Organic Chemistry

Marc Loudon, Jim Parise

Chapter 9

The Chemistry of Alkyl Halides - all with Video Answers

Educators

Chapter Questions

Problem 1

What is the expected nucleophilic substitution product when
(a) methyl iodide reacts with $\mathrm{Na}^{+} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{~S}^{-} ?$
(b) ethyl iodide reacts with ammonia?

Aadit Sharma

Aadit Sharma

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Problem 2

Write the equation for the reaction in Problem 9.la as a net ionic equation.

Aadit Sharma

Numerade Educator

Problem 3

What product(s) are expected in the ethoxide-promoted $\beta$ -elimination reaction of each of the following compounds?
(a) 2 -bromo- 2,3 -dimethylbutane
(b) 1 -chloro-1-methylcyclohexane

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Problem 4

What substitution and elimination products (if any) might be obtained when each of the following alkyl halides is treated with sodium methoxide in methanol?
(a) trans-l-bromo-3-methylcyclohexane
(b) methyl iodide
(c) (bromomethyl)cyclopentane

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Problem 5

Tell whether each of the following reactions favors reactants or products at equilibrium. (Assume that all reactants and products are soluble.)

Aadit Sharma

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Problem 6

For each of the following reactions, (1) what is the overall kinetic order of the reaction, (2) what is the order in each reactant, and (3) what are the dimensions of the rate constant?
(a) an addition reaction of bromine to an alkene with the rate law
$$
\text { rate }=k[\text { alkene }]\left[\mathrm{Br}_{2}\right]^{2}
$$
(b) a substitution reaction of an alkyl halide with the rate law
rate $=k[$ alkyl halide $]$

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Problem 7

(a) What is the ratio of rate constants $k_{\mathrm{A}} / k_{\mathrm{B}}$ at $25^{\circ} \mathrm{C}$ for two reactions $\mathrm{A}$ and $\mathrm{B}$ if the standard free energy of activation of reaction $\mathrm{A}$ is $14 \mathrm{~kJ} \mathrm{~mol}^{-1}\left(3.4 \mathrm{kcal} \mathrm{mol}^{-1}\right)$ less than that of reaction $\mathrm{B}$ ?
(b) What is the difference in the standard free energies of activation at $25^{\circ} \mathrm{C}$ of two reactions $\mathrm{A}$ and $\mathrm{B}$ if reaction $\mathrm{B}$ is 450 times faster than reaction $\mathrm{A}$ ? Which reaction has the greater $\Delta G^{\circ}$ ? $?$

Aadit Sharma

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Problem 8

What prediction does the rate law in Eq. 9.18 make about how the rate of the reaction changes as the reactants $D$ and $E$ are converted into $F$ over time? Does the rate increase, decrease, or stay the same? Explain. Use your answer to sketch a plot of the concentrations of starting materials and products against time.

Aadit Sharma

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Problem 9

The reaction of acetic acid with ammonia is very rapid and follows the simple rate law shown in the following equation. Propose a mechanism that is consistent with this rate law.

Aadit Sharma

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Problem 10

What rate law would be expected for the reaction of cyanide ion ( : CN ) with ethyl bromide by the $S_{\mathrm{N}} 2$ mechanism?

Aadit Sharma

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Problem 11

Methyl iodide $(0.1 M)$ and hydriodic acid $(\mathrm{HI}, 0.1 M)$ are allowed to react in ethanol solution with $0.1 M$ sodium ethoxide. What products are observed?

Aadit Sharma

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Problem 12

Ethyl bromide $(0.1 M)$ and $\mathrm{HBr}(0.1 M)$ are allowed to react in aqueous THF with $1 M$ sodium cyanide $\left(\mathrm{Na}^{+}-\mathrm{CN}\right)$. What products are observed? Are any products formed more rapidly than others? Explain.

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Problem 13

What is the expected substitution product (including its stereochemical configuration) in the $S_{\mathrm{N}} 2$ reaction of potassium iodide in acetone solvent with the following compound? (D $={ }^{2} \mathrm{H}=$ deuterium, an isotope of hydrogen.)

Aadit Sharma

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Problem 14

When methyl bromide is dissolved in ethanol, no reaction occurs at $25^{\circ} \mathrm{C}$. When excess sodium ethoxide is added, a good yield of ethyl methyl ether is obtained. Explain.

Aadit Sharma

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Problem 15

(a) Give the structure of the $S_{N} 2$ reaction product between ethyl iodide and potassium acetate.
(b) In which solvent would you expect the reaction to be faster: acetone or ethanol? Explain.

