Organic Chemistry 2 Quiz: Reaction Workup And Yield Calculations
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Reaction Workup And Yield CalculationsQuestion 1 of 20

In a Williamson ether synthesis, a student reacts 11.61 g of sodium phenoxide (MW = 116.1 g/mol) with 17.15 g of ethyl iodide (MW = 155.9 g/mol). After workup, 9.77 g of phenetole (the ether product, MW = 122.16 g/mol) is isolated. What is the percent yield of the reaction?

56.9%
70.8%
80.0%
84.1%
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Organic Chemistry 2 Quiz

Organic Chemistry 2 Quiz: Reaction Workup And Yield Calculations

Practice Reaction Workup And Yield Calculations in Organic Chemistry 2 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Reaction Workup And Yield Calculations, giving you a quick way to practice the rules, question types, and explanations that matter most for Organic Chemistry 2.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

In a Williamson ether synthesis, a student reacts 11.61 g of sodium phenoxide (MW = 116.1 g/mol) with 17.15 g of ethyl iodide (MW = 155.9 g/mol). After workup, 9.77 g of phenetole (the ether product, MW = 122.16 g/mol) is isolated. What is the percent yield of the reaction?

  1. 56.9%
  2. 70.8%
  3. 80.0% (correct answer)
  4. 84.1%
Explanation: First, find the limiting reagent. Moles sodium phenoxide = 11.61 g / 116.1 g/mol = 0.100 mol. Moles ethyl iodide = 17.15 g / 155.9 g/mol = 0.110 mol. The stoichiometry is 1:1, so sodium phenoxide is the limiting reagent. The theoretical yield is based on 0.100 mol of product. Theoretical mass = 0.100 mol × 122.16 g/mol = 12.216 g. Percent yield = (Actual Mass / Theoretical Mass) × 100 = (9.77 g / 12.216 g) × 100 ≈ 80.0%.

Question 2

Aldol: From 0.150 mol limiting reagent, 22.5 g chalcone isolated (MW 208.26); percent yield?

  1. 48.0%
  2. 72.0% (correct answer)
  3. 86.4%
  4. 104%
Explanation: This question tests intermediate organic chemistry skills, focusing on reaction workup and yield calculations. Yield calculations compare actual to theoretical mass, using limiting moles and MW. In this aldol scenario, proper isolation ensures accurate actual mass. The correct answer reflects the percent yield formula application. A common distractor involves miscalculating theoretical yield. To help students, instructors should emphasize precision in measurements. Practice percent yield problems with aldol data.

Question 3

Esterification: Which step best improves ester purity before distillation?

  1. Add boiling chips
  2. Dry organic layer with MgSO₄ (correct answer)
  3. Use excess alcohol
  4. Add more acid catalyst
Explanation: This question tests intermediate organic chemistry skills, focusing on reaction workup and yield calculations. Yield calculations benefit from high purity, achieved through proper pre-distillation steps. In this esterification scenario, drying removes water that could impure the ester. The correct answer reflects drying the layer to improve purity. A common distractor involves confusing reaction conditions with workup. To help students, instructors should emphasize purification techniques. Practice esterification labs with emphasis on purity assessment.

Question 4

Esterification: After separation, which layer is typically retained for ethyl acetate (density 0.90 g/mL)?

  1. Bottom aqueous layer
  2. Top organic layer (correct answer)
  3. Middle emulsion layer
  4. Any layer; density irrelevant
Explanation: This question tests intermediate organic chemistry skills, focusing on reaction workup and yield calculations. Yield calculations require isolating the correct layer during extraction based on density. In this esterification scenario, retaining the organic layer is key for product recovery. The correct answer reflects the lower density ester forming the top layer. A common distractor involves assuming the aqueous is retained. To help students, instructors should emphasize density in separations. Practice extraction simulations with density data.

Question 5

Nitration workup: Which step primarily removes residual strong acids from crude nitrobenzene?

  1. Wash organic layer with NaHCO₃(aq) (correct answer)
  2. Dry with MgSO₄
  3. Distill under reduced pressure
  4. Cool reaction in ice bath
Explanation: This question tests intermediate organic chemistry skills, focusing on reaction workup and yield calculations. Yield calculations depend on removing impurities like acids during workup. In this nitration scenario, washing neutralizes residuals. The correct answer reflects bicarbonate wash for acid removal. A common distractor involves confusing with drying or distillation. To help students, instructors should emphasize workup chemistry. Practice nitration workup sequences.

