Organic Chemistry : Help with Substitution Reactions

Study concepts, example questions & explanations for Organic Chemistry

varsity tutors app store varsity tutors android store

Example Questions

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 #126 : Organic Chemistry

Question 8

Which of these molecules would have a faster SN2 reaction?

Possible Answers:

They would have the same rate. 

1-bromobutane

1-iodobutane

Neither would react. 

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 #127 : Organic Chemistry

Question 10

Which of these would undergo a faster SN2 reaction?

Possible Answers:

Neither would undergo the reaction.

1-bromopropane

2-bromobutane

They would react at the same rate. 

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 #128 : Organic Chemistry

Question 10

What is the rate law for this equation? 

Possible Answers:

rate = k[Nucleophile]2

rate = k[R-Leaving Group]2

rate = k[R-Leaving Group]

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

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. 

Learning Tools by Varsity Tutors