Aadit Sharma

Numerade Educator

Problem 16

Which nucleophile, $: \mathrm{N}\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3}$ or $: \mathrm{P}\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{3}$, reacts most rapidly with methyl iodide in ethanol solvent? Explain, and give the product formed in each case.

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Problem 17

The following hydroxide-catalyzed $\beta$ -elimination takes place by a carbon-anion stepwise mechanism. Show the carbon-anion intermediate and explain its stability. Think in terms of a polar effect (Sec. $3.6 \mathrm{C}$ ). Recalling also that resonance structures imply heightened stability (Sec. 1.4), draw a resonance structure for this anion as well.

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Problem 18

We can conceive of a stepwise version of the $\mathrm{S}_{\mathrm{N}} 2$ reaction consisting of a Lewis acid-base dissociation followed by a Lewis acid-base association. (Nuc: $=$ a nucleophile.)
(a) Why should the stepwise process be slower than the concerted process?
(b) For what type of alkyl halide is the stepwise process likely to be observed?

Aadit Sharma

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Problem 19

In each of the following series, arrange the compounds in order of increasing reactivity in the $\mathrm{E} 2$ reaction with $\mathrm{Na}^{+} \mathrm{EtO}^{-}$.

Aadit Sharma

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Problem 20

(a) The rate-limiting step in the hydration of styrene $\left(\mathrm{Ph}-\mathrm{CH}=\mathrm{CH}_{2}\right)$ is the initial transfer of the proton from $\mathrm{H}_{3} \mathrm{O}^{+}$ to the alkene (Sec. $4.9 \mathrm{~B}$ ). How would you expect the rate of the reaction to change if the reaction were run in $\mathrm{D}_{2} \mathrm{O} / \mathrm{D}_{3} \mathrm{O}^{+}$ instead of $\mathrm{H}_{2} \mathrm{O} / \mathrm{H}_{3} \mathrm{O}^{+} ?$ Would the product be the same?
(b) How would the rate of styrene hydration in $\mathrm{H}_{2} \mathrm{O} / \mathrm{H}_{3} \mathrm{O}^{+}$ differ from that of an isotopically substituted styrene $\mathrm{Ph}-\mathrm{CH}=\mathrm{CD}_{2} ?$ Explain.

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Problem 21

Predict the products, including their stereochemistry, from the $\mathrm{E} 2$ reactions of the following diastereomers of stilbene dibro mide with sodium ethoxide in ethanol. Assume that one equivalent of $\mathrm{HBr}$ is eliminated in each case.

Aadit Sharma

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Problem 22

Draw the structure of the starting material that would undergo anti-elimination to give the $E$ isomer of the alkene product in the $\mathrm{E} 2$ reaction of Eq. $9.40 \mathrm{a}$.

Aadit Sharma

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Problem 23

What nucleophile or base and what type of solvent could be used for the conversion of isobutyl bromide into each of the following compounds?

Aadit Sharma

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Problem 24

Arrange the following four alkyl halides in descending order with respect to the E2 elimination to $\mathrm{S}_{\mathrm{N}} 2$ substitution product ratio expected in their reactions with sodium ethoxide in ethyl alcohol. Explain your answers.

Aadit Sharma

Numerade Educator

Problem 25

Arrange the following four bases in descending order with respect to the $\mathrm{E} 2$ elimination to $\mathrm{S}_{\mathrm{N}} 2$ substitution product ratio expected when they react with isobutyl bromide. Explain your answers.

Aadit Sharma

Numerade Educator

Problem 26

Give all the products that might be formed when 3 -chloro-2,2-dimethylbutane (the alkyl halide in Eq. 9.60 ) undergoes solvolysis in aqueous ethanol. Of the alkenes formed, which should be the major one(s)?

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Problem 27

Write a curved-arrow mechanism for formation of the rearrangement product shown in Eq. 9.60 .

Aadit Sharma

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Problem 28

The optically active alkyl halide in Eq. 9.61 reacts at $60^{\circ} \mathrm{C}$ in anhydrous methanol solvent to give a methyl ether $A$ plus alkenes. The substitution reaction is reported to occur with $66 \%$ racemization and $34 \%$ inversion. Give the structure of ether $A$ and state how much of each enantiomer of $A$ is formed.

