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Example Questions
Example Question #1 : Alkene Chemistry
The stereochemical pathway for the hydrogenation of an alkene with a metal catalyst, such as platinum, occurs via __________.
anti-Markovnikov addition
anti addition
syn addition
formation of a bridged carbocation
Markovnikov addition
syn addition
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.
Example Question #1 : Organic Chemistry
What is(are) the product(s) in the Pd-catalyzed hydrogenation if 1,2-dimethylcyclopentene?
1,1-dimethylcyclopentene
Trans-1,2-dimethylcyclopentane
Dimethylcyclopentane
Cis-1,2-dimethylcyclopentane
A mixture of trans- and cis-1,2-dimethylcyclopentane
Cis-1,2-dimethylcyclopentane
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 #1 : Organic Chemistry
What reagent will complete this reaction?
None of these
PBr3
N-bromosuccinimide (NBS) / light
SOBr2
Br2 / light
N-bromosuccinimide (NBS) / light
N-bromosuccinimide (NBS) brominates at allylic positions. Br2 will not complete this reaction with the presence of the double bond.
Example Question #1 : Hydrocarbon Reactants
Predict the major product of the given reaction.
IV
I
III
None of these
II
II
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 #1 : Reactions By Reactant
reacts with the given molecule to form what final product?
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.
2.
3.
Two of these
Two of these
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 #1 : Reactions By Reactant
Each of the following reagents, or sets of reagents, elicit Markovnikov addition to an alkene except __________.
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?
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 #3 : Help With Alkene Reactions
When reacted with 1 equivalent of , which of these molecules could form multiple products?
1 and 3
2 and 4
2, 3 and 4
1, 4 and 5
1 and 4
2, 3 and 4
This is a classic acid catalyzed alkene reaction. Because of resonance, several reactions have multiple products.
Example Question #11 : Hydrocarbon Reactants
Which of these alkene compounds would react with hydrobromic acid the fastest?
Ethene
1-Butene
1-Pentene
2-Butene
2-Methyl-2-butene
2-Methyl-2-butene
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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