Question 6

Esterification: CH₃CO₂H + HOCH₂CH₃ ⇌ CH₃CO₂CH₂CH₃ + H₂O; theoretical ester from 0.200 mol acid?

  1. 8.81 g
  2. 17.6 g (correct answer)
  3. 22.0 g
  4. 35.2 g
Explanation: This question tests intermediate organic chemistry skills, focusing on reaction workup and yield calculations. Yield calculations involve computing theoretical product mass from the balanced equation and limiting reagent. In this esterification scenario, workup steps like extraction isolate the ester for measurement. The correct answer reflects using the moles of acid and MW of ester for theoretical yield. A common distractor involves incorrect MW application or ignoring the equilibrium. To help students, instructors should emphasize stoichiometry in reversible reactions. Practice theoretical yield problems for various esterifications to build confidence.

Question 7

Aldol: Acetone limiting at 0.200 mol; theoretical mass of chalcone (MW 208.26)?

  1. 20.8 g
  2. 31.2 g
  3. 41.7 g (correct answer)
  4. 52.1 g
Explanation: This question tests intermediate organic chemistry skills, focusing on reaction workup and yield calculations. Yield calculations involve theoretical mass from limiting moles and product MW. In this aldol scenario, workup isolates the condensed product for weighing. The correct answer reflects multiplication for the given limiting reagent. A common distractor involves using the wrong MW. To help students, instructors should emphasize calculation steps. Practice theoretical yields for condensation reactions.

Question 8

Grignard workup: After ether reaction, which reagent quenches excess RMgX to alcohol-compatible products?

  1. Anhydrous MgSO₄
  2. Saturated NaCl
  3. Dilute HCl(aq) (correct answer)
  4. NaHCO₃(aq)
Explanation: This question tests intermediate organic chemistry skills, focusing on reaction workup and yield calculations. Yield calculations involve accurate product isolation, which depends on proper workup to avoid losses or impurities. In this Grignard workup scenario, the steps such as quenching excess reagent are crucial for converting intermediates to the desired alcohol without side reactions. The correct answer reflects an understanding of using dilute acid to safely quench excess Grignard to compatible byproducts. A common distractor involves confusing quenching with drying or washing steps. To help students, instructors should emphasize the role of each workup reagent in Grignard reactions. Practice sequencing workup steps and explaining their purposes to enhance comprehension.

Question 9

In the workup for the reduction of a ketone with LiAlH₄, the reaction is cautiously quenched by the sequential addition of H₂O, then 15% aq. NaOH, then more H₂O (Fieser workup). What is the purpose of adding aqueous NaOH in this procedure?

  1. To ensure the final product, an alcohol, is deprotonated to its alkoxide form for easier extraction.
  2. To hydrolyze any unreacted LiAlH₄ into lithium hydroxide and aluminum hydroxide.
  3. To neutralize the acidic alcohol product, which would otherwise be difficult to isolate.
  4. To convert the aluminum salts into a granular, easily filterable solid (Al(OH)₃). (correct answer)
Explanation: When you encounter questions about LiAlH₄ workup procedures, focus on understanding what happens to the aluminum-containing species formed during the reaction. The Fieser workup is specifically designed to handle the complex mixture of aluminum salts that result from hydride reductions. During LiAlH₄ reduction, aluminum forms various alkoxide complexes with the alcohol product. When water is first added, these complexes are partially hydrolyzed, creating a mixture of aluminum salts, hydroxides, and alkoxides that form a gelatinous, difficult-to-filter mess. The key insight is that adding 15% aqueous NaOH converts all these aluminum species into aluminum hydroxide (Al(OH)₃), which precipitates as a granular, crystalline solid that filters easily and completely separates from your organic product. Looking at the wrong answers: (A) incorrectly suggests the goal is to deprotonate the alcohol product—but alcohols are actually easier to extract in their neutral form, and alkoxides would complicate workup. (B) misidentifies the timing—unreacted LiAlH₄ is destroyed by the initial water addition, not the NaOH step. (C) wrongly characterizes alcohols as acidic products needing neutralization—alcohols are actually neutral compounds that don't require acid-base treatment for isolation. The correct answer is (D): the NaOH converts aluminum salts into easily filterable Al(OH)₃ solid. Study tip: Remember that workup procedures are designed to solve practical separation problems. For LiAlH₄ reductions, the main challenge is dealing with messy aluminum salts—the Fieser workup's NaOH step specifically addresses this by creating a clean, filterable precipitate.

