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Solutions to Problems Alkyl Halides

Building Bridges to Knowledge

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1. Suggest syntheses for the following from the indicated starting material and any necessary inorganic and/or organic materials.

(a) 2-bromopropane from 1-propanol

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(b) 1-bomopropane from 1-propanol

(c)

(d)

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(e) methyl propyl ether from propene

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(f) 2-methylpentane from propene as the only source of organic material

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2. The reaction of 3-bromo-2,2-dimethylbutane with ethanol exhibits the properties of a first order reaction in which the rate of the reaction depends on the concentration of 3-bromo-2,2-dimethylbutane and is independent of the concentration of ethanol. The major product of the reaction is 2-ethoxy-2,3-dimethylbutane. In addition, 2,3-dimethyl-2-butene, 2,3-dimethyl-1-butene, and 3,3-dimethyl-1-butene are produced as minor products. Suggest a mechanism that would be compatible with these observed data.

1.

2.

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3.

2-ethoxy-2,3-dimethylbutane

Minor products

3,3-dimethyl-1-butene

2,3-dimethyl-2-butene

2,3-dimethyl-1-butene

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3. If 2-bromo-3,3-dimethylbutane is treated with sodium ethoxide, the only product isolated is 3,3-dimethyl-1-butane. Suggest a rationale for this observation. Write a rate expression that would account for this observation.

The only hydrogen atoms that are available for abstraction are located on the methyl group of the carbon atom to the right of the carbon atom containing the bromine atom. Therefore, elimination must take place on that carbon atom anti to the bromine atom:

Let

then

4. Arrange the following alcohols in order of reactivity toward HBr.

2-butanol, 1-bromo-2-butanol, 1,1-dibromo-2-butanol, 1,1,1-tribromo-2-butanol

rate = k RBr[ ] CH3CH2O-Na+⎡⎣ ⎤⎦

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1,1,1-tribromo-2-butanol>1,1-dibromo-2-butanol>1-bromo-2-butanol>2-butanol

5. Suggest the major product anticipated in each of the following reactions.

(a)

(b)

(c)

No reaction

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(d)

(e)

(f)

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6. The rate of the reaction between 2-methyl-2-butanol with HCl is unimolecular. Predict the product or products of the reaction, and write the series of elementary steps that rationalize the formation of product.

1.

2.

3.

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7. The rate of the reaction of 1-pentanol with HI is bimolecular. Predict the product of the reaction, and write the series of elementary steps that rationalize the formation of product.

where X = I

8. For the following reaction:

Where x + y = 1

Suggest structures for A and B

Suggest the series of elementary steps that would rationalize the formation of A and B.

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1.

2.

3.

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9. Write a series of elementary steps that rationalize the formation of the designated products.

(a)

1.

2.

3.

secondary carbocation tertiary carbocation

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4.

(b)

1.

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2.

3.

4.

(c)

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1.

2.

3.

4.

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Formation of the five-member ring structure:

1.

2.

3.

4.

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10.

(a) Describe the conditions that would favor substitution nucleophilic bimolecular reactions.

Primary alkyl halides, strong nucleophiles, and protic solvents favor substitution bimolecular reactions.

(b) Describe the conditions that would favor substitution nucleophilic unimolecular reactions.

Tertiary alkyl halides, weak nucleophiles, and protic solvents with charged reactants favor substitution nucleophilic unimolecular reactions.

(c) Describe the conditions that would favor elimination bimolecular reactions.

Tertiary alkyl halides, strong nucleophiles, and aprotic solvents in charged reactants favor elimination bimolecular reactions.

(d) Describe the conditions that would favor elimination unimolecular reactions.

Tertiary alkyl halides, weak nucleopliles, and protic solvents favor substitution nucleophilic unimolecular reactions.

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11. Predict the products for the following chemical reactions;

(a)

(b)

12. The following reaction was observed:

In addition, other alkenes are produced.

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Suggest a mechanism for the formation of the two indicated alkenes

1.

2.

3.

4.

or

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13. Suggest a synthesis for bromocyclohexane from 1,3-butadiene and any other necessary inorganic and organic material.

14. Suggest syntheses for the following from butane as the only organic starting material and any necessary inorganic material.

(a) n-octane

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(b) 1-bromobutane

(c) 2-bromobutane

(d) 3-methylheptane

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15. Suggest the product expected in the following reaction, and suggest a mechanism for the reaction.

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16. Suggest the major product for the reaction of the indicated materials with 1-iodobutane.

(a) NaOH

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(b) Cold sulfuric acid

(c) hot concentrated sulfuric acid

(d) Li, then copper (I) iodide, then ethyl bromide

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(e) Mg/ether, then deuterium oxide

(f) Sodium bromide in acetone

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17. 3,3-Dimethyl-1-butene reacts with hydrogen iodide to produce 3-iodo-2,2-dimethylbutane and 2-iodo-2,3-dimethylbutane. Give a rational for the formation of these two products.

3-iodo-2,2-dimethylbutane

1,2-methide shift:

2-iodo-2,3-dimethylbutane

18. The following reaction proceeds slowly. Give a rationale for this observation.

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19. Write structural formulas for

(a) 2,6-dibromo-4-methyl-4-octanol

(b) 1-bromo-2-cyclobutylethane

(c) trans-4-tert-butylbromocyclohexane

(d) 2-bromo-2-cyclopentylpropane

(e) 5-bromo-2,3-dimethylhexane

20.Methyl bromide depletes the ozone in the atmosphere; therefore, its use in agriculture as a soil fumigant is no longer desirable. Methyl bromide can be prepared industrially from methanol and hydrogen bromide. Following are hypothetical kinetic data for this reaction at

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fifty degrees centigrade.

[Methanol],moles/liters

[HBr],moles/liters Time,seconds

0.10 0.10 0 0.050 0.050 60 0.025 0.025 177 0.0125 0.0125 412

(a)

Use Microsoft Excel to determine if the reaction follows a unimolecular nucleophilic substitution reaction or a bimolecular nucleophilic substitution reaction.

A plot of x/[ao(a

o -x)] versus times gives a straight line. This indicates

that the reaction is second order, i.e., first order in methanol and first order in HBr.

x/[ao(a

o -x)] time (second)

0 0 10 60 30 177 66 412

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The equation for the line is

Therefore, a plot of

gives a straight line with a slope equal to 0.16 L mole-1 s-1, and the intercept equals 0.

Note that the units are 1/Ms or M-1 s-1 or L mol-1 s-1. The units are obtained from the change in concentration (M/M2 = 1/M) divided by the change in time (seconds).

y"="0.1617x"

0"

10"

20"

30"

40"

50"

60"

70"

0" 50" 100" 150" 200" 250" 300" 350" 400" 450"

x/[a(a&x)])

*me,)second)

xao (ao -x)

= kt

xao (ao -x)

versus t

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(b) Determine the rate constant for the reaction.

The rate constant is the slope of the line; therefore, k is

0.16 L mole-1 s-1.

(c) Write the series of elementary steps that would account for the formation of the soil fumigant, methyl bromide as a product.

Where X is Br