All GRE Subject Test: Chemistry Resources
Example Question #2 : Hydrocarbons
What is the best reagent for abstracting a hydrogen from ethyne?
None of these
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 #1 : Alkyne Chemistry
What is the product of the compound when it reacts with two equivalents of base?
None of these
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 : Help With Alkyne Reactions
Starting with an alkyne, synthesis of a cis alkene is driven upon addition of which of the following reagents?
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.