John McMurry, Professor Emeritus

10 • Additional Problems

Visualizing Chemistry

Problem 10-14

Give IUPAC names for the following alkyl halides (green = Cl):

(a)

The ball and stick model of an alkyl halide comprises single carbon-carbon bonds in a ring. Black, gray, and green spheres represent carbon, hydrogen, and chlorine, respectively.

(b)

The ball and stick model of an alkyl halide that comprises a seven-carbon chain and one double bond. Black, gray, and green spheres represent carbon, hydrogen, and chlorine, respectively.

Problem 10-15

Show the product(s) of reaction of the following alkenes with NBS:

(a)

The ball and stick model of alkene that comprises a six-carbon chain. C2 and C3 are double bonded and black and gray spheres represent carbon and hydrogen, respectively.

(b)

The ball and stick model of alkene that comprises two cyclopentane rings joined by a double bond. Black and gray spheres represent carbon and hydrogen, respectively.

Problem 10-16

The following alkyl bromide can be prepared by reaction of the alcohol (S)-2-pentanol with PBr₃. Name the compound, assign (R) or (S) stereochemistry, and tell whether the reaction of the alcohol results in the same stereochemistry or a change in stereochemistry (reddish brown = Br).

The ball and stick model of an alkyl bromide that comprises a five-carbon chain joined by single bonds. Black, gray, and red spheres represent carbon, hydrogen, and bromine, respectively.

Mechanism Problems

Problem 10-17

In light of the fact that tertiary alkyl halides undergo spontaneous dissociation to yield a carbocation plus halide ion (see Problem 10-41), propose a mechanism for the following reaction.

The hydrolysis reaction of t-butyl bromide at 50 degrees Celsius forms t-butyl alcohol and hydrogen bromide.

Naming Alkyl Halides

Problem 10-18

Name the following alkyl halides:

(a)

A seven-carbon alkyl chain with a CH3 group on C2 and C6 positions and a Br group on C4 and C5 positions.

(b)

A six-carbon alkene chain with alkene group on C2 and iodine group on C5.

(c)

A six-carbon alkyl chain with Br and CH3 on the C2 position, Cl on the C4 position, and CH3 on the C5 position.

(d)

A six-carbon alkyl chain with a C H 2 Br group on C3 position.

(e)

Problem 10-19

Draw structures corresponding to the following IUPAC names:

(a)

2,3-Dichloro-4-methylhexane

(b)

4-Bromo-4-ethyl-2-methylhexane

(c)

3-Iodo-2,2,4,4-tetramethylpentane

(d)

cis-1-Bromo-2-ethylcyclopentane

Problem 10-20

Draw and name all the monochlorination products you might obtain from radical chlorination of the following compounds. Which of the products are chiral? Are any of the products optically active?

(a)

2-methylbutane

(b)

methylcyclopropane

(c)

2,2-dimethylpentane

Synthesizing Alkyl Halides

Problem 10-21

How would you prepare the following compounds, starting with cyclopentene and any other reagents needed?

(a)

Chlorocyclopentane

(b)

Methylcyclopentane

(c)

3-Bromocyclopentene

(d)

Cyclopentanol

(e)

Cyclopentylcyclopentane

(f)

1,3-Cyclopentadiene

Problem 10-22

Predict the product(s) of the following reactions:

(a)

Cyclohexane with CH3 and OH groups on the C1 position reacts with HBr and ether to form an unknown product represented by a question mark.

(b)

(c)

Cyclohexene fused with cyclohexane reacts with NBS, light, and carbon tetrachloride to form an unknown product represented by a question mark.

(d)

Cyclohexanol reacts with phosphorus tribromide and ether to form an unknown product represented by a question mark.

(e)

(f)

(g)

Problem 10-23

A chemist requires a large amount of 1-bromo-2-pentene as starting material for a synthesis and decides to carry out an NBS allylic bromination reaction. What is wrong with the following synthesis plan? What side products would form in addition to the desired product?

Problem 10-24

What product(s) would you expect from the reaction of 1-methylcyclohexene with NBS? Would you use this reaction as part of a synthesis?

1-methylyclohexene reacts with N-bromosuccinimide, light, and carbon tetrachloride to form an unknown product represented by a question mark.

Problem 10-25

What product(s) would you expect from the reaction of 1,4-hexadiene with NBS? What is the structure of the most stable radical intermediate?

Problem 10-26

What product would you expect from the reaction of 1-phenyl-2-butene with NBS? Explain.

Oxidation and Reduction

Problem 10-27

Rank the compounds in each of the following series in order of increasing oxidation level:

(a)

The chemical structures of 2-butene, 1-butene, butanal, and butanoic acid.

(b)

The chemical structures of propylamine, 1-bromopropane, 1-chloropropan-2-one, and 1-bromo-3-chloropropane.

Problem 10-28

Which of the following compounds have the same oxidation level, and which have different levels?

