5.P: Organohalogen and Organometallic Compounds (Problems)

Problem 5-1 Deduce the structures of the two compounds whose NMR and infrared spectra are shown in Figure 5-1. Assign as many of the infrared bands as you can and analyze the NMR spectra in terms of chemical shifts and spin-spin splittings.

Figure 5-1: Infrared and proton NMR spectra of substance \(\ce{C_4H_7Br}\) and substance \(\ce{C_5H_8Br_2}\) (see Problem 5-1). NMR spectra are at \(60 \: \text{MHz}\) with reference to TMS at \(0.0\).

Problem 5-2 

a. Methyl iodide can be prepared from potassium iodide and dimethyl sulfate. Why is dimethyl sulfate preferable to methanol in reaction with potassium iodide?

b. 1-Bromobutane can be prepared from 1-butanol and sodium bromide in concentrated sulfuric acid. What is the function of the sulfuric acid?

c. Some people like to put salt in their beer. Assess the possibility of \(\ce{CH_3CH_2Cl}\) poisoning from the reaction of \(\ce{NaCl}\) with the ethanol in beer. Give your reasoning.

d. Both isopropyl bromide and tert-butyl bromide react with sodium ethoxide in ethanol. Which bromide would give the most alkene? Which bromide would give the most alkene on solvolysis in \(60\%\) aqueous ethanol? Of the two reagents, sodium ethoxide in ethanol or \(60\%\) aqueous ethanol, which would give the most alkene with each bromide? Give your reasoning.

Problem 5-3 The reaction of \(\ce{N}\)-bromosuccinimide (NBS) with alkenes to produce allylic bromides is though to involve molecular bromine produced by the reaction

Show the propagation steps that probably are involved in the radical-chain bromination of cyclohexane with NBS, assuming that bromine atoms are produced from NBS in the initiation steps. What by-products would you anticipate?

Problem 5-4 Give a reasonable method for the synthesis of the following compounds from organic compounds not containing a halogen. Indicate the structures of any major by-products expected.

a. 

b. \(\ce{HC \equiv CCH_2Br}\)

c. 

d. \(\ce{CH_3-CH(Cl)-CH=CH-CH_2CH_3}\)

e. \(\ce{(CH_2=CH)_2CHCl}\)

Problem 5-5 In the presence of only traces of ionizing agents, either pure 1-chloro-2-butene or 3-chloro-1-butene is converted slowly to a 50-50 equilibrium mixture of the two chlorides. Explain.

Problem 5-6 

a. Write the initiation and propagation steps involved in the radical bromination of methylbenzene (toluene) with bromine. Write the low-energy valence-bond structures of the intermediate phenylmethyl radical.

b. Calculate \(\Delta H^0\) for the following reactions of the radical, using the \(\ce{C-Br}\) bond strength of \(\ce{C_6H_5CH_2Br}\) \(\left( 55 \: \text{kcal} \right)\), and any other necessary bond energies. Assume that stabilization arising from electron delocalization is \(38 \: \text{kcal}\) for a phenyl group (Section 6-5A) and \(5 \: \text{kcal}\) for the triene structure \(3\).

What can you conclude from these calculations about the stability of \(3\) and the likelihood of its formation in this kind of bromination?

Problem 5-7 Explain the following observations.

a. 1,2-Propadiene gives 3-chloropropyne on radical chlorination with either \(\ce{Cl_2}\) or tert-butyl hypochlorite.

b. 1-Chloro-2-propanone could be regarded as a kind of allylic chloride, but it is very unreactive under \(S_\text{N}1\) conditions although it is highly reactive in \(S_\text{N}2\) reactions.

c. The enantiomers of 3-chloro-1-butene racemize somewhat more rapidly than they give solvolysis products, under conditions that favor \(S_\text{N}1\) reactions when a good ionizing but weakly nucleophilic solvent is used.

Problem 5-8 Would you expect the behavior of 3-chloropropyne to more nearly resemble 1-chloropropane or 3-chloropropane in nucleophilic displacement reactions? Give your reasoning.