Aadit Sharma

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Problem 29

In light of the ion-pair hypothesis, how would you expect the stereochemical outcome of an $\mathrm{S}_{\mathrm{N}}$ 1 reaction (percent racemization and inversion) to differ from the result discussed in this section for an alkyl halide that gives a carbocation intermediate which
(a) more stable or is considerably
(b) less stable than the one involved in Eq. $9.61 ?$

Aadit Sharma

Numerade Educator

Problem 30

Predict the products expected in each of the following situations, and show the mechanism of any reaction that takes place using the curved-arrow notation.
(a) 1-bromobutane in methanol containing a large excess of sodium methoxide
(b) 2-bromobutane in tert-butyl alcohol containing a large excess of potassium tert-butoxide
(c) 2 -bromo -1,1 -dimethylcyclopentane in ethanol
(d) bromocyclohexane in methanol, heat

Aadit Sharma

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Problem 31

Give an equation showing the preparation of each of the following organometallic compounds.

Aadit Sharma

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Problem 32

Complete each of the following equations.

Ramesh Singh

Numerade Educator

Problem 33

Give the products of the following reactions. Show the curved-arrow notation for each.

Aadit Sharma

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Problem 34

(a) Give the structures of two isomeric alkylmagnesium bromides that would react with water to give propane.
(b) What compounds would be formed from the reactions of the reagents in (a) with $\mathrm{D}_{2} \mathrm{O} ?$

Aadit Sharma

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Problem 35

What alkyl halide and what alkene would yield each of the following cyclopropane derivatives in the presence of a strong base?

Aadit Sharma

Numerade Educator

Problem 36

Predict the products that result when each of the following alkenes reacts with chloroform and potassium tert-butoxide. Give the structures of all product stereoisomers, and, if more than one stereoisomer is formed, indicate whether they are formed in the same or different amounts.
(a) cyclopentene
(b) $(R)-3$ -methylcyclohexene

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Problem 37

Give the structure of the organic product expected when $\mathrm{CH}_{2} \mathrm{I}_{2}$ reacts with each of the following alkenes in the presence of a $\mathrm{Zn}-$ Cu couple:

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Problem 38

From which alkene could each of the following cyclopropane derivatives be prepared using the Simmons-Smith reaction?

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Problem 39

Give the free-radical chain mechanism for the formation of ethyl bromide from ethane and bromine in the presence of light.

Aadit Sharma

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Problem 40

Explain why butane is formed as a minor by-product in the free-radical bromination of ethane.

Aadit Sharma

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Problem 41

Given the product distribution in Eq. $9.88,$ calculate the relative rate of abstraction (per hydrogen) of a tertiary and a primary hydrogen by a chlorine atom.

Aadit Sharma

Numerade Educator

Problem 42

Two monobromination products are obtained in roughly equal amounts when pentane is treated with bromine and light. What are they? Explain your reasoning.

Aadit Sharma

Numerade Educator

Problem 43

(a) What is the major monobromination product obtained when methylcyclohexane is treated with bromine and light? Explain your reasoning.
(b) The tertiary/primary relative reactivity per hydrogen at $-15^{\circ} \mathrm{C}$ in the photochlorination of triptane $(2,2,3$ -trimethylbutane) is $4.5 .$ What are the relative amounts formed of the three possible monochlorination products?

Aadit Sharma

Numerade Educator

Problem 44

Choose the alkyl halide(s) from the following list of $\mathrm{C}_{6} \mathrm{H}_{13} \mathrm{Br}$ isomers that meet each criterion below.
(1) 1 -bromohexane
(2) 3 -bromo- 3 -methylpentane
(3) 1 -bromo- 2,2 -dimethylbutane
(4) 3-bromo-2-methylpentane
(5) 2 -bromo-3-methylpentane
(a) the compound(s) that can exist as enantiomers
(b) the compound(s) that can exist as diastereomers
(c) the compound that gives the fastest $\mathrm{S}_{\mathrm{N}} 2$ reaction with sodium methoxide
(d) the compound that is least reactive to sodium methoxide in methanol
(e) the compound(s) that give only one alkene in the $\mathrm{E} 2$ reaction with $\mathrm{K}^{+}$ - $\mathrm{O} t \mathrm{Bu}$
(f) the compound(s) that give an $\mathrm{E} 2$ but no $\mathrm{S}_{\mathrm{N}} 2$ reaction with sodium methoxide in methanol
(g) the compound(s) that undergo an $\mathrm{S}_{\mathrm{N}} 1$ reaction to give rearranged products
(h) the compound that gives the fastest $\mathrm{S}_{\mathrm{N}} 1$ reaction

