Textbook Question
Show two ways you could synthesize each of the following secondary alcohol by adding an appropriate Grignard reagent to an aldehyde.
(c)
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13. Alcohols and Carbonyl Compounds
Grignard Reaction
3:21 minutesProblem 39d
Textbook Question
Show how you would use Grignard syntheses to prepare the following alcohol from the indicated starting material and any other necessary reagents.
(d) 2-cyclohexylethanol from bromocyclohexane
Verified step by step guidance1
Step 1: Begin by preparing the Grignard reagent. React bromocyclohexane with magnesium (Mg) in anhydrous ether to form cyclohexylmagnesium bromide. This reaction involves the insertion of magnesium into the carbon-bromine bond.
Step 2: Identify the electrophile needed to react with the Grignard reagent to form the desired alcohol. In this case, you need an aldehyde or ketone that will lead to 2-cyclohexylethanol upon reaction. Ethylene oxide (an epoxide) is a suitable choice because it will open up to form a primary alcohol with the Grignard reagent.
Step 3: React the Grignard reagent (cyclohexylmagnesium bromide) with ethylene oxide in anhydrous ether. The nucleophilic attack of the Grignard reagent on the epoxide ring opens the ring, forming a magnesium alkoxide intermediate.
Step 4: Perform an acidic workup by adding dilute acid (e.g., HCl or H2SO4) to the reaction mixture. This step protonates the alkoxide intermediate, converting it into the desired alcohol, 2-cyclohexylethanol.
Step 5: Purify the product by standard organic chemistry techniques, such as extraction, drying, and distillation, to isolate 2-cyclohexylethanol in its pure form.
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3mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Grignard Reagents
Grignard reagents are organomagnesium compounds, typically represented as R-MgX, where R is an organic group and X is a halogen. They are highly reactive and can act as nucleophiles, allowing them to react with electrophiles such as carbonyl compounds. In the synthesis of alcohols, Grignard reagents can add to carbonyl groups, leading to the formation of alcohols after hydrolysis.
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Carbonation of Grignard Reagents
Nucleophilic Addition
Nucleophilic addition is a fundamental reaction mechanism in organic chemistry where a nucleophile attacks an electrophilic carbon atom, typically found in carbonyl groups. This process is crucial in the formation of alcohols from carbonyl compounds, as the nucleophile (like a Grignard reagent) adds to the carbonyl carbon, resulting in an alkoxide intermediate that can be protonated to yield the final alcohol product.
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Synthesis of Alcohols
The synthesis of alcohols can be achieved through various methods, including the reduction of carbonyl compounds or the reaction of Grignard reagents with carbonyls. In the case of preparing 2-cyclohexylethanol from bromocyclohexane, the Grignard reagent formed from bromocyclohexane can react with an appropriate carbonyl compound, followed by hydrolysis, to yield the desired alcohol.
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