Advanced Organic Chemistry. Part A. Structure and Mechanisms

Francis A. Carey, Richard J. Sundberg

Chapter 6 Carbanions and Other Carbon Nucleophiles - all with Video Answers

Educators

Chapter Questions

Problem 1

Predict the order of increasing thermodynamic acidity in each series of compounds and explain the basis of your prediction.
a. benzene, 1,4 -cyclohexadiene, cyclopentadiene, cyclohexane
e. $9-(m$-chlorophenyl)fluorene, 9-( $p$-methoxyphenyl)fluorene,
9-phenylfluorene,
$9-(m$-methoxyphenyl)fluorene,
9-( $p$-methylphenyl)fluorene.

Ian Kaigh

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05:09

Problem 2

Indicate the most acidic hydrogen in each of the following molecules. Explain your reasoning.

Jorge Villanueva

Numerade Educator

03:13

Problem 3

Offer an explanation for the following observations.
a. Base-catalyzed exchange rates indicate that the hydrocarbon cubane is much more acidic than cyclobutane, and even more acidic than cyclopropane.
b. Cyclopropyl phenyl ketone $(\mathrm{p} K=28.2)$ is less acidic than acetophenone ( $\mathrm{p} K=24.7$ ) and undergoes $\mathrm{C}-\mathrm{H}$ exchange more slowly than phenyl $i$-propyl ketone, despite the fact that its most acidic hydrogen is located on a cyclopropyl ring.
c. The order of acidity for cyclopentadiene, indene, and fluorene in DMSO is indicated below. The gas phase acidity is in the opposite direction, as measured by the proton affinity. Why does the fusion of benzene rings decrease the solution acidity relative to cyclopentadiene?
d. The rotational barrier of the allyl anion in THF, as measured by NMR, is dependent on the identity of the cation present.

Shazia Naz

Numerade Educator

06:41

Problem 4

a. The relative rates of hydroxide ion-catalyzed deuterium exchange at $\mathrm{C}(3)$ have been measured for bicyclo[2.1.1]hexan-2-one, bicyclo[2.2.1]heptan-2one, and bicyclo[2.2.2]octan-2-one. What factor(s) could account for the much smaller rate of exchange for bicyclo[2.1.1]hexan-2-one?
b. Treatment of $(+)$-camphenilone with $\mathrm{K}^{+.-}-\mathrm{O}-t$-Bu in $t$-BuOD at $185^{\circ} \mathrm{C}$ results in incorporation of D, as indicated below. Racemization occurs at the same rate. Suggest a mechanism that could account for these observations.

Dr. Satish Ingale

Numerade Educator

05:26

Problem 5

Using data from Tables $6.1$ (p. 580 ) and $6.2$ (p. 583 ), estimate the extent of deprotonation for each hydrocarbon-base combination below. Discuss the uncertainties that could affect the calculation.
a. indene by $0.01 \mathrm{MaCH}_{3}$ in $1: 1 \mathrm{DMSO}-\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}$
b. fluorene by $0.01 M \mathrm{NaOC}_{2} \mathrm{H}_{5}$ in 20:1 DMSO-C $_{2} \mathrm{H}_{5} \mathrm{OH}$
c. triphenylmethane by $5 \mathrm{M} \mathrm{KOCH}_{3}$ in $\mathrm{CH}_{3} \mathrm{OH}$

Adriano Chikande

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01:27

Problem 6

The rates of removal of axial and equatorial protons from 4- $t$-butylcyclohexanone in $\mathrm{NaOD} /$ dioxane have been compared by an NMR technique. The rate of removal of an axial proton is $5.5$ times faster than for an equatorial proton. How do you explain the difference?

Madi Sousa

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Problem 7

The acidity of various substituted acetophenones in DMSO is given below. Would you expect the $\rho$ value to correlate best with $\sigma, \sigma^{+}$, or $\sigma^{-}$? Explain, considering each of the $\sigma$ parameters explicitly. Do a plot of the $\mathrm{p} K / \mathrm{p} K_{0}$ values for each $\sigma$ parameter.

Victor Salazar

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Problem 8

Victor Salazar

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07:44

Problem 9

Suggest mechanisms for the following reactions:

Tom Rutherford

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07:26

Problem 10

Meldrum's acid, $\mathrm{p} K=7.3$, is exceptionally acidic in comparison with acyclic analogs, such as dimethyl malonate $(\mathrm{p} K=15.9)$. 5,5-Dimethyl1,3 -cyclohexadione is only moderately more acidic than pentane-2,4-dione. ( $\mathrm{p} K=11.2$ versus 13.43). (All $\mathrm{p} K$ values in DMSO). It is found that the enhanced acidity of Meldrum's acid derivatives decreases as the ring size increases, with the larger ring compounds being similar in acidity to dimethyl malonate. Analyze the factors that contribute to the enhanced acidity of Meldrum's acid.

Tom Rutherford

Numerade Educator

05:58

Problem 11

In some solvents, such as $\mathrm{K}^{+.-}$OR-DMSO, it can be shown that the internal return equilibrium characterized by $k_{1} / k_{-1}$ is fast relative to the dissociation process characterized by $k_{2}$. In this process, the base returns the proton to the carbanion faster than the proton donor exchanges with other molecules from solution. If internal return is important under a given set of conditions, how might that affect the correlation between observed kinetic and thermodynamic acidity? How can the occurrence of internal return be detected experimentally?