Problem 5-9 Arrange the following halides in order of expected increasing reactivity towards (a) sodium iodide in acetone and (b) silver nitrate in ethanol. Indicate your reasoning.

\[\ce{C_6H_5CH_2CH_2Cl}, \: \ce{C_5H_5C \equiv CCl}, \: \ce{C_6H_5C \equiv CCH_2Cl}, \: \ce{C_6H_5CH=CHCl}\]

Problem 5-10 Write a reasonable mechanism for the formation of phenylethanoic acid on heating phenylbromoethyne with potassium hydroxide in aqueous alcohol:

\[\ce{C_6H_5C \equiv CBr} \underset{\ce{KOH}}{\overset{\ce{H_2O-C_2H_5OH}}{\longrightarrow}} \: \overset{\ce{H}^\oplus}{\longrightarrow} \ce{C_6H_5CH_2CO_2H}\]

Problem 5-11 

a. Write resonance structures analogous to structures \(5a\) through \(5d\) to show the activating effect of \(\ce{-C \equiv N}\) and \(\ce{-SO_2R}\) groups in nucleophilic substitution of the corresponding 4-substituted chlorobenzenes.

b. How would you expect the introduction of methyl groups ortho to the activating group to affect the reactivity of 4-bromonitrobenzene and 4-bromocyanobenzene toward ethoxide ion? (Investigate the geometry of the anion intermediate.)

Problem 5-12 Would you expect 4-bromonitrobenzene or (4-bromophenyl)-trimethylammonium chloride to be more reactive in bimolecular replacement of bromine by ethoxide ion?

Problem 5-13* Would you expect 4-chloromethoxybenzene and 4-chlorotrifluoromethylbenzene to be more, or less, reactive than chlorobenzene toward methoxide ion? Explain.

Problem 5-14* Whereas the order of reactivity of alkyl halides toward a given nucleophile is \(\ce{I} > \ce{Br} > \ce{Cl} \gg \ce{F}\), the reverse order of reactivity frequently is observed with aryl halides \(\left( \ce{F} \gg \ce{Cl} \cong \ce{Br} \cong \ce{I} \right)\). What does this signify regarding the relative rates of the addition and elimination steps (Equations 5-3 and 5-4) in this kind of aromatic substitution?

Problem 5-15 The reactions of several 1-substituted 2,4-dinitrobenzenes with piperidine (azacyclohexane), Equation 5-5, proceed at nearly the same rate, independent of the nature of \(\ce{X}\). Rationalize this observation in terms of a mechanism of nucleophilic aromatic substitution.

\(\tag{5-5}\)

Problem 5-16* The reaction of 1-fluoro-2,4-dinitrobenzene with dimethylamine is catalyzed by weak bases. How may this observation be explained? (Consider possible intermediates, rate-determining steps, etc.)

Problem 5-17 Write the products of the following reactions:

a. 

b. 

c. 

Problem 5-18 Two valence-bond structures are possible for benzyne:

How do these differ from the Kekulé structures usually written for benzene? Devise an atomic-orbital model for benzyne.

Problem 5-19 The intervention of benzyne in the amination of chlorobenzene, bromobenzene, and iodobenzene with sodium amide in liquid ammonia originally was demonstrated by J. D. Roberts using \(\ce{^{14}C}\)-labeled halobenzenes. Show explicitly how the use of a chlorobenzene-\(\ce{^{14}C}\) label could differentiated between amination by addition-elimination (Section 5-6B) versus amination by elimination-addition (benzyne mechanism).

Problem 5-20 In the hydrolysis of chlorobenzene-1-\(\ce{^{14}C}\) with \(4 \: \text{M}\) aqueous sodium hydroxide at \(340^\text{o}\), the products are \(58\%\) benzenol-1-\(\ce{^{14}C}\) and \(42\%\) benzenol-2-\(\ce{^{14}C}\). Calculate the percentage of reaction proceeding (a) by an elimination-addition mechanism, and (b) by direct nucleophilic displacement. Would you expect the amount of direct displacement to increase, or decrease, if the reaction were carried out (a) at \(240^\text{o}\) and (b) with lower concentrations of sodium hydroxide? Give you reasoning.

Problem 5-21 Explain the following observations:

a. 2,6-Dimethylchlorobenzene does not react with potassium amide in liquid ammonia at \(-33^\text{o}\).

b. Fluorobenzene, labeled with deuterium in the 2- and 6-positions, undergoes rapid exchange of deuterium for hydrogen in the presence of potassium amide in liquid ammonia, but does not form benzenamine (aniline).

