GRE Subject Test: Chemistry : Aldehyde Chemistry

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

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Example Questions

Example Question #1 : Reactions With Ketones And Aldehydes

Acetaldehyde  undergoes a Wolf-Kishner reaction, which is the addition of hydrazine  with subsequent addition of a base and heat. In this reaction, the aldehyde is __________, resulting in a(n) __________ product.

Possible Answers:

oxidized . . . amide

oxidized . . . carboxylic acid

reduced . . . alcohol

reduced . . . alkane

Correct answer:

reduced . . . alkane

Explanation:

The correct answer is that the aldehyde is reduced to an alkane. In viewing the final product, we see that acetaldehyde would be reduced to ethane. The reaction of any aldehyde or ketone with hydrazine and the subsequent addition of base and heat will result in that aldehyde or ketone being reduced to an alkane, and is referred to as the Wolf-Kishner reaction. The Wolf-Kishner reagent is a commonly tested reducing agent.

Example Question #16 : Molecular Properties

Grinard

What reagent(s) will successfully complete the synthesis reaction shown above?

Possible Answers:

Methyl-MgCl; hydronium ions

N-propyl-MgBr; hydronium ions

Isopropyl-MgBr; hydronium ions

Ethyl-MgI; hydronium ions

Correct answer:

N-propyl-MgBr; hydronium ions

Explanation:

This is an example of a Grignard reagent reaction. Because we are adding three carbons to our chain, the Grignard reagent we need must have three carbons on it. We can therefore rule out methyl grignard and ethyl grignard.

N-propyl is the straight-chained 3-carbon alkane, while isopropyl is branched. Looking at our final product, we can see the carbon chain we have added is straight-chained, and thus N-propyl Grignard is the best option. Because Grignard reagents are relatively basic, we must add an hydronium ion workup to protonate our alcohol.  

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