Question 10

A student performs an aldol condensation of acetone, which produces mesityl oxide (C₆H₁₀O) upon dehydration. Starting with 11.6 g of acetone (MW = 58.08 g/mol), the student isolates 6.50 g of purified mesityl oxide (MW = 98.14 g/mol). What is the percent yield?

  1. 33.1%
  2. 56.0%
  3. 66.2% (correct answer)
  4. 75.5%
Explanation: First, determine the stoichiometry. Two moles of acetone react to form one mole of mesityl oxide. Calculate the starting moles of acetone: 11.6 g / 58.08 g/mol = 0.200 mol. Based on the 2:1 stoichiometry, the maximum moles of mesityl oxide that can be formed is 0.200 mol / 2 = 0.100 mol. This corresponds to a theoretical yield of 0.100 mol × 98.14 g/mol = 9.814 g. The percent yield is (Actual / Theoretical) × 100 = (6.50 g / 9.814 g) × 100 ≈ 66.2%. A common mistake is to assume a 1:1 ratio, which would give half the correct yield.

Question 11

A student prepares a Grignard reagent from bromobenzene and then reacts it with excess acetone. After an acidic workup, the isolated crude product weighs 125% of the theoretical yield. Which of the following is the most plausible explanation for this observation?

  1. The student misidentified the limiting reagent, leading to an erroneously low theoretical yield calculation.
  2. The Grignard reagent reacted with atmospheric CO₂ to form a byproduct, increasing the final mass.
  3. The reaction is highly exothermic, which causes a systematic error in favor of product formation.
  4. The crude product is contaminated with residual solvent (e.g., diethyl ether or water) from the workup. (correct answer)
Explanation: When you encounter a question about yields exceeding 100%, you're dealing with experimental error rather than miraculous chemistry. This scenario tests your understanding of practical laboratory considerations in Grignard reactions. The correct answer is D because crude products commonly retain solvents from the reaction and workup process. Grignard reactions typically use anhydrous diethyl ether as solvent, and the acidic workup involves aqueous solutions. Even after standard isolation procedures, the crude product often contains residual ether, water, or other solvents that haven't been completely removed. This extra mass makes the apparent yield exceed the theoretical value based on pure product alone. Let's examine why the other options don't explain yields over 100%. Option A involves calculation errors that would affect your computed theoretical yield, but wouldn't actually increase the physical mass of isolated product beyond what's theoretically possible. Option B suggests CO₂ contamination forming byproducts - while Grignard reagents do react with CO₂, this would typically be minimized under proper inert atmosphere conditions, and wouldn't commonly produce 25% excess mass. Option C incorrectly implies that exothermic reactions somehow create extra product, which violates stoichiometry and conservation of mass. Study tip: When you see yields significantly above 100% in organic chemistry problems, immediately think "contamination" rather than reaction mechanisms. The most common culprits are residual solvents, water, or unreacted starting materials. This pattern appears frequently on exams testing practical laboratory knowledge.

Question 12

An ether solution contains benzoic acid (pKa ≈ 4.2), p-cresol (a phenol, pKa ≈ 10.2), and anisole (neutral). Which sequence of aqueous extractions would most effectively separate these three components?

  1. First extract with 5% aq. HCl to remove p-cresol, then with 5% aq. NaOH to remove benzoic acid.
  2. First extract with 5% aq. NaHCO₃ to remove benzoic acid, then with 5% aq. NaOH to remove p-cresol. (correct answer)
  3. First extract with 5% aq. NaOH to remove both acidic compounds, then add acid to precipitate them together.
  4. First extract with deionized water to remove the most polar compound, then with 5% aq. NaCl to salt out the others.
Explanation: To separate acids of different strengths, one should use bases of corresponding strengths. Sodium bicarbonate (NaHCO₃) is a weak base that is strong enough to deprotonate the carboxylic acid (pKa 4.2) but not the much weaker acid, phenol (pKa 10.2). After removing the benzoic acid, a strong base like sodium hydroxide (NaOH) can be used to deprotonate and extract the remaining phenol, leaving the neutral anisole in the ether.

Question 13

A student performs a reduction of acetophenone with NaBH₄ in ethanol. The workup involves adding dilute HCl, extracting with ether, washing with brine, and drying with anhydrous MgSO₄. What is the primary role of the anhydrous MgSO₄?