Aadit Sharma

Numerade Educator

Problem 45

Give the products expected when isopentyl bromide (1-bromo- 3 -methylbutane) or the other substances indicated react with the following reagents.
(a) KI in aqueous acetone
(b) KOH in aqueous ethanol
(c) $\mathrm{K}^{+}\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{O}^{-}$ in $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{OH}$
(d) product of part (c) $+\mathrm{HBr}$
(e) CsF in $N, N$ -dimethylformamide (a polar aprotic solvent)
(f) product of part (c) $+$ chloroform + potassium tertbutoxide
(g) product of part (c) $+\mathrm{CH}_{2} \mathrm{I}_{2}$ in the presence of a $\mathrm{Zn}-$ Cu couple
(h) Li in hexane, then ethanol
(i) sodium methoxide in methanol
(j) $\mathrm{Mg}$ and anhydrous ether, then $\mathrm{D}_{2} \mathrm{O}$

Aadit Sharma

Numerade Educator

Problem 46

Give the products expected when 2 -bromo- 2 -methylhexane or the other substances indicated react with the following reagents.
(a) warm 1:1 ethanol-water
(b) sodium ethoxide in ethanol
(c) KI in aqueous acetone
(d) product(s) of part (b) $+\mathrm{HBr}$ in the presence of peroxides
(e) product(s) of part (b) $+\mathrm{Hg}(\mathrm{OAc})_{2}$ in THF-water, followed by $\mathrm{NaBH}_{4}$
(f) product(s) of part (b) $+\mathrm{BH}_{3}$ in THF, followed by alkaline $\mathrm{H}_{2} \mathrm{O}_{2}$

Aadit Sharma

Numerade Educator

Problem 47

Identify the gas evolved in each of the following reactions.

Aadit Sharma

Numerade Educator

Problem 48

Give the structure that meets the criteria given in each of the following cases:
(a) A compound $\mathrm{C}_{6} \mathrm{H}_{14}$ that gives only two products of monochlorination, one of which is chiral.
(b) Four stereoisomeric compounds $\mathrm{C}_{4} \mathrm{H}_{\mathrm{g}} \mathrm{O},$ all optically active, that contain no double bonds and evolve a gas when treated with EtMgBr.

Aadit Sharma

Numerade Educator

Problem 49

Rank the following compounds in order of increasing $\mathrm{S}_{\mathrm{N}} 2$ reaction rate with KI in acetone.

Aadit Sharma

Numerade Educator

Problem 50

In each of the following series, order the atoms, compounds, or ions in order of increasing polarizability, and explain your choices.
(a) $\mathrm{Se}, \mathrm{O}, \mathrm{S}$
(b) chloroform, fluoroform, iodoform
(c) $\mathrm{I}^{-}, \mathrm{Br}^{-}, \mathrm{Cl}^{-}, \mathrm{F}^{-}$

Aadit Sharma

Numerade Educator

Problem 51

Rank the following compounds in order of increasing $\mathrm{S}_{\mathrm{N}} 2$ reaction rate with KI in acetone.
methyl bromide $\quad$ sec-butyl bromide
$A$
B
3-(bromomethyl)-3-methylpentane
$C$
1-bromopentane $\quad$ 1-bromo-2-methylbutane
$D$

Aadit Sharma

Numerade Educator

Problem 52

Give the structure of the nucleophile that could be used to convert iodoethane into each of the following compounds in an $\mathrm{S}_{\mathrm{N}} 2$ reaction.

Aadit Sharma

Numerade Educator

Problem 53

Give all of the product(s) expected, including pertinent stereochemistry, when each of the following compounds reacts with sodium ethoxide in ethanol. $(\mathrm{D}=$ deuterium $=$ ${ }^{2} \mathrm{H},$ an isotope of hydrogen.

Aadit Sharma

Numerade Educator

Problem 54

Tell which of the following alkyl halides can give only one alkene, and which can give a mixture of alkenes, in the $\mathrm{E} 2$ reaction.

Aadit Sharma

Numerade Educator

Problem 55

In the Williamson ether synthesis, an alkoxide reacts with an alkyl halide to give an ether.
$$
\mathrm{R}-\ddot{\mathrm{O}}:^{-}+\mathrm{R}^{\prime}-\ddot{\mathrm{X}}: \longrightarrow \mathrm{R}-\ddot{\mathrm{O}}-\mathrm{R}^{\prime}+: \ddot{\mathrm{X}}:^{-}
$$

Aadit Sharma

Numerade Educator

Problem 56

Propose a synthesis of ethyl neopentyl ether, $\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OCH}_{2} \mathrm{C}\left(\mathrm{CH}_{3}\right)_{3},$ from an alkyl bromide and any
other reagents.