Problem 5-22* Bromobenzene reacts rapidly with potassium tert-butoxide in \(\ce{(CH_3)_2SO}\) (methylsulfinylmethane, dimethyl sulfoxide, DMSO) to give tert-butyl phenyl ether:

A comparable reaction does not take place in tert-butyl alcohol as solvent (see Section 8-7F). Suggest a mechanism for the reaction and explain why DMSO is a better solvent for the reaction than tert-butyl alcohol. What products would you expect to be formed using 4-bromo-1-methylbenzene in place of bromobenzene?

Problem 5-23* Both 2,4-D and 2,4,5-T are herbicides that have been used for weed control and as defoliating agents in jungle warfare. Apart from the arguments for or against the use of chemicals for such purposes, there have been reports of serious dermatitis among the industrial workers who produce these substances.

The cause finally was traced to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which is produced as an impurity in the manufacture of 2,4,5-T.

This substance (TCDD) is very toxic. In addition to the dermatitis in produces, it is a potent teratogen (induces birth abnormalities). The lethal does is less than \(10^{-6} \: \text{g}\) for guinea pigs. Its presence in 2,4,5-T can be eliminated, but the conditions by which it is formed are pertinent to our present discussion.

The production of 2,4,5-T involves the substitution of one chlorine of 1,2,4,5-tetrachlorobenzene with hydroxide ion to give \(12\). This is followed by a second displacement reaction, this time on chloroethanoate by the sodium salt of \(12\):

If the temperature of the first step exceeds \(160^\text{o}\), then two molecules of \(12\) react in a double nucleophilic displacement to give TCDD.

a. Write reasonable mechanisms for the steps by which two molecules of \(12\) are converted to TCDD.

b. Would you expect TCDD to be formed in the preparation of 2,4-D from 1,2,4-trichlorobenzene? Explain.

Problem 5-24 What products would you expect from the reaction of bromoform, \(\ce{CHBr_3}\), with potassium tert-butyl alcohol in the presence of (a) trans-2-butene and (b) cis-2-butene?

Problem 5-25* Devise atomic-orbital models of the singlet and triplet forms of \(\colon \ce{CH_2}\). Of these one has a much greater \(\ce{H-C-H}\) angle than the other. Deduce whether the triplet or the singlet form should have the wider \(\ce{H-C-H}\) angle. (Remember the Pauli principle, Section 6-1.)

Problem 5-26 Write the structures of the products of the following equations:

a. \(\ce{C_6H_5CH_2CH_2MgBr} + \ce{(CH_3)_2SO_4} \rightarrow\)
b. \(\ce{C_2H_5MgBr} + \ce{CH_3C \equiv C-CH_2Br} \rightarrow\)
c. \(\ce{CH_2=CH-CH_2Li} + \ce{CH_2=CH-CH_2Cl} \rightarrow\)
d. \(\ce{CH_3CH_2CH_2MgBr} + \ce{ClCH_2OCH_3} \rightarrow\)
e. \(\ce{C_4H_9Na} + \ce{C_4H_9Br} \rightarrow\)

Problem 5-27* Show the steps involved in the formation of 2,3-pentadiene from cis-2-butene (1 mole), carbon tetrabromide (1 mole), and methyllithium (2 moles). (See Section 5-7B for pertinent reactions.)

Problem 5-28* Show how the following transformations might be achieved. Define the reaction conditions as closely as possible. More than one step may be required.

a. 

b. 

c. 

Problem 5-29 Addition of Grignard reagents, \(\ce{RMgX}\), to diethyl carbonate, \(\ce{O=C(OC_2H_5)_2}\), gives tertiary alcohols, \(\ce{R_3COH}\), on hydrolysis. Write the steps involved in this reaction.

Problem 5-30 Write structures for the products of the following reactions involving Grignard reagents. Show the structures of both the intermediate substances and the substances obtained after hydrolysis with \(\ce{NH_4Cl}\) solution. Unless otherwise specified, assume that sufficient Grignard reagent is used to effect those addition reactions that occur readily at room temperature.

a. \(\ce{C_6H_5MgBr} + \ce{C_6H_5CHO}\)
b. \(\ce{CH_3MgI} + \ce{CH_3CH_2CO_2C_2H_5}\)
c. \(\ce{CH_3CH_2MgBr} + \ce{ClCO_2C_2H_5}\)
d. \(\ce{C_6H_5MgBr} + \ce{(CH_3O)_2C=O}\)
e. 

