Organic Chemistry 2 Quiz: Organometallic Additions Grignard Organolithium
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Organometallic Additions Grignard OrganolithiumQuestion 1 of 14

During the reaction of acetone with methylmagnesium bromide (CH₃MgBr) in anhydrous ether, a key intermediate is formed. Which structure represents this intermediate that exists in the solution immediately before the addition of aqueous acid?

The magnesium salt of tert-butoxide, (CH₃)₃C-O⁻ MgBr⁺
The final product, tert-butanol, (CH₃)₃C-OH
A radical anion intermediate, (CH₃)₂C(•)-O⁻
A four-membered ring including Mg, Br, O, and the carbonyl carbon.
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Organic Chemistry 2 Quiz

Organic Chemistry 2 Quiz: Organometallic Additions Grignard Organolithium

Practice Organometallic Additions Grignard Organolithium in Organic Chemistry 2 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

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This quiz focuses on Organometallic Additions Grignard Organolithium, giving you a quick way to practice the rules, question types, and explanations that matter most for Organic Chemistry 2.

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Question 1

During the reaction of acetone with methylmagnesium bromide (CH₃MgBr) in anhydrous ether, a key intermediate is formed. Which structure represents this intermediate that exists in the solution immediately before the addition of aqueous acid?

  1. The magnesium salt of tert-butoxide, (CH₃)₃C-O⁻ MgBr⁺ (correct answer)
  2. The final product, tert-butanol, (CH₃)₃C-OH
  3. A radical anion intermediate, (CH₃)₂C(•)-O⁻
  4. A four-membered ring including Mg, Br, O, and the carbonyl carbon.
Explanation: The nucleophilic carbon of the Grignard reagent attacks the electrophilic carbonyl carbon of acetone, pushing the pi electrons onto the oxygen. This forms a tetrahedral intermediate which is an alkoxide. In the ether solution, this alkoxide is stabilized as a magnesium salt, specifically a halomagnesium alkoxide. The final alcohol is not formed until the proton source (aqueous acid) is added in a separate workup step.

Question 2

When optically pure (R)-3-methyl-2-pentanone is treated with excess ethylmagnesium bromide followed by an aqueous workup, a new stereocenter is created at the carbonyl carbon. Which statement best describes the resulting product mixture?

  1. A single, optically active stereoisomer is formed.
  2. A racemic mixture (equal amounts of two enantiomers) is formed.
  3. A mixture of diastereomers is formed in unequal amounts. (correct answer)
  4. A single, achiral meso compound is formed.
Explanation: The starting ketone is chiral. The nucleophilic attack of the Grignard reagent can occur from either the Re or Si face of the prochiral carbonyl, creating a new stereocenter. Because the existing stereocenter creates a chiral environment, the transition states for attack from the two faces are diastereomeric and have different energies. This results in a mixture of two diastereomeric products, formed in an unequal ratio.

Question 3

A student attempts to synthesize 4-phenyl-2-butanol from 4-phenyl-2-butanone using methylmagnesium bromide (CH₃MgBr). Prior to the reaction, the ketone is purified via column chromatography using a solvent system containing a trace amount of water. The collected fractions are immediately used. The reaction fails to yield any significant amount of the desired alcohol. What is the most likely cause for this failure?

  1. Trace water from the chromatography quenched the Grignard reagent through an acid-base reaction. (correct answer)
  2. The Grignard reagent acted as a base, causing the ketone to enolize rather than undergo nucleophilic addition.
  3. The silica gel from the column remained in the solution and catalyzed the decomposition of the Grignard reagent.
  4. Methylmagnesium bromide is not nucleophilic enough to react efficiently with a sterically hindered ketone like 4-phenyl-2-butanone.
Explanation: Grignard reagents are extremely strong bases and are highly sensitive to protic substances. Any trace amount of water (pKa ≈ 15.7) will rapidly and irreversibly react with the Grignard reagent (the conjugate acid of which is methane, pKa ≈ 50) in an acid-base reaction, destroying the nucleophile before it can react with the ketone.

Question 4

A sample of benzaldehyde is specially prepared such that its carbonyl oxygen is isotopically labeled with ¹⁸O. This labeled compound is treated with excess methylmagnesium bromide, followed by a workup with normal water (H₂¹⁶O). What is the location of the ¹⁸O label in the resulting 1-phenylethanol product?

  1. The ¹⁸O label is completely lost and replaced by ¹⁶O from the water during workup.
  2. The ¹⁸O label is found exclusively in the hydroxyl group of the 1-phenylethanol. (correct answer)
  3. The ¹⁸O label becomes part of the magnesium oxide byproduct of the reaction.
  4. The ¹⁸O label is randomly distributed between the product's hydroxyl group and the solvent.
Explanation: In a Grignard reaction, the nucleophile adds to the carbonyl carbon, and the C=O π-bond breaks, placing a negative charge on the oxygen. This oxygen atom, which was originally the carbonyl oxygen (¹⁸O), is then protonated during the workup step. The oxygen atom itself is never exchanged. The proton comes from the water, but the oxygen atom of the resulting hydroxyl group is the same oxygen atom that was in the starting carbonyl.

