15.
Which of the options listed below show the most appropriate set of reagents to carry out
the following reaction?
a. i. EtMgBr, ii. HCl
b. i. HCl, ii. EtMgBr
c. i. C2H5Li, ii. HCl
d. i. (C2H5)2CuLi, ii. HCl
Most appropriate reagents for Grignard addition to an α,β‑unsaturated aldehyde
Understanding the reaction
The starting compound is an α,β‑unsaturated aldehyde (like propenal), and the product is a secondary alcohol bearing an extra ethyl group and a terminal chloro group. Grignard reagents add to aldehydes to give alcohols after acidic work‑up. The reagent sequence must therefore first introduce the chloro group, then use the Grignard reagent to form the alcohol.
Why option (b) is correct
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Step i: HCl
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The double bond of the α,β‑unsaturated aldehyde undergoes electrophilic addition of HCl.
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H⁺ adds to the β‑carbon and Cl⁻ to the terminal carbon, producing a 3‑chloro aldehyde without disturbing the carbonyl group.
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Step ii: EtMgBr
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Ethylmagnesium bromide (a Grignard reagent) attacks the aldehyde carbonyl carbon, forming a magnesium alkoxide.
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On aqueous work‑up (the implicit final step), protonation of the alkoxide gives the observed secondary alcohol (–CH(OH)–) while retaining the –CH₂Cl group.
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Thus, HCl creates the required chloro‑substituted aldehyde, and EtMgBr converts that aldehyde into the target secondary alcohol, matching the product structure.
Why the other options are wrong
Option (a): i. EtMgBr, ii. HCl
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If EtMgBr is added first to the α,β‑unsaturated aldehyde, it attacks the carbonyl carbon, giving a secondary allylic alcohol after acidic work‑up, not a chloro‑substituted alcohol.
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Subsequent treatment with HCl would protonate the OH and may cause substitution or dehydration, but would not selectively give a 3‑chloro‑substituted alcohol as shown. Therefore this sequence cannot furnish the required product.
Option (c): i. C₂H₅Li, ii. HCl
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Ethyllithium is an organolithium reagent that behaves similarly to a very reactive Grignard, adding to the carbonyl carbon of the α,β‑unsaturated aldehyde to give an alcohol after work‑up.
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However, there is no source of chlorine in this sequence, so the required –CH₂Cl group in the product cannot form.
Option (d): i. (C₂H₅)₂CuLi, ii. HCl
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(C₂H₅)₂CuLi is a Gilman reagent (organocuprate), which preferentially undergoes 1,4‑ (conjugate) addition to α,β‑unsaturated carbonyls, not simple 1,2‑addition to the carbonyl carbon.
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Conjugate addition of C₂H₅ to the β‑carbon gives a saturated aldehyde with the ethyl group at the β‑position; subsequent HCl treatment might protonate or add elsewhere, but it will not produce the specific 3‑chloro‑2‑alkanol shown. Hence this sequence is inconsistent with the product.
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SEO‑friendly introduction:
In many organic chemistry exams, questions ask for the most appropriate set of reagents to transform an α,β‑unsaturated aldehyde into a specific alcohol using Grignard chemistry. Understanding how HCl, Grignard reagents like EtMgBr, organolithium, and organocuprate reagents behave toward conjugated carbonyl systems is essential for choosing the correct reagent sequence and avoiding common pitfalls.