Problem 5-31 Show how each of the following substances could be prepared from the indicated organic halide and any other appropriate organic compounds:

a.  from 

b. \(\ce{CH_2=CHC(CH_3)_2OH}\) from \(\ce{CH_3I}\)

c. \(\ce{C_6H_5C \equiv CCH_2OH}\) from \(\ce{C_2H_5Cl}\)

d. \(\ce{CH_3CH_2CH(OH)CH_3}\) from \(\ce{C_2H_5Cl}\)

Problem 5-32 Write structures for the addition, enolization, and reduction products possible for the following reactions:

a. \(\ce{CH_3COCH_3} + \ce{(CH_3)_3CMgX}\)
b. \(\ce{C_6H_5COC_6H_5} + \ce{CH_3CH_2MgX}\)
c. \(\ce{C_6H_5CH=CHCO_2C_2H_5} + \ce{C_6H_5MgX}\)

Problem 5-33 Grignard reagents, such as \(\ce{CH_3MgI}\), often add to the triple bond of nitriles, \(\ce{RC \equiv N}\), to give adducts that, on hydrolysis, yield ketones, \(\ce{RCO_CH_3}\). Show the possible steps involved.

Problem 5-34 Show the products expected from the following combinations of reagents. Write the structures of the initial adducts and also the products they give on acid hydrolysis:

a. \(\ce{(CH_3)_3CMgCl} + \ce{CO_2}\)

b. 

c. 

d. \(\ce{CH_3-COCl} + \ce{CD_3Li} + \ce{CuI}\)

Problem 5-35 Show how the following compounds could be synthesized by reasonable reactions from the indicated compound and any other needed reagents:

a.  from chlorocyclohexane

b. \(\ce{C_6H_5C \equiv CCO_2H}\) from phenylethyne

c. \(\ce{ClC \equiv CCO_2H}\) from ethyne

Problem 5-36 Predict the products expected from the reactions of the following compounds:

a. \(\ce{CH_2=CH-CO-CH_3} + \ce{C_2H_5MgBr}\)

b. 

c. 

d. 

Problem 5-37 Would you expect the overall energy change (after hydrolysis) to be more favorable for 1,2 or 1,4 addition of a Grignard reagent to \(\ce{CH_2=CH-COCH_3}\)? Give your reasoning.

Problem 5-38 What products do you expect from the following reactions? Give your reasoning. Show the structures of the intermediate compounds in sequences of the type \(\rightarrow \rightarrow\).

a. 

b. \(\ce{CH_2=CH_2} \overset{\ce{ICl}}{\longrightarrow} \: \underset{\text{heat}}{\overset{\ce{KOH}}{\longrightarrow}}\)

c. 

d. \(\ce{CH_3CH_2OH} + \ce{KI} + \ce{H_3PO_4} \overset{\text{reflux}}{\longrightarrow}\)

e. 

Problem 5-39 In the reaction of 5-38e, when the aqueous acid is mixed with 2-methyl-2-butanol, the mixture is initially homogenous, but it soon separates into two phases. Explain why two phases appear. On separation of the phases using a separatory funnel, which layer (upper or lower) would contain the organic product? If you were unsure, how could you quickly find out?

Problem 5-40 Suppose one could hydrolyze pure cis-1-chloro-2-butene exclusively by (a) the \(S_\text{N}1\) mechanism or (b) the \(S_\text{N}2\) mechanism. Would you expect the 2-butenol formed in each case to be the cis isomer, the trans isomer, or a mixture? Explain.

Problem 5-41 The intermediate carbocation formed by \(S_\text{N}1\) reactions of either 3-chloro-3-methyl-1-butene or 4-chloro-2-methyl-2-butene reacts with water to give a mixture of 2-methyl-3-buten-2-ol and 3-methyl-2-buten-1-ol. Which alcohol would you expect to predominate under conditions of equilibrium control? On the bases of steric hindrance, charge distribution in the cation, and so on, which alcohol should be favored under conditions of kinetic control? (Review Sections 6-5C, 10-4A, and 11-3.) Give your reasoning.

Problem 5-42 Explain why 2-chloropyridine is more reactive than 3-chloropyridine in nucleophilic substitution reactions.

Problem 5-43 Explain why 2-chloropyridine reacts with potassium amide \(\left( \ce{KNH_2} \right)\) in liquid ammonia solution at \(-33^\text{o}\) to give 2-aminopyridine, whereas 3-chloropyridine under the same conditions gives a mixture of \(65\%\) 4-amino- and \(35\%\) 3-aminopyridine.

