown structure?

(b)

How many triple bonds are present?

(c)

How many double bonds are present?

(d)

How many rings are present?

(e)

Draw a structure that fits the data.

Problem 9-47

Compound A (C₉H₁₂) absorbed 3 equivalents of H₂ on catalytic reduction over a palladium catalyst to give B (C₉H₁₈). On ozonolysis, compound A gave, among other things, a ketone that was identified as cyclohexanone. On treatment with NaNH₂ in NH₃, followed by addition of iodomethane, compound A gave a new hydrocarbon, C (C₁₀H₁₄). What are the structures of A, B, and C?

Problem 9-48

Hydrocarbon A has the formula C₁₂H₈. It absorbs 8 equivalents of H₂ on catalytic reduction over a palladium catalyst. On ozonolysis, only two products are formed: oxalic acid (HO₂CCO₂H) and succinic acid (HO₂CCH₂CH₂CO₂H). Write the reactions, and propose a structure for A.

Problem 9-49

Occasionally, a chemist might need to invert the stereochemistry of an alkene—that is, to convert a cis alkene to a trans alkene, or vice versa. There is no one-step method for doing an alkene inversion, but the transformation can be carried out by combining several reactions in the proper sequence. How would you carry out the following reactions?

(a)

(b)

Problem 9-50

Organometallic reagents such as sodium acetylide undergo an addition reaction with ketones, giving alcohols:

Acetone reacts in the presence of sodium acetylide and hydronium ion to form an alcohol.

How might you use this reaction to prepare 2-methyl-1,3-butadiene, the starting material used in the manufacture of synthetic rubber?

Problem 9-51

The oral contraceptive agent Mestranol is synthesized using a carbonyl addition reaction like that shown in Problem 9-50. Draw the structure of the ketone needed.

Problem 9-52

1-Octen-3-ol, a potent mosquito attractant commonly used in mosquito traps, can be prepared in two steps from hexanal, CH₃CH₂CH₂CH₂CH₂CHO. The first step is an acetylide-addition reaction like that described in Problem 9-50. What is the structure of the product from the first step, and how can it be converted into 1-octen-3-ol?

Problem 9-53

Erythrogenic acid, C₁₈H₂₆O₂, is an acetylenic fatty acid that turns a vivid red on exposure to light. On catalytic hydrogenation over a palladium catalyst, 5 equivalents of H₂ are absorbed, and stearic acid, CH₃(CH₂)₁₆CO₂H, is produced. Ozonolysis of erythrogenic acid gives four products: formaldehyde, CH₂O; oxalic acid, HO₂CCO₂H; azelaic acid, HO₂C(CH₂)₇CO₂H; and the aldehyde acid OHC(CH₂)₄CO₂H. Draw two possible structures for erythrogenic acid, and suggest a way to tell them apart by carrying out some simple reactions.

Problem 9-54

Hydrocarbon A has the formula C₉H₁₂ and absorbs 3 equivalents of H₂ to yield B, C₉H₁₈, when hydrogenated over a Pd/C catalyst. On treatment of A with aqueous H₂SO₄ in the presence of mercury(II), two isomeric ketones, C and D, are produced. Oxidation of A with KMnO₄ gives a mixture of acetic acid (CH₃CO₂H) and the tricarboxylic acid E. Propose structures for compounds A–D, and write the reactions.

The structure of a C7 tricarboxylic acid labeled E.

Problem 9-55

A cumulene is a compound with three adjacent double bonds. Draw an orbital picture of a cumulene. What kind of hybridization do the two central carbon atoms have? What is the geometric relationship of the substituents on one end to the substituents on the other end? What kind of isomerism is possible? Make a model to help see the answer.

Problem 9-56

Which of the following bases could be used to deprotonate 1-butyne?

(a) (b) The structure of the sodium salt of the methylene anion connected to a sulfonyl group that is further connected to a methyl group. (c) (d)

Problem 9-57

Arrange the following carbocations in order of increasing stability.

(a)

The structure of three cyclopentene carbocations with positive charges at C 4, C 1, and C 3.

(b)

The structure of three 1-pentene carbocations, with positive charges at C 3, C 2, and C 4.

(c)

The structure of three 4-methylcyclohexene carbocations with positive charges at C 1, C 4, and C 5.