Organic Chemistry : Help with Elimination Reactions

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

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Example Question #1 : Help With Elimination Reactions

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

Possible Answers:

Weak nucleophile

Aprotic solvent

Strong base

Strong nucleophile

Weak base

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 #2 : Help With Elimination Reactions

Which of the following conditions favors an E2 mechanism?

Possible Answers:

Strong base

Weak nucleophile

Strong nucleophile

Weak base

Strong electrophile

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 #3 : Help With Elimination Reactions

In an elimination reaction __________.

Possible Answers:

one sigma bond is broken and one sigma bond is formed

one pi bond is broken and one pi bond is formed

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

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

two pi bonds are 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

Aprotic solvent

Weak base

Unimolecular reaction kinetics

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 #5 : Help With Elimination Reactions

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

Possible Answers:

Two-step mechanism

Aprotic solvent

Bimolecular reaction kinetics

Tertiary substrate

Strong base

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.

Example Question #41 : Reactions Types

The molecule shown reacts with . Which type of reaction will ensue?

Possible Answers:

Correct answer:

Explanation:

These conditions are perfect for an  reaction. We see a secondary halide so first instinct might be to say that an  reaction would ensue. However, anytime we have a secondary halide reacting with a strong, bulky base, the laws of sterics dictates that an  reaction will ensue.

Example Question #1 : Help With Elimination Reactions

Chloropropane reacts with   to form propene.

Through which mechanism did this reaction occur?

Possible Answers:

Correct answer:

Explanation:

The given conditions are perfect for an  reaction. Here we have a primary haloalkane reacting with a big, bulky base. Big, bulky bases such as , and alkene products are classic indicators that the reaction was an  reaction.

Example Question #65 : Organic Concepts

Which of the following hydrogens are removed upon treatment of the pictured molecule with potassium tert-butoxide?

Screen shot 2015 11 09 at 3.15.31 pm

Possible Answers:

II

I

IV

III

Correct answer:

III

Explanation:

Treatment with strong base indicates E2 mechanism. The halide has two beta-carbons (II and III), and potassium tert-butoxide is a bulky base. The favored product is the Hofmann product (less substituted alkene). Therefore III is the hydrogen that's removed. 

Example Question #66 : Organic Concepts

What is the product of the reaction shown?

 Screen shot 2015 11 14 at 10.45.18 am

Screen shot 2015 11 14 at 10.41.22 am

Possible Answers:

I and II

I only

I, II, and III

III only

II only

Correct answer:

II only

Explanation:

E2 reaction mechanism requires antiperiplanar orientation between the leaving group and the hydrogen. Given the stereochemistry of the methyl group, forming an alkene bond (to give the more substituted product) is impossible.

Example Question #2 : Help With Elimination Reactions

What is the major product in the following reaction?

Image2

Possible Answers:

Correct answer:

Explanation:

This is an acid-catalyzed dehydration of a tertiary alcohol. The first step is protonation of the alcohol oxygen which will create a good leaving group (), and subsequent loss of water from the molecule. The molecule will then contain a carbocation (which will not move due to the positive being on the most substituted carbon atom in its immediate vicinity), and the conjugate base (water in this case) will remove a hydrogen from a carbon next to the carbocation. The base will preferentially remove a hydrogen from the one that will produce the more substituted alkene, as this is generally the more stable product. This is known as Zaitsev's rule for the formation of alkenes.

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