Problem 5-44 Write a structural formula for a compound that fits each of the following descriptions:

a. an aryl halogen compound that reacts with sodium iodide in acetone but not with aqueous silver nitrate solution
b. an organic fluoro compound that is more reactive in displacement reactions than the corresponding iodo compound
c. an aryl bromide that cannot undergo substitution by the elimination-addition (benzyne) mechanism
d. the monobromonitronaphthalene expected to be least reactive toward ethoxide ion in ethanol

Problem 5-45 Explain why the substitution reactions of the following halonaphthalenes give about the same ratio of 1- and 2-naphthyl products independently of the halogen substituent and the nucleophilic reagent. Show the steps involved.

Problem 5-46 For each of the following pairs of compounds describe a chemical test, preferably a test-tube reaction, that will distinguish between the two compounds. Write a structural formula for each compound and equations for the reactions involved.

a. chlorobenzene and phenylmethyl chloride
b. 4-nitrochlorobenzene and 3-nitrochlorobenzene
c.  and 

d. \(\ce{CH_3C \equiv C-Br}\) and \(\ce{BrCH_2C \equiv CH}\)

Problem 5-47 Show how benzyne can be formed from the following reagents:

a. fluorobenzene and phenyllithium

b.  \(+ \ce{KNH_2}\)

c. 1,2-dibromobenzene + butyllithium

Problem 5-48 Indicate the steps involved in each of the following transformations:

a. 

The reaction does not occur unless more than one mole of \(\ce{KNH_2}\) per mole of \(\ce{C_6H_5Br}\) is used.

b. 

c. 

Problem 5-49 Predict the products of the following reactions:

a. 

b. 

c. 

Problem 5-50 Nucleophilic displacement of the halogen of 3,5-dimethyl-4-nitrobromobenzene is much slower than with the corresponding compound lacking the methyl groups. Give a reasonable explanation of this observation. (Construction of molecular models will help.)

Problem 5-51 Methylmagnesium halides have been employed as analytical reagents for the determination of the number of acidic hydrogens in a molecule (the Zerewitinoff determination). The method involves measuring the amount of methane produced from a given weight of compound (such as \(\ce{RH}\), with an acidic hydrogen) by the following reaction:

\[\ce{CH_3MgI} + \ce{RH} \rightarrow \ce{CH_4} + \ce{RMgI}\]

Excess methylmagnesium iodide and \(0.1776 \: \text{g}\) of Compound A (formula \(\ce{C_4H_{10}O_3}\)) react to give \(84.1 \: \text{mL}\) of methane collected over mercury at \(740 \: \text{mm}\) and \(25^\text{o}\). How many acidic hydrogens does Compound A possess per molecule? Suggest a possible structure on the basis that spectral data indicate (a) there is no \(\ce{C=O}\) group in the molecule and (b) A is achiral.

Problem 5-52 From the nature of the carbon-metal bonds in organometallic compounds, predict the products of the following reactions. Give your reasoning.

a. \(\ce{CH_3MgCl} + \ce{ICl}\)
b. \(\ce{C_6H_5Li} + \ce{CH_3OH}\)
c. \(\ce{CH_3Li} + \ce{HC \equiv CH}\)
d. \(\ce{C_6H_5Li} + \ce{CuI}\)

Problem 5-53 

a. Show the steps and reaction intermediates by which the product is formed in the following reaction sequence:

b. Draw structures for the products in each step of the following sequence:

\[\ce{C_2H_5Cl} \overset{\ce{Mg}, \: \text{ether}}{\longrightarrow} \: \overset{\ce{C_6H_5C \equiv CH}}{\longrightarrow} \: \overset{\ce{CO_2}}{\longrightarrow} \: \overset{\ce{CH_3Li}}{\longrightarrow} \: \overset{\ce{H}^\oplus, \: \ce{H_2O}}{\longrightarrow}\]

Problem 5-54 The following experimental observations have been reported:

1. tert-Butyl chloride was added to lithium metal in dry ether at \(35^\text{o}\). A vigorous reaction ensued with evolution of hydrocarbon gases. After all the lithium metal was consumed, the mixture was poured onto dry ice. The only acidic product that could be isolated (small yield) was 4,4-dimethylpentanoic acid.

