Difficulty: Intermediate | Prerequisites: General chemistry (atomic orbitals, electronegativity), intro organic chemistry (functional groups, nucleophiles/electrophiles, Lewis structures).
The Grignard reaction is one of the most important carbon-carbon bond-forming reactions in organic chemistry. It sits at the heart of the second-semester organic lab sequence because it teaches you how to build molecular complexity from simple aryl halides. In this lab, you take a brominated aromatic compound, convert it into a highly reactive organometallic intermediate (the Grignard reagent), and then use that reagent to make a carboxylic acid via reaction with CO₂. If you are comfortable with nucleophilic addition and understand why water kills organometallics, you have the conceptual foundation for this entire experiment.
You react a brominated aromatic (e.g. bromobenzene) with magnesium metal in anhydrous diethyl ether to form an aryl Grignard reagent (ArMgBr). That reagent attacks dry ice (solid CO₂) to give a carboxylate salt, which you protonate with acid to get a carboxylic acid (e.g. benzoic acid). Everything must stay bone-dry because water destroys the Grignard reagent on contact.
Grignard reagent (RMgX)
An organomagnesium halide formed by inserting magnesium metal into a carbon-halogen bond in an anhydrous ether solvent. Think of it as a "carbanion delivery system": the carbon bonded to Mg carries a strong partial negative charge, making it an excellent nucleophile.
Aryl halide
An aromatic ring bearing a halogen substituent (Br, Cl, I). In this lab, bromobenzene (C₆H₅Br) is the model substrate. In simple terms, it is the starting material whose C-Br bond gets swapped for a C-Mg bond.
Anhydrous conditions
A reaction environment completely free of water. For Grignard chemistry this is non-negotiable, because even trace moisture protonates the reagent and kills it. In simple terms, "anhydrous" means "no water allowed, at all."
Carboxylation
The process of introducing a carboxyl group (-COOH) into a molecule. Here it happens when the Grignard reagent attacks CO₂ (dry ice). Think of it as the Grignard reagent grabbing a carbon from CO₂ and bolting a new carboxylic acid onto the ring.
Reflux
Heating a liquid so that its vapour rises, condenses in a water-cooled condenser, and drips back into the flask. This keeps the reaction at or near the solvent's boiling point without losing material. In simple terms, it is a way of cooking a reaction at a steady temperature.
Acid hydrolysis (protonation workup)
The step where you add aqueous acid (HCl) to convert the magnesium carboxylate salt into the free carboxylic acid and dissolve the magnesium salts into the aqueous layer. Think of it as "breaking up" the organometallic complex so you can isolate your organic product.
Recrystallisation
A purification technique in which a crude solid is dissolved in a hot solvent and allowed to slowly cool, forming purer crystals while impurities stay in solution. In simple terms, you dissolve the messy solid, then let the clean product crystallise back out.
The oxidative addition step: Magnesium metal inserts into the C-Br bond of the aryl halide. The carbon that was bonded to bromine is now bonded to magnesium, giving ArMgBr.
Why diethyl ether? Ether's lone pairs coordinate to the Mg centre, stabilising the Grignard reagent in solution. Without ether (or THF), the reagent does not form properly.
Anhydrous requirement: Water, alcohols, or any protic source will protonate the carbanion (Ar⁻) instantly, giving ArH + Mg(OH)Br. Every piece of glassware must be flame-dried and drying tubes must cap every opening.
Initiating the reaction: The solution turns brown or cloudy and the flask warms when insertion begins. If nothing happens, gentle steam heating or an iodine crystal can activate the Mg surface.
Controlling the rate: Once started, the aryl bromide/ether solution is added dropwise (1 to 3 drops per second) so the exothermic reaction maintains a gentle reflux without boiling over. If the reflux ring climbs above the lower third of the condenser, slow down.
Excess magnesium: The lab uses a slight excess of Mg (28.81 mmol vs. 25 mmol aryl bromide) to push the reaction toward completion.
Adding dry ice (solid CO₂): The Grignard reagent is decanted onto freshly crushed dry ice. The nucleophilic carbon of ArMgBr attacks the electrophilic carbon of CO₂, forming a magnesium carboxylate salt (ArCOO⁻ MgBr⁺).
Why crush the dry ice immediately before use? Dry ice condenses moisture from the air. If the surface is wet, the Grignard reagent reacts with water instead of CO₂, lowering your yield.
The mixture thickens: The magnesium carboxylate is not very soluble. You add 15 mL of extra ether to keep things stirrable.
Acid hydrolysis (Day 2): Re-dissolve the product in 25 mL ether, then add crushed ice plus 5 mL of 6 M HCl. This protonates the carboxylate (ArCOO⁻ → ArCOOH) and dissolves the Mg salts into the aqueous layer.
Layer separation: The ether (organic) layer contains your carboxylic acid product plus unreacted aryl halide and biphenyl side-product. The aqueous layer holds MgBrCl and other inorganic salts.
Starting material: 25 mmol bromobenzene (C₆H₅Br, FW 157.01) = 3.93 g
Magnesium: 28.81 mmol (0.7 g, FW 24.3), a ~15% molar excess
Dry ice (CO₂): 10 g (227 mmol), a large excess to ensure complete carboxylation
Theoretical product: 25 mmol benzoic acid (C₆H₅COOH, FW 122.12) = 3.05 g
Typical yield: Historically around 50%. For planning your extraction quantities, assume 75% yield (≈ 2.3 g, ≈ 18.8 mmol) so you use enough reagent without massive excess.