Five compounds, numbered one through five: one is 2-ethyloxirane, two is but-2-en-1-ol, three is but-3-en-2-one, four is tetrahydrofuran, five is 1,3-butadiene.

Problem 10-29

Tell whether each of the following reactions is an oxidation, a reduction, or neither:

(a)

Ethanol reacts with chromium trioxide to form methanaldehyde.

(b)

But-3-en-2-one reacts with ammonia form 4-aminobutan-2-one.

(c)

2-bromobutane reacts first with magnesium and then with water to form butane.

General Problems

Problem 10-30

Arrange the following radicals from most stable to least stable.

(a)

(b)

Three structures of cyclohexene radical with the radical located on C3, C1, and C4 positions, respectively.

(c)

Three structures of isopentane radical with the radical on the C3, C2, and C1 positions, respectively.

Problem 10-31

Alkylbenzenes such as toluene (methylbenzene) react with NBS to give products in which bromine substitution has occurred at the position next to the aromatic ring (the benzylic position). Explain, based on the bond dissociation energies in Table 6.3.

Toluene reacts with N-bromosuccinimide, light, and carbon tetrachloride to form benzyl bromide.

Problem 10-32

Draw resonance structures for the benzyl radical, C₆H₅CH₂·, the intermediate produced in the NBS bromination reaction of toluene (Problem 10-31).

Problem 10-33

Draw resonance structures for the following species:

(a)

(b)

A cyclopentadiene ring with a lone pair and a negative charge on C5.

(c)

Problem 10-34

(S)-3-Methylhexane undergoes radical bromination to yield optically inactive 3-bromo-3-methylhexane as the major product. Is the product chiral? What conclusions can you draw about the radical intermediate?

Problem 10-35

Assume that you have carried out a radical chlorination reaction on (R)-2-chloropentane and have isolated (in low yield) 2,4-dichloropentane. How many stereoisomers of the product are formed, and in what ratio? Are any of the isomers optically active? (See Problem 10-34.)

Problem 10-36

How would you carry out the following syntheses?

Cyclohexene, cyclohexanol, and cyclohexane eacg react with unknown reagents represented by question marks to form n-butylcyclohexane.

Problem 10-37

The syntheses shown here are unlikely to occur as written. What is wrong with each?

(a)

(b)

1-methyl-2-methylidenecyclohexane reacts with N-bromosuccinimide, light, and carbon tetrachloride to form 3-bromo-1-methyl-2-methylidenecyclohexane.

(c)

Fluorocyclohexane reacts with lithium dimethylcopper to form toluene.

Problem 10-38

Why do you suppose it’s not possible to prepare a Grignard reagent from a bromo alcohol such as 4-bromo-1-pentanol? Give another example of a molecule that is unlikely to form a Grignard reagent.

4-bromo-1-pentanol does not react with magnesium to form 4-magnesium bromide pentanol.

Problem 10-39

Addition of HBr to a double bond with an ether (−OR) substituent occurs regiospecifically to give a product in which the −Br and −OR are bonded to the same carbon. Draw the two possible carbocation intermediates in this electrophilic addition reaction, and explain using resonance why the observed product is formed.

1-methoxycyclohexene reacts with hydrogen bromide to form 1-bromo-1-methoxycyclohexane.

Problem 10-40

Identify the reagents a–c in the following scheme:

Ethylidene cyclohexane reacts with reagent a to form 1-cyclohexylethanol. The formed product reacts with reagent b to form (1-bromoethyl)cyclohexane which further reacts with reagent c to form isopropylcyclohexane.

Problem 10-41

Tertiary alkyl halides, R₃CX, undergo spontaneous dissociation to yield a carbocation, R₃C⁺, plus halide ion. Which do you think reacts faster, (CH₃)₃CBr or H₂C $\text{=}$ CHC(CH₃)₂Br? Explain.

Problem 10-42

Carboxylic acids (RCO₂H; pK_a ≈ 5) are approximately 10¹¹ times more acidic than alcohols (ROH; pK_a ≈ 16). In other words, a carboxylate ion (RCO₂⁻) is more stable than an alkoxide ion (RO⁻). Explain, using resonance.

Problem 10-43

How might you use a Suzuki–Miyaura reaction to prepare the biaryl compounds below? In each case, show the two potential reaction partners.

(a)

Diphenyl with methoxy groups on C5 and C4', and C O O C H 3 on C2.

(b)

Diphenyl with N H 2 on C3 and a methyl group on C3'.

(c)

Diphenyl with methoxy groups on C3, C4, and C5 and an O C H 2 O group connecting C3' and C4'

Problem 10-44

The relative rate of radical bromination is 1 : 82 : 1640 for 1° : 2° : 3° hydrogens, respectively. Draw all of the monobrominated products that you might obtain from the radical bromination of the compounds below. Calculate the relative percentage of each.

(a)

methylcyclobutane

(b)

3,3-dimethylpentane

(c)

3-methylpentane

Problem 10-45

Choose the alcohol from each pair below that would react faster with HX to form the corresponding alkyl halide.

(a)