Aadit Sharma

Numerade Educator

Problem 57

Three alkyl halides, each with the formula $\mathrm{C}_{7} \mathrm{H}_{15} \mathrm{Br}$, have different boiling points. One of the compounds is optically active. Following reaction with $\mathrm{Mg}$ in ether, then with water, each compound gives 2,4 -dimethylpentane. After the same reaction with $\mathrm{D}_{2} \mathrm{O}$, a different product is obtained from each compound. Suggest a structure for each of the three alkyl halides.

Aadit Sharma

Numerade Educator

Problem 58

When 2,3 -dimethylbutane is treated with $\mathrm{Br}_{2}$ in the presence of light, the bromine-containing compounds obtained in greatest amount are compound $A\left(\mathrm{C}_{6} \mathrm{H}_{13} \mathrm{Br}\right)$ and compound $B\left(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{Br}_{2}\right)$. Propose structures for these compounds and explain your reasoning.

Aadit Sharma

Numerade Educator

Problem 59

The banned insecticide chlordane is reported to lose some of its chlorine and to be converted into other compounds when exposed to alkaline conditions. Explain.

Aadit Sharma

Numerade Educator

Problem 60

What products are expected, including their stereochemistry, when $(2 S, 3 R)-2$ -bromo -3 -methylpentane is subjected to each of the following conditions? Explain.
(a) methanol containing an excess of sodium methoxide
(b) hot methanol containing no sodium methoxide

Aadit Sharma

Numerade Educator

Problem 61

(a) Explain why the compound given in part (a) of Fig. P9.6l reacts to give a mixture of 2 -butene stereoisomers in which only the $Z$ isomer contains deuterium.
(b) Explain why the compound given in part (b) of Fig. P9.61 reacts to give a mixture of 2 -butene stereoisomers in which only the $E$ isomer contains deuterium. (Hint: Convert the conformations shown into a conformation that allows the elimination to occur with proper stereochemistry.)

Aadit Sharma

Numerade Educator

Problem 62

Which of the following secondary alkyl halides reacts faster with - $\mathrm{CN}$ in the $\mathrm{S}_{\mathrm{N}} 2$ reaction? (Hint: Consider the hybridization and geometry of the $\mathrm{S}_{\mathrm{N}} 2$ transition state.)

Aadit Sharma

Numerade Educator

Problem 63

Explain why 1 -chlorobicyclo[2.2.1]heptane, even though it is a tertiary alkyl halide, is virtually unreactive in the $\mathrm{S}_{\mathrm{N}} 1$ reaction. (It has been estimated that it is $10^{-13}$ times as reactive as tert-butyl chloride!) (Hint: Consider the preferred geometry of the reactive intermediate.)

Hailey Tomashek

Hailey Tomashek

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Problem 64

Explain each of the following observations.
(a) When benzyl bromide $\left(\mathrm{Ph}-\mathrm{CH}_{2}-\mathrm{Br}\right)$ is added to a suspension of potassium fluoride in benzene, no reaction occurs. However, when a catalytic amount of the crown ether [18] -crown- $6($ Sec. $8.7 B)$ is added to the solution, benzyl fluoride can be isolated in high yield.
(b) If lithium fluoride is substituted for potassium fluoride, no reaction occurs even in the presence of the crown ether.

Aadit Sharma

Numerade Educator

Problem 65

Tert-butyl chloride undergoes solvolysis in either acetic acid or formic acid.
<smiles>CC(C)(C)CO</smiles>
Both solvents are protic, donor solvents, but they differ substantially in their dielectric constants $\epsilon$
(a) What is the $\mathrm{S}_{\mathrm{N}}$ 1 solvolysis product in each solvent?
(b) In one solvent, the $\mathrm{S}_{\mathrm{N}} 1$ reaction is 5000 times faster than it is in the other. In which solvent is the reaction more rapid, and why?

Madeline Currie

Madeline Currie

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Problem 66

Suppose that $\mathrm{CH}_{3} \mathrm{I}$ is added to an ethanol solution containing an excess of both $\mathrm{Na}^{+} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}$ and $\mathrm{K}^{+} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}$ in equimolar amounts.
(a) What is the major product that will be isolated from the reaction? Explain.
(b) How would your answer change (if at all) if the experiment were conducted in anhydrous DMSO, a polar aprotic solvent?

Molly Cary

Numerade Educator

Problem 67

(a) Two isomeric $S_{N} 2$ products are possible when sodium thiosulfate is allowed to react with one equivalent of methyl iodide in methanol solution. Give the structures of the two products.
sodium thiosulfate
(Thiosulfate is an example of an ambident, or "twotoothed," nucleophile.)
(b) In fact, only one of the two possible products is formed. Which one is formed, and why?

