GRE Subject Test: Chemistry : Alkane Chemistry

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

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Example Questions

Example Question #1 : Help With Alkane Synthesis

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What is the IUPAC name of the given molecule?

Possible Answers:

3,3,5-trimethylnonane

None of these

4,6-dimethyl-6-ethylpentane

2,2,4-trimethyloctane

3,3,5-trimethyloctane

Correct answer:

3,3,5-trimethyloctane

Explanation:

The longest carbon chain that can be formed is eight carbons. The base molecule is octane.

Using IUPAC rules, substituents should have the lowest possible numbers; thus, we start counting carbons from the right side rather than the left. If you count from the correct side, there are two methyl groups on carbon 3 and one on carbon 5. Thus, the name of the moleculue is 3,3,5-trimethyloctane.

Example Question #1 : Hydrocarbon Products

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How could you brominate the compound?

Possible Answers:

None of these

Bromine and peroxides

Hydrobromic acid

Bromine gas

Bromine and UV light

Correct answer:

Bromine and UV light

Explanation:

The given molecule is an alkane. The only way to brominate an alkane is with bromine gas and UV light. The energy from the light serves to creat two radical bromines. These radicals are capable of bonding with alkanes. If the given compound were an alkene, either hydrobromic acid or bromine and peroxides would work.

Example Question #2 : Alkane Chemistry

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

Possible Answers:

Lithium aluminum hydride (LiAlH4)

H2/Raney nickel

H2/Pd and H2/Raney nickel

Lithium aluminum hydride (LiAlH4) and H2/Pd

H2/Pd

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 #3 : Alkane Chemistry

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

Possible Answers:

2-methyl-1-pentene

cis-3-methyl-2-pentene

2-methyl-2-pentene

3-methyl-1-pentene

None of the other answers

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.

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