Difficulty: Intermediate | Prerequisites: Grignard Reaction Theory (Doc 1), basic extraction and recrystallisation techniques from earlier labs.
This is the practical side of the Grignard reaction: a two-day lab where technique determines whether you get product or a beaker of ruined reagent. The reaction is moisture-sensitive at every stage, so flame-drying, drying tubes, and anhydrous solvent handling are not optional extras. Day 1 covers reagent formation and carboxylation. Day 2 covers the acid workup and a purification scheme you design yourself, drawing on extraction and recrystallisation skills from earlier in the course. If you are behind on extraction technique, revisit those notes first.
Day 1: flame-dry your apparatus, form the Grignard reagent by slowly adding aryl bromide in ether to Mg turnings under reflux, then pour the reagent onto crushed dry ice. Day 2: dissolve the crude product, add ice and HCl to protonate, extract into ether, then design and execute your own purification (extraction followed by recrystallisation). Measure melting point before and after recrystallisation and collect NMR data.
Flame drying
Heating glassware with a Bunsen burner to drive off adsorbed moisture before assembling the apparatus. In simple terms, you bake the water off the glass so it cannot ruin your Grignard reagent.
Drying tube
A tube packed with a desiccant (Drierite, anhydrous CaSO₄) and plugged with cotton at both ends, placed at every opening in the apparatus to keep humid air out. Think of it as a moisture filter for the air entering your setup.
West condenser (water-jacketed reflux condenser)
A glass tube surrounded by a water jacket. Solvent vapour rises into it, condenses, and drips back into the flask. In simple terms, it is the part that stops your ether from escaping.
Addition funnel (separatory funnel repurposed)
Used to add the aryl bromide/ether solution dropwise into the flask at a controlled rate. The stopcock lets you regulate the flow.
Liquid-liquid extraction
Separating compounds based on their different solubilities in two immiscible solvents (here, ether and aqueous acid or base). Think of it as sorting your products by shaking them between two layers that do not mix.
Acid-base workup
Using aqueous acid or base washes to selectively move compounds between the organic and aqueous layers based on whether they are acidic, basic, or neutral.
Melting point determination
Measuring the temperature at which a solid melts as a purity check. A pure compound melts sharply over a narrow range; impurities broaden and depress the range.
Flask preparation: Start with a clean, dry 250 mL two-necked round-bottom flask. Add a stir bar and 0.7 g of magnesium turnings.
Attach glassware: Fit a dry West condenser to one neck and a separatory funnel (acting as addition funnel) to the other. Lightly grease all ground-glass joints to ensure airtight seals.
Drying tubes: Place a drying tube on the top of both the condenser and the addition funnel. Every opening must be protected from atmospheric moisture.
Flame dry: Use a Bunsen burner to heat the entire assembled apparatus. Let it cool to room temperature before proceeding. Do not wear gloves during flame drying, as melted glove material causes severe burns.
Cooling water: Attach water hoses to the condenser (water enters from below, exits from above). Turn on the flow before any ether is introduced.
All burners away: The TA will not distribute diethyl ether until every student has put their Bunsen burner away. Ether is extremely flammable.
Prepare the aryl bromide solution: Obtain 0.025 mol of your assigned brominated aromatic compound and dissolve it in 25 mL of anhydrous diethyl ether.
Load the addition funnel: Close the stopcock, pour in the ether solution, replace the drying tube.
Initiate the reaction: Add approximately 5 mL of solution to the flask and stir. Watch for the solution turning brown, cloudy, or warming up, which signals the reaction has started.
Troubleshooting initiation: If nothing happens, apply gentle steam heat. If refluxing for a long time with no reaction, the TA may add an iodine crystal to activate the Mg surface.
Dropwise addition: Once started, add the remaining solution at 1 to 3 drops per second. Control the rate so the reaction maintains a gentle reflux. If the reflux ring climbs above the lower third of the condenser, slow down.
Final reflux: After all solution is added, reflux with steam for 15 minutes.