Aadit Sharma

Numerade Educator

Problem 68

Consider the following equilibrium:
In each case (a) and (b), choose the solvent in which the equilibrium would lie farther to the right. Explain. (Assume that the products are soluble in all solvents considered.)
(a) ethanol or diethyl ether
(b) $N, N$ -dimethylacetamide (a polar, aprotic solvent, $\epsilon=38$ ) or a mixture of water and methanol that has the same dielectric constant

Aadit Sharma

Numerade Educator

Problem 69

Although Grignard reagents are normally insoluble in hydrocarbon solvents, they can be dissolved in such solvents if a tertiary amine (a compound with the general structure $\mathrm{R}_{3} \mathrm{~N}:$ ) is added. Explain.

Aadit Sharma

Numerade Educator

Problem 70

When methyl iodide at $0.1 M$ concentration is allowed to react with sodium ethoxide at $0.1 M$ concentration in ethanol solution, the product ethyl methyl ether is obtained in good yield. Explain why the reaction is over much more quickly, but about the same yield of the ether is obtained, when the reaction is run with an excess $(0.5 M)$ of sodium ethoxide.

Aadit Sharma

Numerade Educator

Problem 71

When methyl bromide is dissolved in methanol and an equimolar amount of sodium iodide is added, the concentration of iodide ion quickly decreases, and then slowly returns to its original value. Explain.

Aadit Sharma

Numerade Educator

Problem 72

Consider the following experiments with trityl chloride, $\mathrm{Ph}_{3} \mathrm{C}-\mathrm{Cl}$, a very reactive tertiary alkyl halide:
(1) In aqueous acetone, the reaction of trityl chloride follows a rate law that is first order in the alkyl halide, and the product is trityl alcohol, $\mathrm{Ph}_{3} \mathrm{C} \rightarrow \mathrm{OH}$.
(2) In another reaction, when one equivalent of sodium azide $\left(\mathrm{Na}^{+} \mathrm{N}_{3}^{-} ;\right.$ see Table $9.3,$ p. 396 ) is added to a solution that is otherwise identical to that used in experiment (1), the reaction rate is the same as in (1); however, the product isolated in good yield is trityl azide, $\mathrm{Ph}_{3} \mathrm{C}-\mathrm{N}_{3-}$
(3) In a reaction mixture in which both sodium azide and sodium hydroxide are present in equal concentrations, both trityl alcohol and trityl azide are formed, but the reaction rate is again unchanged.

Explain why the reaction rate is the same but the products are different in these three experiments.

Aadit Sharma

Numerade Educator

Problem 73

The first demonstration of the stereochemistry of the $\mathrm{S}_{\mathrm{N}} 2$ reaction was carried out in 1935 by Prof. E. D. Hughes and his colleagues at the University of London. They allowed ( $R$ )-2-iodooctane to react with radioactive iodide ion ( ${ }^{*} \mathrm{I}^{-}$ ).
2-iodooctane $\quad$ 2-iodooctane (radioactive)
The rate of substitution (rate constant $k_{\mathrm{S}}$ ) was determined by measuring the rate of incorporation of radioactivity into the alkyl halide. The rate of loss of optical activity from the alkyl halide (rate constant $k^{\circ}$ ) was also determined under the same conditions.
(a) What ratio $k^{\circ} / k_{\mathrm{S}}$ is predicted for each of the following stereochemical scenarios:
(1) retention; (2) inversion; (3) equal amounts of both retention and inversion? Explain.
(b) The experimental rate constants were found to be as follows:
$k_{\mathrm{S}}=(13.6 \pm 1.1) \times 10^{-4} M^{-1} \mathrm{~s}^{-1}$
$k^{\circ}=(26.2 \pm 1.1) \times 10^{-4} M^{-1} \mathrm{~s}^{-1}$
Which scenario in part (a) is consistent with the data?

Aadit Sharma

Numerade Educator

Problem 74

In a laboratory, two liquids, $A$ and $B$, were found in a box labeled only "isomeric alkyl halides $\mathrm{C}_{5} \mathrm{H}_{11} \mathrm{Br}$." You have been employed to deduce the structures of these compounds from the following data left in an accompanying In a laboratory, two liquids, $A$ and $B$, were found in a box labeled only "isomeric alkyl halides $\mathrm{C}_{5} \mathrm{H}_{11} \mathrm{Br}$." You have been employed to deduce the structures of these compounds from the following data left in an accompanying

Aadit Sharma

Numerade Educator

Problem 75

An optically active compound $A$ has the formula $\mathrm{C}_{8} \mathrm{H}_{13} \mathrm{Br}$. Compound $A$ gives no reaction with $\mathrm{Br}_{2}$ in $\mathrm{CH}_{2} \mathrm{Cl}_{2},$ but it reacts with $\mathrm{K}^{+}\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{O}^{-}$ to give a single new compound $B$ in good yield. Compound $B$ decolorizes $\mathrm{Br}_{2}$ in $\mathrm{CH}_{2} \mathrm{Cl}_{2}$ and takes up hydrogen over a catalyst. When $\mathrm{com}-$ pound $B$ is treated with ozone followed by aqueous $\mathrm{H}_{2} \mathrm{O}_{2}$, dicarboxylic acid $C$ is isolated in excellent yield; notice its cis stereochemistry.

