GRE Subject Test: Chemistry : Alkene Chemistry

Study concepts, example questions & explanations for GRE Subject Test: Chemistry

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Example Questions

Example Question #4 : Functional Group Reactions

Untitled

What is the IUPAC name of the given diene?

Possible Answers:

None of these answers

5-chloro-3,5-dimethyl-1,6-heptadiene

5-chloro-3,6-dimethyl-1,5-heptadiene

3-chloro-2,5-dimethyl-2,6-heptadiene

3-chloro-2,5-dimethyl-1,5-heptadiene

Correct answer:

5-chloro-3,6-dimethyl-1,5-heptadiene

Explanation:

You must begin counting the carbons so that the first functional substituent has the lowest possible number. In this case, C1 is connected to C2 by the double bond, meaning we start counting from the left.

The longest carbon chain is seven carbons so the parent molecule is heptane. With this numbering, there are methyl groups on carbons 3 and 6 and a chlorine on carbon 5.

Substituents are named in alphabetical order and two double bonds result in a diene. Thus, the correct answer is 5-chloro-3,6-dimethyl-1,5-heptadiene.

Example Question #1 : Help With Alkene Reactions

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

Possible Answers:

syn addition

anti-Markovnikov addition

Markovnikov addition

formation of a bridged carbocation

anti 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 #4 : Hydrocarbons

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

Possible Answers:

1,1-dimethylcyclopentene

Trans-1,2-dimethylcyclopentane

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

Cis-1,2-dimethylcyclopentane

Dimethylcyclopentane

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 #5 : Hydrocarbons

Nbs

What reagent will complete this reaction?

Possible Answers:

PBr3

None of these

SOBr2

Br2 / light

N-bromosuccinimide (NBS) / light

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. 

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