Organic Chemistry : Reduction Reactions
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Example Questions
Example Question #2 : Reactions
Acetaldehyde 
oxidized . . . carboxylic acid
reduced . . . alkane
reduced . . . alcohol
oxidized . . . amide
reduced . . . alkane
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 #1 : Reduction Reactions
What reagent(s) will successfully complete the synthesis reaction shown above?
Methyl-MgCl; hydronium ions
Isopropyl-MgBr; hydronium ions
Ethyl-MgI; hydronium ions
N-propyl-MgBr; hydronium ions
N-propyl-MgBr; hydronium ions
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.
Example Question #2 : Reduction Reactions
The reaction of a Grignard reagent with ethylene oxide (oxirane) followed by work-up with dilute acid gives which of the following products?
Ethanol
A primary alcohol
A secondary alcohol
A tertiary alcohol
No alcohol
A primary alcohol
The reaction of a Grignard reagent with oxirane (a type of epoxide) in addition to the work-up with dilute acid will yield a primary alcohol solely because there is a work-up with dilute acid. This shows that there is an excess of hydrogen, which will yield to a primary alcohol versus a secondary or tertiary alcohol.
Example Question #3 : Reduction Reactions
Predict the major product of the given Grignard reaction.
I
IV
II
None of these
III
I
Grignard reagents (often formed in-situ due to their highly non-specific reactivity) act as reducing agents by forming carbanions, which are strong bases and nucleophiles. Nitriles may react with Grignard reagents to form imines. The reaction proceeds in an analogous fashion to a standard nucleophilic carbonyl addition, converting the triple bond to a double bond by forming an intermediate in which nitrogen carries a negative charge. A protic solvent, such as ethanol, is then added to neutralize the intermediate. Thus, the correct answer is the only compound in which an imine is formed, which is found in compound I.
Example Question #3 : Reduction Reactions
What is the product of the reaction shown?
II
III
IV
I
I
Step 1 converts the carboxylic acid into an ester.
Step 2 adds two equivalents of Grignard reagent: one to turn the molecule into acetone, and a second one to turn it into tert-butoxide. There is no way to stop the reaction after the first addition of Grignard reagent.
Step 3 will neutralize the base, leaving only t-butyl alcohol (I).
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