Organic Chemistry : Reactions by Reactant

Study concepts, example questions & explanations for Organic Chemistry

varsity tutors app store varsity tutors android store

Example Questions

← Previous 1 3 4 5 6 7

Example Question #1 : Reactions By Reactant

Which of the following can reduce an alkene to an alkane?

Possible Answers:

Lithium aluminum hydride (LiAlH4)

H2/Raney nickel

H2/Pd

Lithium aluminum hydride (LiAlH4) and H2/Pd

H2/Pd and H2/Raney nickel

Correct answer:

H2/Pd and H2/Raney nickel

Explanation:

Neither lithium aluminum hydride, nor sodium borohydride will reduce C–C double bonds.

H2/Raney nickel and H2/Pd can each (individually) reduce an alkene to an alkane. Since both H2/Raney nickel and H2/Pd can reduce the alkene, the answer is both of those reagents. This is a catalytic hydrogenation reaction, and H2/Raney nickel not only reduces C–C double bonds, but also carbonyl compounds.

Example Question #1 : Organic Chemistry

Identify the major organic product expected from the acid-catalyzed dehydration of 2-methyl-2-pentanol.

Possible Answers:

None of the other answers

3-methyl-1-pentene

2-methyl-2-pentene

cis-3-methyl-2-pentene

2-methyl-1-pentene

Correct answer:

2-methyl-2-pentene

Explanation:

The initial compound is a five-carbon alkane chain with methyl and hydroxy groups on the second carbon. Dehydration involves the hydrogenation of the hydroxy group. That group then leaves, and a double bond is formed. Zaitsev's rule states that double bonds are more stable on more highly substituted carbons. The double bond forms across carbons two and three.

Example Question #1 : Hydrocarbon Reactants

If an alkane is reacted with excess  in a high temperature environment, what are the products formed?

Possible Answers:

 and 

 and 

 and 

 and 

Correct answer:

 and 

Explanation:

Whenever an alkane is reacted with excess oxygen in a high temperature reaction, the reaction is known as combustion. Combustion of an alkane always produces carbon dioxide gas and water vapor.

Therefore the correct answer is:  and .

Example Question #1 : Hydrocarbon Reactants

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

Possible Answers:

Markovnikov addition

anti-Markovnikov addition

anti addition

formation of a bridged carbocation

syn 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 : Organic Chemistry

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

Possible Answers:

Trans-1,2-dimethylcyclopentane

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

Cis-1,2-dimethylcyclopentane

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 #531 : Organic Chemistry

Nbs

What reagent will complete this reaction?

Possible Answers:

N-bromosuccinimide (NBS) / light

Br2 / light

None of these

SOBr2

PBr3

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 : Hydrocarbon Reactants

Screen shot 2015 07 08 at 8.37.25 am

Predict the major product of the given reaction.

Possible Answers:

II

I

None of these

IV

III

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 #1 : Reactions By Reactant

 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 #1 : Reactions By Reactant

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 #531 : Organic Chemistry

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.

← Previous 1 3 4 5 6 7
Learning Tools by Varsity Tutors