Question 5

The reaction of di-tert-butyl ketone with isopropylmagnesium bromide results in a very low yield of the expected tertiary alcohol. The major product is 2,2,4,4-tetramethyl-3-pentanol. This transformation is an example of the Grignard reagent playing an alternative mechanistic role. What is that role?

  1. Acting as a Lewis acid to catalyze rearrangement.
  2. Acting as a single-electron transfer agent to form a ketyl radical.
  3. Acting as a base to cause enolization of the ketone.
  4. Acting as a hydride donor to cause reduction of the ketone. (correct answer)
Explanation: When a sterically hindered ketone reacts with a Grignard reagent that possesses β-hydrogens (like isopropylmagnesium bromide), nucleophilic addition can be slow. A competing pathway, reduction, can become dominant. The Grignard reagent delivers a hydride (H⁻) from its β-position to the carbonyl carbon via a six-membered ring transition state, reducing the ketone to a secondary alcohol. The Grignard reagent is converted to propene.

Question 6

A student attempts to prepare a Grignard reagent from 4-bromoaniline in anhydrous THF by reacting it with magnesium turnings. The reaction does not produce the expected Grignard reagent. What is the most likely reason for this failure?

  1. The aromatic ring is electronically deactivated by the -NH₂ group, preventing insertion of magnesium.
  2. The magnesium acts as a Lewis acid and coordinates too strongly to the lone pair of the nitrogen.
  3. Any Grignard reagent that forms is immediately destroyed by the acidic N-H protons of another 4-bromoaniline molecule. (correct answer)
  4. The C-Br bond in 4-bromoaniline is too strong to react with magnesium under these conditions.
Explanation: This is an example of an incompatible functional group during the formation of a Grignard reagent. The amine (-NH₂) group has acidic protons. Grignard reagents are powerful bases. If any molecule of the 4-bromoaniline Grignard reagent were to form, it would immediately encounter another molecule of the starting material and be quenched in an acid-base reaction. This prevents any significant concentration of the desired reagent from accumulating.

Question 7

Both methyllithium and methylmagnesium bromide are effective for adding a methyl group to cyclohexanone. However, in which of the following reactions would methyllithium be expected to give a significantly different major product or reaction outcome compared to methylmagnesium bromide?

  1. Reaction with benzaldehyde to form 1-phenylethanol.
  2. Reaction with ethyl acetate to form tert-butanol.
  3. Reaction with 1-hexyne. (correct answer)
  4. Reaction with propylene oxide.
Explanation: The primary difference between organolithium and Grignard reagents in an introductory context is basicity; organolithiums are substantially more basic. A terminal alkyne like 1-hexyne has a weakly acidic proton (pKa ≈ 25). While a Grignard reagent can deprotonate it, the reaction may be slow or incomplete. Methyllithium, a much stronger base, will rapidly and quantitatively deprotonate the alkyne in an acid-base reaction. In the other cases (A, B, D), both reagents act primarily as nucleophiles and give analogous addition products.

Question 8

A Grignard reagent is prepared from 1-bromo-3-methylbenzene (m-bromotoluene) and magnesium metal. This reagent is then poured over crushed dry ice (solid CO₂), and the resulting mixture is acidified with dilute H₂SO₄. What is the final major organic product?

  1. 3-Methylbenzaldehyde
  2. 3-Methylbenzoic acid (correct answer)
  3. 1-(m-tolyl)ethanone
  4. (3-Methylphenyl)methanol
Explanation: Grignard reagents act as strong nucleophiles and add to the electrophilic carbon of carbon dioxide. This forms a magnesium carboxylate salt. Subsequent acidification (workup) protonates the carboxylate to yield a carboxylic acid. In this case, the m-tolyl Grignard reagent forms m-toluic acid (3-methylbenzoic acid).

Question 9

In an attempt to synthesize triphenylmethanol, a student adds exactly 1.0 equivalent of phenylmagnesium bromide to a solution of ethyl benzoate. After aqueous workup, the student finds that the major product isolated is benzophenone, not the desired tertiary alcohol. What is the most plausible explanation for this result?

