Organic Chemistry : Help with Alkene Reactions

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

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Example Question #1 : Alkene Chemistry

The stereochemical pathway for the hydrogenation of an alkene with a metal catalyst, such as platinum, occurs via __________.

Possible Answers:

anti-Markovnikov addition

formation of a bridged carbocation

anti addition

syn addition

Markovnikov addition

Correct answer:

syn addition

Explanation:

Hydrogenation of an alkene with a metal catalyst, such as platinum, occurs via syn addition.

It is important to note the three main types of reactions for alkenes. The first type of reaction is a 2-step mechanism in which the electrophile attacks the carbocation nucleophile. This can yield syn or anti products. The second type of reaction is a 2-step mechanism that forms a bridged carbocation as the intermediate. This can yield only anti products. The third and last type of reaction is a 1-step addition. This can only yield syn products.

An example of the third type of reaction is the addition of a hydrogen with palladium, platinum, or nickel as demonstrated in the picture.

1

Example Question #2 : Alkene Chemistry

What is(are) the product(s) in the Pd-catalyzed hydrogenation if 1,2-dimethylcyclopentene?

Possible Answers:

A mixture of trans- and cis-1,2-dimethylcyclopentane

Dimethylcyclopentane

Trans-1,2-dimethylcyclopentane

Cis-1,2-dimethylcyclopentane

1,1-dimethylcyclopentene

Correct answer:

Cis-1,2-dimethylcyclopentane

Explanation:

The product for this hydrogenation is cis-1,2-dimethylcyclopentane.

It is important to note the three main types of reactions for alkenes. The first type of reaction is a 2-step mechanism in which the electrophile attacks the carbocation nucleophile. This can yield syn or anti products. The second type of reaction is a 2-step mechanism that forms a bridged carbocation as the intermediate. This can yield only anti products. The third and last type of reaction is a 1-step addition. This can only yield syn products.

The cis product alone forms because the reagents used were hydrogen and a metal catalyst palladium (other common metal catalysts are platinum and nickel). This type of reagent with an alkene will always be a 1-step addition that yields solely syn products. Cis-1,2-dimethylcyclopentane is the only answer that solely indicates syn products.

Example Question #3 : Alkene Chemistry

Nbs

What reagent will complete this reaction?

Possible Answers:

Br2 / light

SOBr2

PBr3

N-bromosuccinimide (NBS) / light

None of these

Correct answer:

N-bromosuccinimide (NBS) / light

Explanation:

N-bromosuccinimide (NBS) brominates at allylic positions. Br2 will not complete this reaction with the presence of the double bond. 

Example Question #1 : Help With Alkene Reactions

Screen shot 2015 07 08 at 8.37.25 am

Predict the major product of the given reaction.

Possible Answers:

IV

III

I

II

None of these

Correct answer:

II

Explanation:

Reaction of an alkene with osmium tetroxide  results in syn addition of to hydroxyl groups across the double bond, resulting in a 1,2-diol. The correct answer is the option in which hydroxide groups are added to the previously double-bonded carbons on the same side of the ring. The correct answer is compound II.

Example Question #2 : Help With Alkene Reactions

 reacts with the given molecule to form what final product?

Possible Answers:

Correct answer:

Explanation:

This is an alkene electrophilic addition reaction. The alkene first attacks the hydrogen on the . The hydrogen always attaches to the last carbon, never the third. This form of regioselectivity is referred to as the Markovnikov rule. This occurs because it creates a secondary carbocation, rather than a primary carbocation intermediate, which is far more stable. The bromine is then free to attack the third, now positively charged carbon.

Example Question #3 : Help With Alkene Reactions

What is the major product when 4-bromo-2-pentene reacts with ?

1. Img 0643

2. Img 0644  1

3. Img 0645

Possible Answers:

Two of these

Correct answer:

Two of these

Explanation:

When reacting with alkenes, the reagents  add two syn alcohols, meaning that the alcohols face the same way in space. The alcohols can either be configured in the front (wedge) or the back (dash). Therefore, the correct answer is two of these, with the correct products being molecule 1 and molecule 2.

Example Question #4 : Help With Alkene Reactions

Each of the following reagents, or sets of reagents, elicit Markovnikov addition to an alkene except __________.

Possible Answers:

Correct answer:

Explanation:

 are the only reagents that do not elicit Markovnikov addition of the available answer choices. Instead,  elicits an anti-Markovnikov addition of hydrogen and bromine to an alkene.

Example Question #5 : Help With Alkene Reactions

Which of the following show the major product of the addition of hydrobromic acid to 2-methylhex-2-ene?

Possible Answers:

Noname04

Noname06

Noname03

Noname02

Noname05

Correct answer:

Noname06

Explanation:

This is an addition reaction with  being added to an alkene (ionic addition). To reach the correct answer first we need to add the  to one of the carbons by using electrons from the double bond. This next step is crucial because the positive charge (carbocation) that forms needs to be in the most substituted position. However, hydride shifts can occur moving a hydrogen atom over to an adjacent carbon to form the most stable carbocation. In this case, the positive charge on the secondary carbon adjacent to the tertiary carbon will first get the positive charge and then a hydride shift will occur amongst those two carbon leading to a more stable tertiary carbocation. Finally, the  will attack the carbocation from both sides, and then we will have an alkyl halide. The most stable intermediate (carbocation) produces the greatest quantity of the stable product. 

Example Question #6 : Help With Alkene Reactions

When reacted with 1 equivalent of , which of these molecules could form multiple products?

Screen shot 2015 10 24 at 10.11.48 am

Possible Answers:

2 and 4

1, 4 and 5

2, 3 and 4

1 and 3

1 and 4

Correct answer:

2, 3 and 4

Explanation:

This is a classic acid catalyzed alkene reaction. Because of resonance, several reactions have multiple products.

Screen shot 2015 10 24 at 10.11.30 am

Example Question #7 : Help With Alkene Reactions

Which of these alkene compounds would react with hydrobromic acid  the fastest?

Possible Answers:

Ethene

2-Methyl-2-butene

1-Butene

1-Pentene

2-Butene

Correct answer:

2-Methyl-2-butene

Explanation:

2-Methyl-2-butene will react the fastest with hydrobromic acid because when it reacts, it will form the most stable carbocation among all of the alkenes listed. This stability allows for the reaction to proceed much quicker than other compounds, therefore it will react the fastest.

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