Carboxylation: Crush 10 g of dry ice immediately before use (it picks up moisture from the air). Decant the reaction mixture from the flask onto the crushed dry ice, leaving unreacted Mg turnings behind.
Thin the mixture: The slurry will become very thick. Add 15 mL of ether so you can stir it.
Stop for the day: Cover the beaker with a watch glass, label it, and give it to your TA.
Re-dissolve: Add 25 mL of ether to your crude product from Day 1.
Acid hydrolysis: Add a mixture of ~15 g crushed ice and 5 mL of 6 M HCl. This protonates the carboxylate salt to give free carboxylic acid and dissolves the Mg salts into the aqueous layer.
Separate layers: The ether (organic) layer contains your carboxylic acid product, unreacted aryl halide, and biphenyl side-product. The aqueous layer contains MgBrCl and other inorganic salts. Isolate the ether layer.
Design your purification: This is the open-ended part. You need to separate your carboxylic acid from bromobenzene (or its substituted analogue) and biphenyl. A good approach combines acid-base extraction with recrystallisation.
Acid-base extraction strategy: The carboxylic acid is acidic (deprotonated by NaOH into a water-soluble sodium carboxylate), while bromobenzene and biphenyl are neutral and stay in the ether. Wash the ether layer with aqueous NaOH to pull the carboxylic acid into the aqueous layer, then re-acidify with HCl to precipitate it.
Recrystallisation: The lab requires recrystallisation as the final purification step. Choose an appropriate solvent (consult the 241 lab manual for guidance).
Melting point: Measure the melting point of your crude product before recrystallisation and of the purified product after. The comparison tells you how effective your purification was.
NMR: Obtain NMR data on your purified product to confirm its identity.
Use stoichiometry: Calculate how much NaOH or HCl you actually need based on the moles of product you expect, rather than guessing volumes.
Concentration matters: Use 1 M HCl or NaOH for extractions (gentle, controlled). Use 6 M HCl or NaOH only for the acid/base workup steps where you need to shift pH sharply.
Layer volumes: Keep the organic and aqueous layers roughly equal in volume, or at least within a factor of two. Wildly unequal volumes reduce extraction efficiency.
Second extraction: It is standard practice to do a second extraction at half the volume of the first (e.g. first wash with 8 mL, second wash with 4 mL). This captures product left behind in the first pass.
Assume 75% yield for planning: The historical average yield is around 50%, but assume 75% when calculating reagent amounts for your first extraction. This gives you a safety margin without using wasteful excess.
Green chemistry principle: Use the minimum amount of solvent and reagent that will do the job. Excess washes generate more waste without improving purity past a certain point.
Diethyl ether: Extremely flammable. No open flames anywhere in the lab while ether is in use. Ether vapour is heavier than air and can travel to an ignition source across the bench.
Flame drying and gloves: Remove gloves before flame drying. Melted nitrile or latex can cause severe burns that are worse than a brief contact with hot glass.
Cold glass and hot glass look the same: After flame drying, let the apparatus cool fully before touching it or adding ether.
Dry ice: Causes frostbite on contact with skin. Handle with tongs or insulated gloves, never bare hands.
HCl (6 M): Corrosive and produces toxic fumes. Work in the fume hood. If it contacts skin, rinse immediately with water.
NaOH (6 M): Corrosive. Causes chemical burns. Same precautions as HCl.
Magnesium disposal: Unreacted Mg turnings must be quenched with 6 M HCl before disposal. Do not put metallic Mg in the waste without quenching.
No acetone rinse on Day 1: Acetone is a ketone and will react with the Grignard reagent if any residue is present. Use only ether for rinsing on Day 1.
Steam line: The steam line is hot. Handle with care to avoid burns.
Students often think rinsing glassware with acetone before starting is fine. It is not on Day 1: acetone is a carbonyl compound that reacts with Grignard reagents, contaminating your product.
