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Example Questions
Example Question #31 : Reactions Types
What is the product of the reaction shown?
None of these
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?
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
Which of these molecules would have a faster SN2 reaction?
1-bromobutane
They would have the same rate.
Neither would react.
1-iodobutane
1-iodobutane
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
Which of these would undergo a faster SN2 reaction?
2-bromobutane
They would react at the same rate.
1-bromopropane
Neither would undergo the reaction.
1-bromopropane
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
What is the rate law for this equation?
rate = k[Nucleophile]2
rate = k[Nucleophile][R-Leaving Group]
rate = k[R-Leaving Group]
rate = k[R-Leaving Group]2
rate = k[R-Leaving Group]
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?
Weak nucleophile
Weak base
Strong base
Aprotic solvent
Strong nucleophile
Weak base
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?
Weak base
Strong electrophile
Weak nucleophile
Strong base
Strong nucleophile
Strong base
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 __________.
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
two sigma bonds are broken, and one sigma and one pi bond are formed
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?
Tertiary substrate
Formation of a carbocation intermediate
Weak base
Unimolecular reaction kinetics
Aprotic solvent
Aprotic solvent
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?
Strong base
Bimolecular reaction kinetics
Two-step mechanism
Aprotic solvent
Tertiary substrate
Two-step mechanism
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.