Which of the following set of reagents, would provide the greatest yield of (I)?

Question

BlackTom AI · Accepted Answer

Question restatement: The prompt asks which set of reagents would provide the greatest yield of the target compound (I). The answer key indicates that the correct choice is b.

Option a: This set shows an aryl system undergoing what appears to be an acylation using an acid chloride in the presence of a Lewis acid. If this option were to proceed, issues could arise such as competing side reactions or insufficient activation depending on the exact acyl donor and conditions. In some cases, steric or electronic effects could lower yield relative to the optimal choice.

Option b: This set depicts an aryl system reacting with an acyl chloride (likely acetyl chloride) in the presence of FeCl3, which is a classic Friedel–Crafts acylation setup. FeCl3 activates the acyl chloride, generating a strong electrophile that can efficiently acylate the aromatic ring. This pathway is typically high-yielding when the acyl chloride is simple and the ring is suitably activated, provided the reaction conditions are well-controlled to minimize polyacylation or overreaction.

Option c: This option seems to pair the aryl substrate with a different acyl source or possibly a di-chloro or multi-substituted acyl donor, along with FeCl3. Depending on the exact structure, this could lead to lower selectivity or sterically hindered acetylation, reducing overall yield compared to the simpler acetylation in option b.

Option d: This setup shows a different reagent combination, possibly involving hydrogen chloride or a non-activated carbonyl source. Without the correct electrophile being efficiently generated, Friedel–Crafts acylation would be less favorable, leading to a poorer yield compared to option b.

Why option b is the most likely to give the greatest yield: In Friedel–Crafts acylation, the combination of a simple acyl chloride (such as acetyl chloride) with a Lewis acid catalyst (FeCl3) typically provides a clean, high-yielding electrophilic acylation on an activated aromatic ring. The acetyl group is not overly bulky, and FeCl3 efficiently generates the acylium ion to attack the arene. Other options may use less favorable acyl donors, introduce competing side reactions, or suffer from deactivation or over-reaction, all of which can lower yield.

Potential misconceptions and why they’re incorrect:
- If one assumes acetylation will always give the highest yield regardless of reagents, that ignores the need for a suitable electrophile and catalyst system; some combinations may be less active or cause polyacylation, lowering yield.
- If a reagent set uses a less activated or more hindered acyl donor, the rate of acylation may drop, or regioselectivity may suffer, reducing overall yield.
- If the reaction conditions promote poly-substitution or rearrangements, the desired mono-acylated product yield will drop, making other reagent combinations comparatively better or worse depending on their propensity for side reactions.

In summary, option b aligns with the classic, efficient Friedel–Crafts acylation mechanism using a simple acyl chloride and FeCl3, which is why it is indicated as providing the greatest yield for the target (I).

My LMS Subjects Quiz 2

Which of the following set of reagents, would provide the greatest yield of (I)?

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