Organic Chemistry : Reactions Types

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #31 : Reactions Types

What is the product of the reaction shown?


Image8

Possible Answers:

None of these

Correct answer:

Explanation:

The reaction shown is a nucleophilic substitution reaction. The molecule shown is a primary alkyl bromide, and the nucleophile that will be used is the iodide anion (). In this type of reaction the iodide will displace the bromide on the organic molecule, generating iodoethane ().

Example Question #32 : Reactions Types

What is the product of the reaction shown?

Image10

Possible Answers:

Correct answer:

Explanation:

This reaction is an example of a nucleophilic substitution on a secondary alkyl halide. The nucleophile in this case is cyanide (), and the atom that attacks in the cyanide ion is the carbon. Because cyanide is a good nucleophile, the reaction will occur via  mechanism. The answer is thus the molecule where the bromide is replaced with cyanide.

Note: when nucleophilic substitution is performed using a nucleophile that contains carbon (Grignard reagents, acetylide reagents, cyanide, etc.) it is often easy to incorrectly count the number of carbons in the final product.

Example Question #32 : Reactions Types

Question 8

Which of these molecules would have a faster SN2 reaction?

Possible Answers:

1-bromobutane

They would have the same rate. 

Neither would react. 

1-iodobutane

Correct answer:

1-iodobutane

Explanation:

 is a better leaving group because it is a larger atom and thus has its negative charge spread more evenly. It is more stable and a weaker base than . The molecule with the leaving group that would be most stable, or the weakest base, is the molecule that would react fastest in an SN2 reaction. 

Example Question #33 : Reactions Types

Question 10

Which of these would undergo a faster SN2 reaction?

Possible Answers:

2-bromobutane

They would react at the same rate. 

1-bromopropane

Neither would undergo the reaction.

Correct answer:

1-bromopropane

Explanation:

There would be more steric hindrance for a nucleophile in the 2-bromobutane because the leaving group, bromine, is located on a secondary carbon atom. A secondary carbon atom is attached to two other carbon atoms. The leaving group on the 1-propane is located on a primary carbon atom, which is attached to only one other carbon atom. 

Example Question #34 : Reactions Types

Question 10

What is the rate law for this equation? 

Possible Answers:

rate = k[Nucleophile]2

rate = k[Nucleophile][R-Leaving Group]

rate = k[R-Leaving Group]

rate = k[R-Leaving Group]2

Correct answer:

rate = k[R-Leaving Group]

Explanation:

This reaction would occur using an SN1 mechanism because the leaving group is attached to a tertiary carbon, a carbon atom with three of the carbon atoms attached to it. The rate laws for SN1 mechanisms do not depend on the nucleophile concentration. The slow-step occurs unimolecularly within the molecule with the leaving group. 

Example Question #1 : Help With Elimination Reactions

Which of the following reaction conditions favors an E1 reaction mechanism?

Possible Answers:

Weak nucleophile

Weak base

Strong base

Aprotic solvent

Strong nucleophile

Correct answer:

Weak base

Explanation:

E1 reactions occur in two steps. First, the leaving group is removed, yielding a carbocation. Second, a weak base removes a proton from the carbon adjacent to the carbocation carbon. Thus, to favor E1, a protic solvent is desired in order to stabilize the carbocation. Weak bases favor an E1 mechanism.

Example Question #1 : Help With Elimination Reactions

Which of the following conditions favors an E2 mechanism?

Possible Answers:

Weak base

Strong electrophile

Weak nucleophile

Strong base

Strong nucleophile

Correct answer:

Strong base

Explanation:

E2 reaction occur in only one step. The strong base removes the beta-hydrogen, while the leaving group simultaneously leaves. The double bond forms simultaneously. 

Example Question #2 : Help With Elimination Reactions

In an elimination reaction __________.

Possible Answers:

two pi bonds are broken and one sigma bond is formed

one sigma bond and one pi bond are broken, and two sigma bonds are formed

one pi bond is broken and one pi bond is formed

two sigma bonds are broken, and one sigma and one pi bond are formed

one sigma bond is broken and one sigma bond is formed

Correct answer:

two sigma bonds are broken, and one sigma and one pi bond are formed

Explanation:

Elimination reactions involve the use of bases, which remove hydrogen atoms. The leaving group, which is bound via a sigma bond is removed, along with a hydrogen (thus two sigma bonds are broken). The result is a double bond, which consists of one sigma bond and one pi bond. 

Example Question #4 : Help With Elimination Reactions

Which of the following characteristics does not reflect an E1 reaction mechanism?

Possible Answers:

Tertiary substrate

Formation of a carbocation intermediate

Weak base

Unimolecular reaction kinetics

Aprotic solvent

Correct answer:

Aprotic solvent

Explanation:

E1 reactions occur in two steps, forming a carbocation intermediate, which is most stable if there is a protic solvent present. Furthermore, the use of a weak base favors E1.

Example Question #3 : Help With Elimination Reactions

Which of the following characteristics does not reflect an E2 reaction mechanism?

Possible Answers:

Strong base

Bimolecular reaction kinetics

Two-step mechanism

Aprotic solvent

Tertiary substrate

Correct answer:

Two-step mechanism

Explanation:

E2 reactions occur in one step; thus no carbocation intermediate is formed, and an aprotic solvent is favored. E2 reactions are favored by strong bases and higher temperatures.

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