GRE Subject Test: Chemistry : Alkyne Chemistry

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

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Example Questions

Example Question #1 : Alkyne Chemistry

What is the best reagent for abstracting a hydrogen from ethyne?

Possible Answers:

None of these

Correct answer:

Explanation:

The triple bond in ethyne makes the hydrogens slightly more acidic than those found in ethane. A very strong base, such as the conjugate base of ammonia, would be able to abstract that hydrogen. The abstraction turns the base into ammonia. It also creates a carbanion that can be used for chain extension and alkyne synthesis.

Example Question #2 : Alkyne Chemistry

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What is the product of the compound when it reacts with two equivalents of base?

Possible Answers:

None of these

2-pentene

1-pentyne

1-pentene

2-pentyne

Correct answer:

2-pentyne

Explanation:

For each equivalent of base, a pi bond is formed between the carbons initially bound to the bromine atoms. For each bond formed, a bromine leaving group leaves the hydrocarbon. One equivalent of base abstracts a hydrogen. The electrons from the bond to the hydrogen create a pi bond. This occurs twice, and a triple bond is formed. The result is a 5-carbon chain with a triple bond between the second and third carbons. This molecule is 2-pentyne.

Example Question #1 : Alkyne Chemistry

Starting with an alkyne, synthesis of a cis alkene is driven upon addition of which of the following reagents?

Possible Answers:

Correct answer:

Explanation:

Reduction of an alkyne with hydrogen and Lindlar's catalyst will result in a cis alkene. While  is a reducing agent, when added to an alkyne, a trans alkene is formed. Potassium permanganate is an oxidizing agent and thus will not reduce the triple bond. The Grignard reagent is used to add organic substituents onto carbonyls. Addition of one equivalent of chlorine in carbon tetrachloride solvent yields a trans alkene; addition of a second equivalent of chlorine yields a tetrachloro alkane.

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