  1. To quench any unreacted NaBH₄ remaining in the ether layer.
  2. To act as a Lewis acid and catalyze the removal of borate salts.
  3. To neutralize any residual HCl that may have carried over into the organic layer.
  4. To remove trace amounts of dissolved water from the ether solution by forming solid hydrates. (correct answer)
Explanation: Anhydrous salts like MgSO₄, Na₂SO₄, or CaCl₂ are used as drying agents. They are hygroscopic and react with water dissolved in the organic solvent to form solid hydrates, which can then be removed by filtration. This step ensures the final isolated product is free from water. The other options describe functions that are either incorrect or are accomplished by other steps in the workup (e.g., HCl quenches NaBH₄).

Question 14

A reaction workup requires the separation of a desired neutral product from an acidic byproduct in an ether solution. A student mistakenly adds 5% aqueous HCl to the separatory funnel instead of the prescribed 5% aqueous NaHCO₃. What is the most immediate consequence of this error?

  1. Both the neutral product and the acidic byproduct will remain in the ether layer, resulting in a failed separation. (correct answer)
  2. The neutral product will become protonated and move to the aqueous layer, causing loss of product.
  3. The acidic byproduct will be protonated by the HCl and will move to the aqueous layer.
  4. An exothermic reaction will occur, potentially causing dangerous pressure buildup in the separatory funnel.
Explanation: Acid-base extractions rely on converting compounds between neutral and ionic forms to control their solubility in organic versus aqueous layers. When you have a neutral product and an acidic byproduct in ether, you normally add base (like NaHCO₃) to deprotonate the acid, making it ionic and water-soluble so it moves to the aqueous layer. By adding HCl instead of NaHCO₃, you've created an acidic aqueous layer that cannot deprotonate the acidic byproduct. Since the byproduct is already in its neutral, protonated form, adding more acid won't change its charge or solubility. Both compounds remain neutral and stay in the organic ether layer, making separation impossible. Answer A correctly identifies this failed separation. Answer B incorrectly assumes the neutral product will become protonated - but neutral organic products typically aren't basic enough to be protonated by dilute HCl under these conditions. Answer C wrongly suggests the acidic byproduct will be "protonated by HCl" - this is backwards since the byproduct is already protonated (that's why it's called an acid). Answer D mentions pressure buildup, but simply mixing dilute HCl with an ether solution containing typical organic compounds won't cause a dangerous exothermic reaction. The key study tip: In acid-base extractions, acids need bases to become ionic and move to water, while bases need acids to become ionic. Using the wrong reagent (acid instead of base, or vice versa) prevents the charge conversion that drives the separation, leaving everything in the original layer.

Question 15

During an extraction with a separatory funnel, a student observes a persistent emulsion at the interface of the ether and aqueous layers. Which of the following actions is the LEAST likely to be effective in breaking the emulsion?

  1. Adding a small amount of saturated aqueous NaCl (brine) and swirling gently.
  2. Allowing the funnel to sit undisturbed in a ring stand for 20-30 minutes.
  3. Filtering the entire mixture through a coarse glass frit to mechanically separate the layers. (correct answer)
  4. Gently stirring the emulsified layer with a glass rod to encourage droplet coalescence.
Explanation: An emulsion is a stable mixture of two immiscible liquids. Filtration separates solids from liquids, not two inter-dispersed liquids. Attempting to filter an emulsion would be ineffective as both liquid phases would simply pass through the frit. Adding brine (A), waiting (B), and gentle stirring (D) are all standard and effective techniques for breaking emulsions by changing the properties of the interface or allowing time for gravity to act.

Question 16

A student performs a Fischer esterification and purifies the product by distillation. An IR spectrum of the final product shows a strong C=O stretch at 1735 cm⁻¹, but also a significant, broad absorption centered at 3400 cm⁻¹. This indicates the product is contaminated with an O-H containing compound. How does this impurity affect the calculated percent yield?

