Organic Chemistry Elementary Steps General Chemistry Reaction Types

Experiment 19: Reactivities of Some Alkyl Halides Overview Use this document to record your observations, analyze the results, and reflect on the mechanisms of the substitution reactions observed in Experiment 19. Some of the questions in this document will refer to the elementary steps in organic reactions as well as the reaction types that were taught in general chemistry. You may refer to the lists below. Table 1. Common elementary steps for organic compounds and inorganic reaction types.

Organic Chemistry Elementary Steps General Chemistry Reaction Types

• Coordination

• Heterolysis

• Bimolecular nucleophilic substitution

• Bimolecular elimination

• Nucleophilic addition

• Nucleophile elimination

• Electrophilic addition

• Electrophile elimination

• Carbocation Rearrangement

• Proton Transfer

• Precipitation

• Single Replacement

• Double Replacement

• Acid/Base neutralization

• Combustion

• Redox

• Synthesis

• Decomposition

Part A. Sodium Iodide in Acetone Introduction

1) Before we analyze the data associated with Experiment 19, Part A, let’s be sure that we understand the chemical reactions being observed. Although we will not be testing alkyl bromide X, shown below, it would be expected to undergo substitution under the reaction conditions of the experiment.

a. First, X would undergo an “organic chemistry elementary step”. On the figure below, draw the curved arrows for this step, provide the structures of all products (organic and inorganic) in the box, and name the elementary step.

b. One of products of step 1 undergoes a reaction type that you considered in general chemistry. On the figure below, name the type of reaction and provide the identity of the white powder that would be observed had we studied alkyl halide X in the experiment.

Figure 1. How alkyl bromide X would react under the conditions of Exp. 19, Part A.

 

Br

NaI

acetone

“organic” chemistry elementary step 1:

“general” chemistry reaction type:

product(s) of step 1 white precipitate alkyl bromide X

a) b)

 

 

2) If a reaction occurs for an alkyl halide in Part A, it will be a (circle one): SN1 or SN2 reaction? How do

each of the following factors affect whether or not (or how fast) a reaction will occur in Exp. 19, Part A? a. Solvent (same for all alkyl halides in Exp 19, Part A)

 

b. Nucleophile (same for all alkyl halides in Exp 19, Part A)

c. Structure of the alkyl halide (will vary in Exp 19, Part A)

d. Leaving group (may vary in Exp 19, Part A)

Results 3) Now we are ready to consider the alkyl halides that were tested in Exp 19, Part A.

a. Draw the bond-line structures of each alkyl halide tested (note: crotyl chloride, tube 6, was omitted).

b. **Read the procedure**, view the video of the experiment and record the results. Be sure to include the results at room temperature AND elevated temperature when appropriate.

c. For the alkyl halides that reacted, draw the bond-line structures of the major organic product. d. Finally, for each tube (regardless of whether or not a reaction occurred), explain the results.

Your analysis should be based on structure, the mechanism of the organic reaction occurring (or not occurring) and the factors that affect the rate law of that reaction type. You may find words and drawings are helpful for a complete explanation.

Tube #

Bond-line Structure of Alkyl Halide

Result(s) Bond-line Structure of Product (or “NA” if no reaction)

Analysis ( = an Explanation, based on structure)

1

 

 

2

3

4

5

 

 

7

8

9

10

 

Reflect 4) Refer back to Figure 1. Would we have been able to analyze whether or not the organic reaction

occurred without the “general chemistry reaction” (part b)? Why or why not? (Answer in one sentence.)

 

 

5) Consider crotyl chloride, shown below. a. Draw a complete curved arrow mechanism for how crotyl chloride could react under the

conditions of this experiment. b. Provide the structures of all products (organic and inorganic). c. Then, predict: would you expect to observe a reaction for crotyl chloride under these

conditions? Explain your answer in one sentence.

 

6) For each pair of reactions: draw the structure of the substitution product, circle the reaction that would proceed with the fastest rate (if they have the same rate, circle “same”), and explain your answer in one sentence, referring to structure, mechanism, and rate law.

