Given below are two statements: Statement I : Ethyl pent-4-yn-oate on reaction with CH\(_3\)MgBr gives a 3\(^\circ\)-alcohol. Statement II : In this reaction one mole of ethyl pent-4-yn-oate utilizes two moles of CH\(_3\)MgBr. In the light of the above statements, choose the most appropriate answer from the options given below :
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Always look for acidic protons first when dealing with Grignard reagents! Functional groups like –OH, –NH, –SH, terminal alkynes (–C\(\equiv\)CH), and –COOH will react in an acid-base manner before any nucleophilic attack occurs. Each acidic proton consumes one equivalent of the Grignard reagent.
Step 1: Understanding the Question:
We need to analyze the reaction of ethyl pent-4-yn-oate with a Grignard reagent (CH\(_3\)MgBr) and evaluate the correctness of two statements regarding the product and stoichiometry. Step 2: Analysis of the Reactant and Reagent:
- Reactant: Ethyl pent-4-yn-oate, HC\(\equiv\)C–CH\(_2\)–CH\(_2\)–COOEt. This molecule has two sites that can react with a Grignard reagent:
1. The acidic proton of the terminal alkyne (–C\(\equiv\)C–H).
2. The electrophilic carbonyl carbon of the ester group (–COOEt).
- Reagent: CH\(_3\)MgBr is a strong nucleophile and a strong base. Step 3: The Reaction Sequence:
1. Acid-Base Reaction: The first reaction that occurs is the fastest one: the acid-base reaction. The Grignard reagent (base) deprotonates the terminal alkyne (acid). This consumes one equivalent of CH\(_3\)MgBr.
HC\(\equiv\)C–R + CH\(_3\)MgBr \(\rightarrow\) MgBr\(^+\)\(^{-}\)C\(\equiv\)C–R + CH\(_4\) \(\uparrow\)
2. Nucleophilic Acyl Substitution/Addition: After the acid-base reaction is complete, the Grignard reagent acts as a nucleophile and attacks the ester carbonyl group. This reaction with an ester consumes two equivalents of the Grignard reagent to produce a tertiary alcohol. The first equivalent adds to the carbonyl, eliminating the ethoxide group to form a ketone intermediate. The second equivalent immediately adds to this highly reactive ketone.
R'–COOEt + 2 CH\(_3\)MgBr \(\rightarrow\) R'–C(CH\(_3\))\(_{2}\)–OMgBr (after 2 additions)
Upon acidic workup (H\(_3\)O\(^+\)), this gives the tertiary alcohol, R'–C(CH\(_3\))\(_{2}\)–OH. Step 4: Evaluating the Statements:
- Statement I: Ethyl pent-4-yn-oate on reaction with CH\(_3\)MgBr gives a 3\(^\circ\)-alcohol.
The reaction at the ester group indeed converts it to a tertiary alcohol (specifically, 2-methylhex-5-yn-2-ol after workup). So, Statement I is true.
- Statement II: In this reaction one mole of ethyl pent-4-yn-oate utilizes two moles of CH\(_3\)MgBr.
The molecule utilizes a total of 1 (for the alkyne proton) + 2 (for the ester group) = three moles of CH\(_3\)MgBr. Therefore, the statement that it utilizes two moles is false. Step 5: Final Answer:
Statement I is true but Statement II is false.