Students sometimes assume a faster drip rate will save time. Adding the aryl bromide too quickly causes the reaction to boil over, losing product and creating a safety hazard.
Students frequently skip the second extraction wash, thinking one pass is enough. A second wash at half volume recovers a meaningful amount of product that stays behind in the first aqueous layer.
Students sometimes treat the melting point measurement as a formality. It is the primary evidence that your purification worked: a sharp melting range close to the literature value confirms purity.
⚠️ Know the purpose of every piece of the apparatus: drying tubes (moisture exclusion), condenser (prevent ether loss), addition funnel (controlled reagent addition). Exam questions ask you to explain what happens if you remove one.
⚠️ Understand why you use 6 M acid for the workup but 1 M for extractions. Concentration choice affects both the chemistry and the layer volumes.
⚠️ Be prepared to design a purification scheme on paper. Exam questions may present a mixture of carboxylic acid, neutral organic, and inorganic salt and ask how you would separate them using extraction.
⚠️ The melting point comparison (crude vs. recrystallised) is a classic exam topic. Know that impurities lower and broaden the melting range.
⚠️ Green chemistry principles (minimising waste, using minimal reagent) appear in lab practical exams and write-up rubrics.
True or False: You should wear gloves while flame-drying the apparatus.
False. Melted gloves cause worse burns than brief contact with hot glass.
Fill in the blank: Diethyl ether boils at approximately ______ °C, which is why only gentle steam heating is needed.
35 °C
True or False: The aqueous layer after acid hydrolysis contains the carboxylic acid product.
False. The product is in the ether (organic) layer.
Fill in the blank: A second extraction is typically done at ______ the volume of the first.
Half
True or False: You can rinse your glassware with acetone between Day 1 steps.
False. Acetone reacts with the Grignard reagent.
Q: Why must the dry ice be crushed and used immediately rather than prepared in advance?
A: Dry ice condenses moisture from the air onto its surface. If the surface is wet when the Grignard reagent contacts it, the reagent reacts with the water instead of the CO₂, reducing your yield.
Q: After the Day 2 acid hydrolysis, what does the aqueous layer contain and why do you discard it?
A: The aqueous layer contains dissolved magnesium salts (MgBrCl), excess HCl, and water. These are inorganic byproducts with no value for the synthesis, so the layer is discarded.
Q: Outline a purification strategy to isolate pure benzoic acid from a mixture that also contains bromobenzene and biphenyl in ether.
A: Wash the ether layer with aqueous NaOH. Benzoic acid (a carboxylic acid) is deprotonated and moves into the aqueous layer as sodium benzoate, while bromobenzene and biphenyl (neutral) stay in the ether. Separate the layers. Acidify the aqueous layer with HCl to precipitate benzoic acid. Filter, dry, and recrystallise.
Q: Why is 1 M NaOH preferred over 6 M NaOH for the extraction washes?
A: 1 M NaOH provides enough base to deprotonate the carboxylic acid without generating excessive heat or forming emulsions that make layer separation difficult. 6 M is reserved for the sharp pH shifts needed in the acid/base workup steps.
Q: Your crude benzoic acid melts over 115 to 119 °C. After recrystallisation it melts at 121 to 122 °C. The literature value is 122.4 °C. What do these results tell you?
A: The crude product contained impurities that depressed and broadened the melting range. Recrystallisation removed most of them, giving a sharp melting point close to the literature value, which confirms high purity.
The extraction and purification skills here build directly on earlier labs in the CHEM 241 sequence. If acid-base extraction feels unfamiliar, revisit the extraction lab notes, because the Grignard purification is essentially the same technique applied to a different mixture.
Recrystallisation technique connects to your earlier recrystallisation lab. The difference here is that you choose the solvent and conditions yourself rather than following a recipe.
The NMR analysis at the end ties into spectroscopy topics. You should be able to identify the carboxylic acid O-H peak, the aromatic C-H signals, and (for substituted products) the methyl peak in a ¹H NMR spectrum.
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