Help with Elimination Reactions

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Organic Chemistry › Help with Elimination Reactions

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1

Screen shot 2016 02 21 at 6.39.46 am

Classify the type of reaction given.

Elimination

Rearrangement

Substitution

Addition

Explanation

An elimination reaction occurs when there is a release of atoms in a given compound to produce two or more products. In the reaction given a hydrogen and chloride atom are eliminated from the original compound to form hydrochloric acid and ethylene.

2

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

Weak base

Strong base

Weak nucleophile

Strong nucleophile

Aprotic solvent

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.

3

What is the major product in the following reaction?

Image2

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.

4

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

III

I

II

IV

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.

5

Which of the following conditions favors an E2 mechanism?

Strong base

Weak base

Strong nucleophile

Weak nucleophile

Strong electrophile

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.

6

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

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.

7

Chloropropane reacts with to form propene.

Through which mechanism did this reaction occur?

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.

8

Screen shot 2016 02 21 at 7.20.05 am

Classify the type of reaction given.

Elimination

Addition

Catalysis

Homolytic

Explanation

An elimination reaction occurs when there is a release of atoms in a given compound to produce two or more products. In the reaction given a hydrogen and chloride atom are eliminated from the original compound to form one 2-butene, potassium chloride and water molecule.

9

Screen shot 2016 02 21 at 7.20.17 am

Classify the type of reaction given.

Heterolytic bond breaking

Homolytic bond breaking

Addition

Substitution

Explanation

Heterolytic bond breaking occurs in polar compounds to form to products of opposite charges. In these types of reaction two electrons from the original bond stays with one fragment upon cleavage. In the reaction given, the bond between the hydrogen and chlorine atom is broken with the two electrons from the original bond staying with the chlorine atom. The resulting products are hydrogen ion and chloride ion.

10

In an elimination reaction __________.

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

one sigma bond is 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

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.

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