 

 

Cl

NaI

acetone

all products (organic and inorganic)

crotyl chloride

complete curved-arrow mechanism leading to the proposed products:

Prediction (select one): A reaction would NOT be observed at any temperature. A reaction would only be observed at elevated temperature. A reaction would be observed at BOTH room and elevated temperatures

Explanation for prediction:

Cl

NaI

acetone

NaCN

acetone

I I

NaCN

acetone

same

explanation:

explanation:

Br

NaI

acetone same

a)

b)

 

 

Part B. Silver Nitrate in Ethanol Introduction

1) Before we analyze the data associated with Experiment 19, Part B, let’s be sure that we understand the chemical reactions being observed. Although we will not be testing 1-bromo-1-methylcyclopentane, shown below, it would be expected to undergo substitution under the reaction conditions of the experiment.

a. First, the alkyl bromide would undergo an “organic chemistry elementary step.” On the figure below, draw the curved arrows for this step, provide the structures of all intermediates (organic and inorganic) and name the elementary step.

b. One of products of step 1 undergoes a reaction type that you considered in general chemistry class. On the figure below, name the type of reaction and provide the identity of the white powder that would be observed had we studied this alkyl halide in the experiment.

c. The other product of step 1 continues to undergo a series of “organic chemistry elementary steps.” Draw the complete curved-arrow mechanism for these steps, naming each one (see: Organic Chemistry Elementary Steps). Circle the final organic product of this reaction.

Figure 2. How 1-bromo-1-methylcyclopentane would react under the conditions of Exp. 19, Part B.

 

Br

“general” chemistry reaction type:

a)

EtOH, AgNO3(EtOH)

intermediate(s)

“organic” chemistry elementary step 1:

white precipitate

b)

EtOH, AgNO3(EtOH)

the rest of the mechanism (label each elementary step with its name and circle the final organic product):

c)

 

 

2) If a reaction occurs for an alkyl halide in Part B, it will be a (circle one): SN1 or SN2 reaction? How do each of the following factors affect whether or not (or how fast) a reaction will occur in Exp. 19, Part B?

a. Solvent (same for all alkyl halides in Exp 19, Part B)

b. Nucleophile (same for all alkyl halides in Exp 19, Part B)

c. Structure of the alkyl halide (will vary in Exp 19, Part B)

d. Leaving group (may vary in Exp 19, Part B)

Results 3) Now we are ready to consider the alkyl halides that were tested in Exp 19, Part B.

a. Draw the bond-line structures of each alkyl halide tested (note: crotyl chloride, tube 6, was omitted).

b. **Read the procedure**, view the video of the experiment and record the results. Be sure to include the results at room temperature AND elevated temperature when appropriate.

c. For the alkyl halides that reacted, draw the bond-line structures of the major organic product. d. Finally, for each tube (regardless of whether or not a reaction occurred), explain the results.

Your analysis should be based on structure, the mechanism of the organic reaction occurring (or not occurring) and the factors that affect the rate law of that reaction type. You may find words and drawings are helpful for a complete explanation.

Tube #

Bond-line Structure of Alkyl Halide

Result(s) Bond-line Structure of Product (or “NA” if no reaction)

Analysis ( = an Explanation, based on structure)

1

 

 

2

3

4

5

 

 

7

8

9

10

Reflect 4) Refer back to Figure 1. Would we have been able to analyze whether or not the organic reaction

occurred without the “general chemistry reaction” (part b)? Why or why not? (Answer in one sentence.)

 

 

Do you expect that crotyl chloride would react under the conditions of Part B? Why or why not? (Explain your answer with clearly drawn structures, referring to the rate law of this reaction).

6) For each pair of reactions: draw the structure of the product, circle the reaction that would proceed with the fastest rate (if they have the same rate, circle “same”), and explain your answer in one sentence, referring to structure, mechanism, and rate law.

Summarize and Reflect You do not need to submit your answers to these questions, but you should take the time to answer them.

1) What did you learn in this experiment? 2) Overall, did alkyl halides undergo only one type of substitution reaction (only react in either part A or

part B) or did some alkyl halides undergo both types of substitution reactions (react in both parts A and B)? Can you describe any patterns?

3) Consider the factors that affect the rate of substitution reactions (make a list). For each factor consider if it affects the rates of SN1 and SN2 in the same or different ways (or not at all).

4) What questions do you have?

Cl

crotyl chloride

Cl

I I

same

explanation:

explanation:

Br same

a)

b)

EtOH EtOH

EtOH EtOH

5)