Ronald Prasad

Numerade Educator

02:02

Problem 12

Discuss the following comparison of the enol content at equilibrium based on data given in Table 6.12. Discuss the reason for the differing enol content of the pairs of compounds in question.
a. Why does cyclohexanone have a somewhat higher enol content than cyclopentanone?
b. Why do methyl acetate and acetamide have much lower enol content than acetone?
c. Why does 2 -indanone have a much higher enol content than 1 -indanone?
d. Why does 5,5 -dimethyl- 1,3 -cyclohexa-1,3-dione have a higher enol content than acyclic diones such as acetylacetone, even though intramolecular hydrogen bonding is not possible?

Raghvendra Singh

Numerade Educator

02:02

Problem 13

Identify the structural factors that lead to the special stabilization of the enol form noted in each example.
a. The enol content of the 2 -isomer of 3 -(x-pyridyl)-3-oxopropanoate esters is higher than for the 3 -and 4 -isomers.
b. The $K_{\text {enol }}$ of 9-formylfluorene is 17 , that is, the enol form is favored at equilibrium.
c. Monoanilides of 2 -cyanopropane-1,3-dicarboxyate monoesters exist in an enolic form in the solid state and in halogenated solvents.

Raghvendra Singh

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01:11

Problem 14

Certain cyclic compounds exhibit enhanced acidity relative to acyclic models. Offer an explanation for the examples given below.

Alkendra Singh

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01:22

Problem 15

The stereoselectivity of alkylation of 3-acetylbutyrolactones is influenced by alkyl substituents at $\mathrm{C}(4)$ and $\mathrm{C}(5)$. Analyze possible conformations of the enolate and develop an explanation of the observed stereoselectivity.

Grigoriy Sereda

Numerade Educator

01:12

Problem 16

Metal ions such as $\mathrm{Zn}^{2+}, \mathrm{Ni}^{2+}$, and $\mathrm{Cu}^{2+}$ enhance the rate of general basecatalyzed enolization of 2 -acetylpyridine by a several orders of magnitude. Account for this effect.

Nidhi Singhi

Numerade Educator

01:12

Problem 17

The $C(2)$ equatorial hydrogen is selectively removed when 1,3 -dithianes are deprotonated. Furthermore, if the resulting carbanion is protonated, there is a strong preference for equatorial protonation, even though it leads to the less stable axial orientation for the 2 -substituent. Discuss the relevance of these observations to the structure of the sulfur-stabilized carbanion in MO terminology.

Muhammad Ahsan

Numerade Educator

02:02

Problem 18

a. It is found that when 2-methyl-2-butene is converted to a dianion, it first gives the 2-methylbutadiene dianion 18-A, but this is converted to the more stable anion 18-B, which can be referred to as a "methyltrimethylenemethane dianion. Does simple HMO theory offer an explanation for this result?
b. The HMO diagrams of several conceivable dianions that might be formed from double deprotonation of 2-methyl- 1,5-hexadiene are given. On the basis of these diagrams, which dianion would be expected to be most stable?

Colton K

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03:49

Problem 19

Which of the two possible structures for the dimer of methylketene is in best accord with the observed $\mathrm{p} K$ of $2.8$ ? Explain.

Theodore Donnell

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02:05

Problem 20

Predict the products of each of the following halogenation reactions.

Grigoriy Sereda

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03:35

Problem 21

Predict the structure and stereochemistry of the products that would be obtained under the specified reaction conditions. Explain the basis of your prediction.

Madi Sousa

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03:44

Problem 22

The alkylation of 3 -methylcyclohex-2-enone with several dibromides led to the products shown below. Discuss the outcome of each reaction and suggest an explanation for the dependence of the product structure on the identity of the dihalide.

Ian Kaigh

Numerade Educator

05:08

Problem 23

The stereochemistry of base-catalyzed deuterium exchange has been examined for 23-A, where $X=C N$ and $C O P h$. When $X=C N$, the exchange occurs with $99 \%$ retention of configuration, but with $\mathrm{X}=\mathrm{COPh}$, only about $30 \%$ net retention is observed. Explain these contrasting results.

Zubair Abdulla

Numerade Educator

03:40

Problem 24

The distribution of $\alpha$-bromoketones formed in the reaction of acetylcyclopentane with bromine shows an altered product ratio when the 1-position of the ring is deuterated. Assuming that acid-catalyzed enolization is the ratedetermining step in bromination, calculate the primary isotope effect.

Allison Krajewski

Numerade Educator

02:44

Problem 25

Analyze the mechanisms and transition structures for the following alkylation reactions in order to determine the factors that lead to the observed stereoselectivity.

Ronald Prasad

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05:41

Problem 26

Several dialkyl malate esters were alkylated with a benzylic bromide. The dimethyl and diethyl esters show a 8:1 and 9:1 selectivity for 26-A (si face, respectively). The diastereoselectivities are shown for several more bulky esters. Figure 6.P26 gives the $\mathrm{HF} / 6-31 \mathrm{G}^{*}$ structures of the corresponding enolates. Explain the observed stereoselectivity on the basis of structural features present in these enolates.

Sana Riaz

Numerade Educator