  1. The calculated yield is artificially high because the measured mass includes the mass of the impurity. (correct answer)
  2. The calculated yield is accurate, as the IR only provides information on purity, not quantity.
  3. The calculated yield is artificially low because the impurity interferes with the IR signal of the ester.
  4. The effect on yield cannot be determined without knowing the molar mass of the impurity.
Explanation: When analyzing experimental results in organic chemistry, you need to distinguish between what spectroscopic data tells you about product identity versus how impurities affect quantitative measurements like percent yield. The IR spectrum confirms you've made the desired ester (C=O stretch at 1735 cm⁻¹), but the broad absorption at 3400 cm⁻¹ indicates contamination with an O-H containing compound—likely unreacted carboxylic acid or alcohol from the Fischer esterification. Since percent yield is calculated as (actual mass of product/theoretical mass of product) × 100%, any extra mass from impurities will inflate your measured "product" mass, making your calculated yield artificially high. Looking at the wrong answers: Answer B incorrectly assumes IR data doesn't relate to quantitative analysis—but impurities detected by IR directly affect the mass you're weighing. Answer C suggests the yield appears low because IR signals interfere, but spectroscopic interference doesn't change the actual mass measurements used in yield calculations. Answer D claims you need the impurity's molar mass to determine the effect, but you don't need specific molecular weights to know that extra mass from any impurity will increase your measured product mass. The key insight is that percent yield calculations assume your isolated material is pure product. When impurities are present, the "product" mass you measure includes both desired product and contaminants, leading to an overestimated yield. Always consider how impurities affect quantitative measurements, not just qualitative identification.

Question 17

A student aims to synthesize 1-phenyl-1-ethanol by reacting 10.6 g of benzaldehyde (MW = 106.1 g/mol) with 40.0 mL of a 3.0 M solution of methylmagnesium bromide (MeMgBr) in THF. After acidic workup, what is the theoretical yield of the alcohol product (MW = 122.16 g/mol)?

  1. 12.2 g (correct answer)
  2. 13.0 g
  3. 14.7 g
  4. 36.6 g
Explanation: First, determine the limiting reagent. Moles of benzaldehyde = 10.6 g / 106.1 g/mol = 0.100 mol. Moles of MeMgBr = 0.040 L × 3.0 mol/L = 0.120 mol. The reaction stoichiometry is 1:1. Since there are fewer moles of benzaldehyde, it is the limiting reagent. The theoretical yield is based on the moles of the limiting reagent. Theoretical yield = 0.100 mol × 122.16 g/mol = 12.216 g.

Question 18

After carrying out a reaction in an organic solvent, a student needs to remove a byproduct, p-toluic acid. The student washes the organic layer with 5% aq. NaHCO₃. In which layer will the p-toluic acid primarily reside after this wash, and why?

  1. In the organic layer, because as an acid it is more soluble in nonpolar solvents.
  2. In the aqueous layer, because it is converted to its conjugate base, sodium p-toluate, which is an ionic salt. (correct answer)
  3. In the organic layer, because NaHCO₃ is a weak base and is not strong enough to react with p-toluic acid.
  4. In the aqueous layer, because the bicarbonate ion forms a hydrogen-bonded complex with the carboxylic acid.
Explanation: p-Toluic acid is a carboxylic acid. Sodium bicarbonate (NaHCO₃) is a base that is strong enough to deprotonate the carboxylic acid, forming the corresponding carboxylate salt (sodium p-toluate). This salt is ionic and therefore much more soluble in the polar aqueous layer than in the nonpolar organic layer. This acid-base reaction is the basis for extractive separation.

Question 19

Grignard: 0.0600 mol benzaldehyde limiting; theoretical mass of 1-phenyl-1-propanol (MW 136.19)?

  1. 4.09 g
  2. 8.17 g (correct answer)
  3. 10.2 g
  4. 13.6 g
Explanation: This question tests intermediate organic chemistry skills, focusing on reaction workup and yield calculations. Yield calculations involve determining the theoretical maximum product based on the limiting reagent's moles and the product's molecular weight. In this Grignard scenario, the workup steps ensure the product is isolated properly, but here the focus is on theoretical mass computation. The correct answer reflects an understanding of straightforward multiplication of moles by MW for the given limiting reagent. A common distractor involves misapplying the MW or forgetting the limiting aspect. To help students, instructors should emphasize the importance of precise calculations and units in theoretical yields. Practice similar problems with varying limiting reagents to build proficiency.

Question 20

Aldol recrystallization: Which step most ensures purity of crystals before weighing?

  1. Hot gravity filtration
  2. Rinse crystals with cold solvent (correct answer)
  3. Boil until all solvent evaporates
  4. Add NaHCO₃ to recrystallization flask
Explanation: This question tests intermediate organic chemistry skills, focusing on reaction workup and yield calculations. Yield calculations require pure crystals, achieved through effective recrystallization techniques. In this aldol scenario, rinsing removes surface impurities. The correct answer reflects using cold solvent for purity. A common distractor involves improper boiling or addition. To help students, instructors should emphasize recrystallization steps. Practice purification techniques in lab settings.