Identify compounds $A$ and $B,$ and account for all observations. (If you need a refresher on how to solve this type of problem, see Study Guide Links 4.3 and $5.3 .$ )

Travis Maslanik

Travis Maslanik

Numerade Educator

Problem 76

In the laboratories of the firm "Halides 'R'Us," a compound $A$ has been found in a vial labeled only "achiral alkyl halide $\mathrm{C}_{10} \mathrm{H}_{17} \mathrm{Br}$." The management feels that the compound might be useful as a pesticide, but they need to know its structure. You have been called in as a consultant at a handsome fee. Compound $A$, when treated with $\mathrm{KOH}$ in warm ethanol, yields two compounds $(B$ and $C),$ each with the molecular formula $\mathrm{C}_{10} \mathrm{H}_{16}$. Compound $A$ rapidly reacts in aqueous ethanol to give an acidic solution, which, in turn, gives a precipitate of $\mathrm{AgBr}$ when tested with $\mathrm{AgNO}_{3}$ solution. Ozonolysis of $A$ followed by treatment with $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{~S}$ affords $\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{O}$ (acetone) as one of the products plus unidentified halogen-containing material. Catalytic hydrogenation of either $B$ or $C$ gives a mixture of both trans- and cis-l-isopropyl-4-methylcyclohexane. Compound $A$ reacts with one equivalent of $\mathrm{Br}_{2}$ to give a mixture of two separable compounds, $D$ and $E$, both of which can be shown to be achiral compounds. Finally, ozonolysis of compound $B$ followed by treatment with aqueous $\mathrm{H}_{2} \mathrm{O}_{2}$ gives acetone and the diketone $F$.
Propose structures for compounds $A$ through $E$ that best fit the data (and collect your fee).

Dalton Hilovsky

Dalton Hilovsky

Numerade Educator

Problem 77

Which one of the following stereoisomers should undergo $\beta$ -elimination most rapidly with sodium ethoxide in ethanol? Explain your reasoning.

Ian Kaigh

Numerade Educator

Problem 78

When menthyl chloride (see Fig. P9.78) is treated with sodium ethoxide in ethanol, 2 -menthene is the only alkene produet observed. When neomenthyl chloride is subjected to the same conditions, the alkene products are mostly 3-menthene $(78 \%)$ along with some 2 -menthene $(22 \%)$. Explain why different alkene products are formed from the different alkyl halides, and why 3 -menthene is the major product in the second reaction. (Hint: Draw the chair conformations of the starting materials, remember the stereochemistry of the E2 reaction, and don't forget about the chair interconversion of cyclohexanes.)

Narayan Hari

Numerade Educator

Problem 79

(a) Tell whether each of the eliminations shown in Fig. P9.79 is syn or anti.
(b) Reaction (2) shows first-order kinetics. Draw a curved-arrow mechanism for this reaction that is con-
sistent with its kinetic order and with its stereochemistry. (Hint: Be sure to draw out the structure of the acetate group.)

Aadit Sharma

Numerade Educator

Problem 80

Explain why each alkyl halide stereoisomer gives a different alkene in the E2 reactions shown in Fig. P9.80 on
p. 450 . It will probably help to build models or draw out the conformations of the two starting materials.

Aadit Sharma

Numerade Educator

Problem 81

Knowing that carbocations rearrange, chemists H. C. Brown and Glenn A. Russell at Purdue University in the early 1950 s investigated whether free-radical rearrangements might also occur:
<smiles>CCCCCC(CC(C)C)CC(C)C(C)C</smiles>
They carried out light-promoted free-radical chlorination of isobutane- $2 d$, in which the tertiary hydrogen of isobutane was replaced by deuterium $\left({ }^{2} \mathrm{H}=\mathrm{D}\right),$ as shown in Fig. P9.81 on p. $450 .$ The ratio of $\mathrm{DCl}$ to $\mathrm{HCl}$ produced in the reaction was found to be exactly the same as the ratio of $A$ to $B$. (Any isotopically substituted $A$, if present, would not be differentiated from A.)
(a) This result was cited as evidence that free radicals do not rearrange. Explain the logic of this conclusion. (Hint: Compare this result with the result that would occur with rearrangement.)
(b) Assuming the same reaction conditions, how would the relative percentages of $A$ and $B$ compare with the percentages of tert-butyl chloride and isobutyl chloride in Eq. 9.88 (greater or less)? Explain. (Hint: See Sec. $9.5 \mathrm{D} .)$
(c) Suggest a method for the preparation of the isotopically labeled starting material, isobutane-2d. (Hint: See Sec. $9.8 \mathrm{C}$.)

