Organic Chemistry : Help with Grignard Reactions

Study concepts, example questions & explanations for Organic Chemistry

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Example Questions

Example Question #11 : Specific Reactions And Named Reactions

Grinard

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

Possible Answers:

Ethyl-MgI; hydronium ions

Methyl-MgCl; hydronium ions

N-propyl-MgBr; hydronium ions

Isopropyl-MgBr; 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.  

Example Question #11 : Specific Reactions And Named Reactions

The reaction of a Grignard reagent with ethylene oxide (oxirane) followed by work-up with dilute acid gives which of the following products?

Possible Answers:

A secondary alcohol

No alcohol

Ethanol

A tertiary alcohol

A primary alcohol

Correct answer:

A primary alcohol

Explanation:

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 #1 : Reduction Reactions

Screen shot 2015 07 19 at 10.47.03 am

Predict the major product of the given Grignard reaction.

Possible Answers:

None of these

IV

III

II

I

Correct answer:

I

Explanation:

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 #1 : Help With Grignard Reactions

What is the product of the reaction shown?

Screen shot 2015 11 13 at 2.18.46 pm

Screen shot 2015 11 13 at 2.18.40 pm

Possible Answers:

IV

II

III

I

Correct answer:

I

Explanation:

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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