In Each Reaction Box Place Reagent and Conditions from the List Below – Chemistry Guide

In Each Reaction Box Place Reagent and Conditions from the List Below – Full Step-by-Step Organic Chemistry Guide

One of the most common and tricky exam or homework questions students face in organic chemistry is: “In each reaction box place the best reagent and conditions from the list below.” These questions test your ability to track molecules through transformations, recall functional group interconversions, and apply named reactions correctly. In this comprehensive guide, we will break down exactly how to solve in-each-reaction-box-problems, provide worked examples, explain common pitfalls, and show you step-by-step how to master this problem type.

We will also cover related concepts such as molecular polarity (e.g., Is KrF2 polar or nonpolar?, Is AsF3 polar or nonpolar?, Is AsF5 polar or nonpolar?), since these often appear alongside mechanism questions in exams. By the end of this article, you will have a solid framework to tackle reaction box problems confidently.

1. Understanding the Problem: What Does “In Each Reaction Box Place the Best Reagent and Conditions from the List Below” Mean?

When you are given a starting compound and a final product, the reaction box question asks you to fill in the missing steps with the correct reagents and conditions. Each box corresponds to one step in the overall transformation. For example, if the problem shows a bromoalkane converting into an alcohol, one box may contain NaOH (aq) as the best reagent under SN2 conditions.

Common instructions you might see:

• “In each reaction box, place the best reagent and conditions from the list below.”
• “Draw the intermediate compound after each step.”
• “Use reagents only once unless otherwise stated.”
• “You may not need to use all the reagents provided.”

2. Step 1 – Analyze the Reaction

Before filling in any reagent, analyze the overall transformation. Look at the starting compound and the final product. What has changed? What functional groups appear or disappear? What structural differences exist? This analysis is the foundation of your solution.

Example: If the starting material is 2-bromoethane and the final product is ethanol, the functional group has changed from alkyl halide to alcohol. That suggests a substitution reaction, most likely an SN2 reaction under basic conditions.

3. Step 2 – Count the Number of Carbons

Carbon counting is one of the most overlooked yet crucial strategies in organic reaction problems. Before deciding on a reagent, ensure that the carbon skeleton matches from start to finish. If the product has more carbons than the starting material, you need a carbon–carbon bond-forming reaction (e.g., Grignard reagent, organolithium, or alkylation).

Pro Tip: If you see the final compound has one more carbon atom than the starting compound, think Grignard addition to a carbonyl followed by acid workup.

4. Step 3 – Apply IUPAC Naming

Knowing the IUPAC names of both starting materials and products helps you track what’s happening at each step. For instance, converting 2-bromoethane into ethanol indicates a substitution reaction. If the name changes from “-ane” to “-one” (alkane → ketone), you are looking at an oxidation or rearrangement step.

5. Step 4 – Choose the Best Reagent and Conditions

This is where most students hesitate: which reagent from the list below fits best? Always choose the most specific reagent that achieves the transformation in a single step, instead of a general but less controlled option.

Examples of common reagents and their typical uses:

• NaOH (aq) – Converts alkyl halides to alcohols (SN2 substitution).
• KOH (alc) – Causes elimination (E2), producing alkenes.
• H₂/Pd – Hydrogenation of alkenes to alkanes.
• KMnO₄ – Oxidation of alkenes to diols or cleavage products (conditions matter).
• O₃/Zn – Ozonolysis, breaking double bonds into carbonyl compounds.
• SOCl₂ – Converts alcohols to alkyl chlorides.
• RMgX (Grignard) – Adds carbon units to carbonyls (follow with H₃O⁺).

6. Step 5 – Recall Named Reactions

Reaction box questions often include steps based on named reactions. Recognizing these patterns saves time and ensures accuracy. Here are some you should know:

• Friedel–Crafts Alkylation/Acylation – Adds alkyl or acyl groups to benzene.
• Aldol Condensation – Forms C–C bonds between enolate and carbonyl compounds.
• Wittig Reaction – Converts carbonyls to alkenes using a phosphonium ylide.
• Hydroboration–Oxidation – Converts alkenes to alcohols with anti-Markovnikov selectivity.
• Claisen Condensation – Joins two esters into a β-keto ester.

7. Worked Example: Grignard Reaction

Let’s solve a real problem. The starting compound is CH₃CH₂Br and the final product is propan-2-ol. How do we fill in the reaction boxes?

1. Step 1: Convert CH₃CH₂Br into the Grignard reagent CH₃CH₂MgBr using Mg in dry ether.
2. Step 2: React with acetone (CH₃COCH₃).
3. Step 3: Acid workup (H₃O⁺) gives propan-2-ol.

8. Bonus Section – Molecular Polarity Questions

Alongside reagent questions, exams often test molecular geometry and polarity. Here are three classic cases:

Is KrF₂ Polar or Nonpolar?

KrF₂ has a linear geometry with bond dipoles canceling out, making it nonpolar.

Is AsF₃ Polar or Nonpolar?

AsF₃ is trigonal pyramidal due to a lone pair, so its dipoles do not cancel. It is polar.

Is AsF₅ Polar or Nonpolar?

AsF₅ is trigonal bipyramidal, with dipoles canceling. It is nonpolar.

9. Final Tips for Students

• Always start by analyzing functional group changes.
• Check carbon counts before picking reagents.
• Remember that SN1 and SN2 require different conditions (polar protic vs. aprotic solvents).
• Practice with Chegg-like questions, but aim to understand the why, not just the answers.
• Keep a reagent flashcard deck to drill conversions quickly.

10. FAQs

Q: How do I solve “In each reaction box place the best reagent and conditions from the list below” questions?
A: Break down the problem into steps: analyze functional groups, count carbons, name compounds, choose the specific reagent, and recall named reactions. Use practice problems to get faster.

Q: What is the most common mistake students make?
A: Forgetting to check carbon number changes or using overly general reagents instead of the most selective option available.

Q: Are these problems common in exams?
A: Yes. They appear in university midterms, finals, and online homework platforms like Chegg under the label “in each reaction box place the best reagent and conditions from the list below.”

Q: Can I memorize reagent tables instead of understanding mechanisms?
A: Memorization helps, but true mastery comes from understanding the mechanism behind each reagent and transformation.