2. tert-Butyl chloride was added to lithium metal in dry ether at \(-40^\text{o}\). After all the lithium had reacted, the mixture was carbonated and gave a good yield of 2,2-dimethylpropanoic acid.

3. tert-Butyl chloride was added to lithium metal in dry ether at \(-40^\text{o}\). After all the lithium was consumed, ethene was bubbled through the mixture at \(-40^\text{o}\) until no further reaction occurred. Carbonation of this mixture gave a good yield of 4,4-dimethylpentanoic acid.

a. Give a reasonably detailed analysis of the results obtained and show as best you can the mechanisms involved in each reaction.

b. Would similar behavior be expected with methyl chloride? Explain.

c. Would you expect that a substantial amount of 6,6-dimethylheptanoic acid would be found in Observation 3? Explain.

Problem 5-55 Predict the products of each of the following Grignard reactions before and after hydrolysis. Give reasoning or analogies for each.

a. \(\ce{CH_3MgI} + \ce{HCO_2C_2H_5} \rightarrow\)
b. \(\ce{CH_3CH_2CH(MgBr)CH_3} +\) 2,4-dimethyl-3-pentanone \(\rightarrow\)
c. \(\ce{CH_3CH_2MgBr} + \ce{CS_2} \rightarrow\)
d. \(\ce{CH_3CH_2MgBr} + \ce{NH_3} \rightarrow\)

Problem 5-56 Show how each of the following substances can be synthesized from the indicated starting materials by a route that involves organometallic substances in at least one step.

a. \(\ce{(CH_3)_3C-D}\) from \(\ce{(CH_3)_3CCl}\)
b. \(\ce{CH_3C \equiv C-CO_2H}\) from \(\ce{CH \equiv CH}\)
c.  from \(\ce{(CH_3)_4C}\)
d.  (three ways)
e.  from 
f.  from \(\ce{(CH_3)_3CCH_2Cl}\)
g. 1,5-hexadiene from propene

Problem 5-57 Predict the products of each of the following reactions both before and after hydrolysis:

a. 

b. 

c. \(\ce{HC \equiv CCH_2Br} \overset{\ce{Mg-} \text{ether}}{\longrightarrow} \: \overset{\ce{HC \equiv CCH_2Br}}{\longrightarrow}\)

d. \(\ce{(CH_3)_3CBr} \overset{\ce{Mg-} \text{ether}}{\longrightarrow} \: \overset{\ce{O_2}, \: -70^\text{o}}{\longrightarrow}\)

e. 

f. 

Problem 5-58 Each of the following equations represents a "possible" but not actually feasible Grignard synthesis. Consider each equation and determine why it will not proceed satisfactorily as written. Give your reasoning and show what the actual product will be.

a. \(\ce{(CH_3)_3CMgBr} + \ce{[(CH_3)_2CH]_2C=O} \rightarrow \rightarrow \ce{(CH_3)_3C[(CH_3)_2CH]_2COH}\)
b. \(\ce{(CH_3)_3CCH_2MgBr} + \ce{[(CH_3)_3C]_2C=O} \rightarrow \rightarrow \ce{(CH_3)_3CCH_2[(CH_3)_3C]_2COH}\)
c. \(\ce{CH_3MgI} + \ce{CH_3(CH_3)_2COCl} \rightarrow \rightarrow \ce{CH_3(CH_3)_2COCH_3}\)
d. \(\ce{CH_3MgI} + \ce{CH_3CCH=N-CH_3} \rightarrow \rightarrow \ce{CH_3CH_2N(CH_3)_2}\)
e. \(\ce{BrCH_2CH_2O_2CCH_3} \underset{\ce{(CH_2H_5)_2O}}{\overset{\ce{Mg}}{\longrightarrow}}\) Grignard reagent \(\overset{\ce{CH_2O}}{\longrightarrow} \rightarrow \ce{HOCH_2CH_2CH_2O_2CCH_3}\)
f. \(\ce{CH_2=CHCH_2MgCl} + \ce{C_2H_5Br} \rightarrow \ce{CH_2=CH(CH_2)_2CH_3}\)

Problem 5-59 What products would you expect to predominate in the following reactions:

a. \(\ce{C_6H_5MgBr} +\) 

b. \(\ce{CH_3MgBr} + \ce{(CH_3)_3CCOC(CH_3)_3}\)

c. \(\ce{CH_3MgBr} +\) 

Problem 5-60 Suggest possible reactions by which the following compounds could be prepared from ethyne and any other necessary compounds:

a. \(\ce{ClC \equiv C(CH_2)_4C \equiv CCl}\)

b. 

c. 

d. \(\ce{CH_2=CHCH_2C \equiv CCH_3}\)

Problem 5-61* The rate of addition of dimethylmagnesium to excess diphenylmethanone (benzophenone) in diethyl ether initially is cleanly second order, that is, first order in ketone and first order in \(\ce{(CH_3)_2Mg}\). As the reaction proceeds, the rate no longer follows a strictly second-order rate overall. Suggest how the apparent specific rate could change as the reaction proceeds.