Hailey Tomashek

Hailey Tomashek

Numerade Educator

Problem 82

(a) The reagent tributyltin hydride, $\mathrm{Bu}_{3} \mathrm{Sn}-\mathrm{H}$, brings about the rapid conversion of 1 -bromo-1-methylcyclohexane into methylcyclohexane. The reaction is particularly fast in the presence of AIBN (Sec. $5.6 \mathrm{C}$ ). Suggest a mechanism for this reaction. (Hint: The $\mathrm{Sn}-\mathrm{H}$ bond is relatively weak.)
(b) Suggest two other reaction sequences using other reagents that would bring about the same overall transformation.

Aadit Sharma

Numerade Educator

Problem 83

The cis and trans stereoisomers of 4 -chlorocyclohexanol give different products when they react with $\mathrm{OH}^{-}$, as shown in the reactions given in Fig. P9.83.
(a) Give a curved-arrow mechanism for the formation of each product.
(b) Explain why the bicyclic material $B$ is observed in the reaction of the trans isomer, but not in the reaction of the cis isomer.

Nicholas Sacco

Numerade Educator

Problem 84

The reaction of butylamine, $\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{3} \mathrm{NH}_{2},$ with 1-bromobutane in $60 \%$ aqueous ethanol follows the rate law
rate $=k$ [butylamine][1-bromobutane]
The product of the reaction is $\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}\right)_{2} \mathrm{NH}_{2}^{+}$
$\mathrm{Br}^{-}$. The following very similar reaction, however, has a first-order rate law:
<smiles>C=CCNCCCC</smiles>
Give a mechanism for each reaction that is consistent with its rate law and with the other facts about nucleophilic substitution reactions. Use the curved-arrow notation.

Aadit Sharma

Numerade Educator

Problem 85

In $1975,$ a report was published in which the reaction given in Fig. P9.85 was observed. The - OBs (brosylate) group is a leaving group conceptually like halide. (Think of this group as you would $-$ Br.) Notice that the reaction conditions favor an $\mathrm{S}_{\mathrm{N}} 2$ reaction.
(a) This result created quite a stir among chemists because it seemed to question a fundamental principle of the $\mathrm{S}_{\mathrm{N}} 2$ reaction. Explain.
(b) Because the result was potentially very significant, the work was reinvestigated very soon after it was published. In this reinvestigation, it was found that after about 10 hours' reaction time, the product consisted almost completely of trans- $P .$ Only on standing for a much longer time under the reaction conditions did cis- $P$ form (and trans- $P$ disappear) to give the product mixture shown in Fig. P9.85. Furthermore, when the trans isomer of $S$ was subjected to the same conditions, mostly cis- $P$ was formed after $10 \mathrm{~h}$, but, after 5 days, the same 75: 25 cis:trans product mixture was formed as in Fig. P9.85. Finally, subjecting pure cis- $P$ or pure trans- $P$ to the reaction conditions gave, after five days, the same 75: 25 mixture. Explain these results.
(c) Why is cis- $P$ favored in the product mixture?

Alkendra Singh

Numerade Educator

Problem 86

Consider the reaction sequence given in Fig. P9.86. $\left(\mathrm{Bu}-=\right.$ butyl group $\left.=\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2}-\right)$
(a) Use what you know about the stereochemistry of bromine addition to propose the stereochemistry of compound $B$.
(b) Is the $B \rightarrow C$ reaction a syn-or an anti-elimination? Show your analysis.
(c) How would the stereochemistry of products change if the $\mathrm{E}$ stereoisomer of compound $A$ were carried through the same sequence of reactions? Explain.

Madeline Currie

Madeline Currie

Numerade Educator

Problem 87

Account for each of the results, shown in Fig. P9.87, with a mechanism. In part (a), note that the reaction is not observed in the absence of $\mathrm{NaOH}$. In part (b), note that organolithium reagents are strong bases and that the hydrogens on a carbon adjacent to a benzene ring are relatively acidic.

Aadit Sharma

Numerade Educator