  1. The benzophenone intermediate is exceptionally stable and unreactive towards further Grignard addition.
  2. The stoichiometry was incorrect; two equivalents of Grignard reagent are needed to convert the ester to the tertiary alcohol. (correct answer)
  3. The ethoxide leaving group is a strong enough nucleophile to reverse the second addition step.
  4. The reaction requires a Lewis acid catalyst, which was omitted, halting the reaction at the ketone stage.
Explanation: The reaction of an ester with a Grignard reagent produces a ketone as an intermediate. This ketone is also reactive towards the Grignard reagent. To form the tertiary alcohol, two full equivalents of the Grignard reagent are required: one to convert the ester to the ketone, and a second to convert the ketone to the tertiary alcohol. Using only one equivalent will primarily result in the formation of the ketone intermediate.

Question 10

A student is planning a synthesis that uses n-butyllithium (an organolithium reagent) as a strong base and nucleophile. Which of the following would be an inappropriate choice of solvent for this reaction?

  1. Pentane
  2. Diethyl ether
  3. Ethanol (correct answer)
  4. Tetrahydrofuran (THF)
Explanation: Organolithium reagents, like Grignard reagents, are extremely strong bases. They will react destructively with any protic solvent. Ethanol (CH₃CH₂OH) has an acidic hydroxyl proton that will be readily abstracted by n-butyllithium, quenching the reagent and forming butane and lithium ethoxide. Aprotic solvents like alkanes (pentane) and ethers (diethyl ether, THF) are required.

Question 11

The reaction of 4-tert-butylcyclohexanone with methylmagnesium bromide, followed by aqueous workup, yields two separable, stereoisomeric alcohol products. Which statement correctly describes the relationship between these two products?

  1. They are enantiomers, formed by attack on a chiral starting material.
  2. They are diastereomers, resulting from nucleophilic attack on the two different faces of the carbonyl group. (correct answer)
  3. They are constitutional isomers, resulting from a skeletal rearrangement after the initial addition.
  4. They are conformers, which can be interconverted by a simple chair-flip of the cyclohexane ring.
Explanation: The large tert-butyl group effectively locks the cyclohexane ring in a single chair conformation. The carbonyl group is planar, and the methyl Grignard can attack from either the axial face or the equatorial face. Axial attack leads to an equatorial alcohol, and equatorial attack leads to an axial alcohol. These two products have the same connectivity but different 3D arrangements of atoms, and they are not mirror images of each other. Therefore, they are diastereomers.

Question 12

Which of the following reaction combinations is a valid and efficient synthesis of 3-ethyl-3-hexanol following an appropriate aqueous workup?

  1. 3-hexanone with ethylmagnesium bromide (correct answer)
  2. Ethyl pentanoate with two equivalents of methylmagnesium bromide
  3. Butanal with sec-butylmagnesium bromide
  4. 3-heptanone with methylmagnesium bromide
Explanation: The target, 3-ethyl-3-hexanol, is a tertiary alcohol with two ethyl groups and one propyl group attached to the carbinol carbon. Retrosynthetic analysis shows two possible ketone/Grignard pairs: 3-hexanone (an ethyl group and a propyl group on the carbonyl) reacting with an ethyl Grignard, or diethyl ketone (two ethyl groups on the carbonyl) reacting with a propyl Grignard. Option A matches the first valid disconnection.

Question 13

Propylene oxide (methyloxirane) is treated with phenylmagnesium bromide in THF, followed by an acidic workup. What is the major product of this regioselective reaction?

  1. 2-Phenyl-1-propanol
  2. 1-Phenyl-1-propanol
  3. Phenyl propyl ether
  4. 1-Phenyl-2-propanol (correct answer)
Explanation: Under basic or neutral conditions (like with a Grignard reagent), the nucleophile attacks an epoxide at the less sterically hindered carbon atom in an Sₙ2-like fashion. In propylene oxide, the methyl group is on C2, so the less hindered carbon is C1. Attack by the phenyl nucleophile at C1, followed by protonation of the resulting alkoxide at O, yields 1-phenyl-2-propanol.

Question 14

A chemist plans to synthesize 3-phenyl-3-pentanol. Three of the proposed routes are viable. Which one of the following synthetic routes will FAIL to produce the desired tertiary alcohol?

  1. Propiophenone (1-phenyl-1-propanone) treated with ethylmagnesium bromide, followed by workup.
  2. Ethyl benzoate treated with at least two equivalents of ethylmagnesium bromide, followed by workup.
  3. 3-Oxo-3-phenylpropanoic acid treated with excess ethylmagnesium bromide, followed by workup. (correct answer)
  4. 3-Pentanone treated with phenylmagnesium bromide, followed by workup.
Explanation: The starting material in route C, 3-oxo-3-phenylpropanoic acid, contains a highly acidic carboxylic acid proton (pKa ≈ 4-5). The strongly basic Grignard reagent will be consumed by an irreversible acid-base reaction with this proton before it can act as a nucleophile at the ketone carbonyl. The other three routes are all valid disconnections for synthesizing the target tertiary alcohol.