Question

In Chapters 8 and 17 we learn two reactions for the synthesis of the alcohol shown. (a) Show a mechanism for each of the reactions. (b) If you were designing a synthetic route, which would be considered more sustainable? Consider all factors. [Assume the starting organic molecules are equally green.]

Accepted Answer

Step 1: Analyze the reaction from Chapter 8, which involves oxymercuration-demercuration. The starting alkene undergoes oxymercuration with mercuric acetate (Hg(OAc)₂) and water, forming a mercurinium ion intermediate. This intermediate is then attacked by water, leading to the formation of an organomercury alcohol. Finally, sodium borohydride (NaBH₄) is used to reduce the organomercury compound, replacing the mercury with a hydrogen atom, resulting in the alcohol. Step 2: Examine the reaction from Chapter 17, which involves the Grignard reaction. The starting ketone reacts with methylmagnesium bromide (MeMgBr) in tetrahydrofuran (THF) to form a tertiary alcohol. The Grignard reagent acts as a nucleophile, attacking the electrophilic carbonyl carbon of the ketone, forming a new carbon-carbon bond. The reaction is quenched with an acid, typically H₃O⁺, to protonate the alkoxide intermediate, yielding the alcohol. Step 3: Compare the sustainability of both synthetic routes. Consider factors such as the use of toxic reagents, energy consumption, waste generation, and atom economy. The oxymercuration-demercuration method uses mercury, which is toxic and poses environmental hazards. The Grignard reaction, while requiring careful handling of reagents, does not involve toxic metals and may be considered more sustainable. Step 4: Evaluate the atom economy of each reaction. Atom economy is a measure of how well a reaction utilizes its reactants to form the desired product. The Grignard reaction typically has a higher atom economy because it directly forms the alcohol without the need for additional reduction steps. Step 5: Consider the overall efficiency and practicality of each method in a synthetic route. The Grignard reaction is often favored for its straightforward mechanism and ability to form carbon-carbon bonds, which is crucial in organic synthesis. However, the choice of method may depend on specific reaction conditions and the availability of reagents.

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Concetti chiave

Reaction Mechanisms

Sustainability in Organic Synthesis

Green Chemistry Principles