Limiting reagent: The aryl bromide (25 mmol) is the limiting reagent. Both Mg and CO₂ are in excess.
Step 1, Grignard formation: ArBr + Mg → ArMgBr (in anhydrous Et₂O)
Step 2, Carboxylation: ArMgBr + CO₂ → ArCOO⁻ MgBr⁺
Step 3, Acid hydrolysis: ArCOO⁻ MgBr⁺ + H₃O⁺ → ArCOOH + MgBr⁺ (aq)
Overall (bromobenzene): C₆H₅Br → C₆H₅MgBr → C₆H₅COO⁻ → C₆H₅COOH (benzoic acid)
Yield calculation: % Yield = (actual mass of product / theoretical mass of product) × 100 Theoretical mass = (mmol limiting reagent × FW product) / 1000
The Grignard reaction is a workhorse in pharmaceutical and fine-chemical synthesis. Any time a chemist needs to attach a new carbon fragment to an existing molecule, Grignard reagents are one of the first tools considered. Industrial examples include the synthesis of ibuprofen intermediates and the construction of complex natural-product skeletons where a new C-C bond must be formed at a specific position on an aromatic ring.
Students often think a small amount of water is acceptable. It is not. Even trace moisture from humid air or damp glassware will destroy the Grignard reagent and tank your yield.
Students sometimes assume the ether is just a solvent with no special role. Diethyl ether (and THF) actively coordinate to the magnesium centre, which is what keeps the reagent dissolved and reactive. A non-coordinating solvent like hexane will not work.
Students frequently confuse the carboxylate salt (ArCOO⁻ MgBr⁺) with the final carboxylic acid product. The acid hydrolysis step is essential: without it you have a salt, not a carboxylic acid.
Students sometimes think the dry ice is just a cooling agent. It is the actual reagent (CO₂ source) for the carboxylation step.
⚠️ The requirement for anhydrous conditions is one of the most commonly tested points. Be prepared to explain why water is incompatible and what product forms if water is present (ArH, the protonated arene).
⚠️ Know the three-step sequence cold: formation → carboxylation → hydrolysis. Exam questions often ask you to predict the product from a given aryl halide.
⚠️ Understand why Mg is used in excess and why CO₂ is used in large excess. Both are deliberate choices to push the reaction toward completion.
⚠️ Be able to calculate theoretical yield and percent yield from the masses given in the lab. This is a standard exam calculation.
⚠️ The role of the ether solvent (coordination to Mg) comes up frequently. Do not just say "it dissolves things."
True or False: The Grignard reagent can be formed in ethanol instead of diethyl ether.
False. Ethanol is protic and would destroy the Grignard reagent.
Fill in the blank: The overall transformation in this lab converts an aryl ______ into an aryl ______ acid.
bromide; carboxylic
True or False: Dry ice serves as both a cooling agent and a source of CO₂ for the carboxylation.
True, though its primary purpose here is as the CO₂ reagent.
Fill in the blank: The typical yield for the Grignard reaction in this lab is approximately ______%.
50%
True or False: After the acid hydrolysis step, the carboxylic acid product is in the aqueous layer.
False. The carboxylic acid is in the ether (organic) layer.
Q: Starting from ortho-bromotoluene, what carboxylic acid product would you expect from the Grignard/CO₂ reaction sequence?
A: 2-Methylbenzoic acid (ortho-toluic acid). The methyl group stays on the ring; the bromine is replaced by MgBr, which then reacts with CO₂ to give the -COOH group at the position where Br was.
Q: Why is it critical to flame-dry the glassware before beginning the Grignard reaction?
A: Residual water on the glass surface would react with the Grignard reagent as it forms, protonating the carbanion to give ArH instead of ArMgBr. This wastes starting material and lowers yield.
Q: If your percent yield is 3.05 g theoretical and you isolate 1.40 g of purified benzoic acid, what is your percent yield?
A: (1.40 / 3.05) × 100 = 45.9%.
Q: What is the purpose of adding 6 M HCl during the Day 2 workup?
A: It protonates the magnesium carboxylate salt to give the free carboxylic acid and dissolves the magnesium salts into the aqueous phase, making it possible to separate the product by liquid-liquid extraction.
Q: Why is diethyl ether chosen over a solvent like dichloromethane for forming the Grignard reagent?
A: Diethyl ether's oxygen lone pairs coordinate to the electropositive Mg centre, stabilising the Grignard reagent in solution. DCM lacks this coordinating ability and can also react with Grignard reagents.
This lab connects directly to nucleophilic addition reactions (Chapter on carbonyl chemistry): the Grignard reagent is a powerful nucleophile that attacks electrophilic carbons, and CO₂ is simply one example of a carbonyl electrophile. The same reagent can attack aldehydes (giving secondary alcohols) or ketones (giving tertiary alcohols), which you may encounter in later labs or on the final exam.
The purification and workup connect to your earlier extraction labs: liquid-liquid extraction, acid-base partitioning, and recrystallisation are all techniques you have practised before. The difference here is that you design your own purification scheme rather than following a fixed protocol.
The concept of anhydrous technique reappears whenever you work with organolithium reagents, metal hydrides (LiAlH₄, NaBH₄), or any moisture-sensitive chemistry in upper-division and graduate-level courses.
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