Problem 5-62* An interesting isomer of benzene is "benzvalene" (tricyclo[2.1.1.0\(^{5,6}\)]-2-hexene). This substance, which like prismane (Section 12-10) can decompose with explosive violence, has been synthesized by T. Katz from lithium cyclopentadienide, dichloromethane, and methyllithium:

Write a mechanism for this reaction that you can support by analogy with other reactions discussed in this chapter or by reasonable mechanistic arguments.

Problem 5-63 Table 5-4 shows that the addition of \(\ce{RMgX}\) to \(\ce{R'CON(CH_3)_2}\) gives \(\ce{RCOR'}\), although addition of \(\ce{RMgX}\) to \(\ce{R'CO_2CH_3}\) and \(\ce{R'COCl}\) lead to \(\ce{R'R_2COH}\). Assuming that similar mechanistic steps are involved throughout, why might \(\ce{R'CON(CH_3)_2}\) give a different product than \(\ce{R'CO_2CH_3}\) or \(\ce{R'COCl}\)? Show your reasoning.

Problem 5-64 The formation of alcohols with organometallic compounds by the Grignard synthesis can be used to achieve the reaction \(\ce{RH} + \ce{-C=O}\) to give \(\ce{R-C-OH}\), which generally has an unfavorable equilibrium constant. If you were looking for an industrial synthesis of 2-butanol by a reaction that would have a favorable equilibrium constant, which of the following might be better candidates than \(\ce{CH_3CH_3} + \ce{CH_3CH=O} \rightarrow \ce{CH_3CH_2CH(CH_3)OH}\). Give your reasoning.

a. \(\ce{CH_2=CH_2} + \ce{CH_3CH_2OH}\)

b. \(\ce{CH_3CH_3} + \ce{CH_3CH_2OH}\)

c. \(\ce{CH_3CH_2CH_2CH_3} + \frac{1}{2} \ce{O_2}\)

d. 

e. \(\ce{CH_3CH_2CH=CH_2} + \ce{H_2O}\)

Problem 5-65 Compound X, of formula \(\ce{C_3H_5Br_3}\), with methyllithium formed bromocyclopropane and 3-bromopropene. The NMR spectrum of X showed a one-proton triplet at \(5.9 \: \text{ppm}\), a two-proton triplet at \(3.55 \: \text{ppm}\), and a complex resonance centered at \(2.5 \: \text{ppm}\) downfield from TMS. What is the structure of X? Account for the products observed in its reaction with methyllithium.

Problem 5-66* There have been many proposals for the occurrence of a so-called \(S_\text{N}2\ce{'}\) mechanism that would produce a concerted substitution with rearrangement for allylic halides. One possible example is

Consider whether this mechanism is operating for the particular example from the following experimental results. Give your reasoning.

1. Silver ethanoate in ethanoic acid with 1-chloro-2-butene gives \(65\%\) 2-butenyl ethanoate and \(35\%\) 1-methyl-2-propenyl ethanoate.

2. Ethanoate ion \(\left( 1 \: \text{M} \right)\) in ethanoic acid with 1-chloro-2-butene gives a mixture of about \(85\%\) 2-butenyl ethanoate and \(15\%\) 1-methyl-2-propenyl ethanoate. About \(36\%\) of the overall reaction product results from a process that is zero order in ethanoate ion, whereas \(64\%\) comes from a process that is first order in ethanoate ion.

3. Ethanoate ion in 2-propanone reacts with 1-chloro-2-butene to give only 2-butenyl ethanoate, and with 3-chloro-1-butene to give only 1-methyl-2-propenyl ethanoate

(This Problem involves many of the ideas developed in Chapter 8 and you may wish to review Sections 8-4A, 8-4B, and 8-7, especially 8-7F.)

Contributors

John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."