Difficulty: Intermediate | Prerequisites: Theory doc (Suzuki Coupling Theory and Concepts), familiarity with vacuum filtration, TLC, melting point apparatus, IR and 1H NMR interpretation.
This document covers the hands-on lab procedure and product characterisation for Experiment 4. Session 1 is a known reaction (you know what the product should be), so you are learning the technique and confirming your product matches literature data. Session 2 is the real test: you run the same procedure on unknowns and must use analytical data to figure out what you made. Your grade hinges on correctly identifying the unknown product and presenting clean characterisation data, so understanding what each analytical technique tells you is essential.
Dissolve the aryl bromide and boronic acid in ethanol, add Pd catalyst and KOH, stir until product precipitates, then isolate by vacuum filtration, dissolve in ether, dry with MgSO4, filter, and evaporate. Characterise the product using TLC, melting point, IR, and 1H NMR. Session 2 is identical but with unknowns, and you back-calculate yields once you identify the product.
Vacuum filtration
A technique using a Buchner funnel and vacuum flask to quickly separate a solid from a liquid by pulling solvent through filter paper under reduced pressure. Think of it as: a fast, forced version of gravity filtration.
TLC (thin-layer chromatography)
A quick analytical method that separates compounds on a silica-coated plate using a mobile solvent phase. The plate is visualised under UV light. In simple terms, TLC tells you how many compounds are in your mixture and whether your starting material has been consumed.
Rf value (retention factor)
The ratio of the distance a compound travels on a TLC plate to the distance the solvent front travels. Each compound has a characteristic Rf under given conditions. Think of it as: a compound's "fingerprint" on a TLC plate for that solvent system.
Melting point (m.p.)
The temperature at which a solid becomes a liquid. A pure compound has a sharp melting point that matches the literature value. An impure sample melts over a broader range and at a lower temperature. In simple terms, melting point is a quick purity check.
IR spectroscopy (infrared spectroscopy)
An analytical technique that identifies functional groups by measuring which infrared frequencies a molecule absorbs. Think of it as: a way to see which bonds are in the molecule (C=O, O-H, N-H, etc.) based on their stretching and bending vibrations.
1H NMR spectroscopy (proton nuclear magnetic resonance)
An analytical technique that reveals the hydrogen environment in a molecule: how many types of protons, how many of each, and what their neighbours are. In simple terms, NMR gives you a detailed map of the molecule's hydrogen atoms.
MgSO4 (anhydrous magnesium sulphate)
A drying agent added to an organic solution to absorb residual water. Think of it as: a molecular sponge that soaks up water from your ether layer so the product is dry.
Percent yield
The ratio of the actual mass of product obtained to the theoretical (maximum possible) mass, expressed as a percentage. Percent yield = (actual mass / theoretical mass) x 100%.
Step 1, Dissolve reagents: Dissolve 1.0 mmol (0.199 g) of 4'-bromoacetophenone in 5 mL ethanol in a 25 mL round-bottom flask. Add 1.05 mmol (0.128 g) of phenylboronic acid and stir to dissolve.
Step 2, Add catalyst and base: Add 10 drops of PdCl2 catalyst solution (1000 ppm in 5% HCl), then add 2 mL of 1 M KOH.
Step 3, Stir and wait for precipitation: Stir rapidly. The solid product typically precipitates within 5 to 20 minutes. Once precipitation begins, stir for an additional 2 minutes. If the precipitate makes stirring difficult, add 1 to 2 mL of ethanol.
Step 4, Dilute and stir: After 2 minutes of post-precipitation stirring, add 10 mL of water to the mixture and stir for 10 minutes.
Step 5, Vacuum filtration: Isolate the solid by vacuum filtration. Rinse with 1 mL of water. Pull air through the solid for 10 minutes to dry.
Step 6, Dissolve in ether: Dissolve the solid in 10 mL of diethyl ether.
Step 7, Dry: Add anhydrous MgSO4 to the ether solution to remove residual water. Swirl and let sit briefly.
Step 8, Filter and transfer: Filter to remove the solid MgSO4 and transfer the ether solution to a pre-weighed flask.
Step 9, Evaporate: Blow air or nitrogen gently over the solution until the ether evaporates. The residue is your product.
Step 10, Weigh and characterise: Weigh the product. Calculate percent yield. Run TLC, measure melting point, acquire IR and 1H NMR spectra.
Use the exact same procedure as Session 1.
Use the same masses of unknowns as you used of 4'-bromoacetophenone and phenylboronic acid (you do not know the MW of the unknowns yet, so exact mmol cannot be calculated upfront).
Retain your unknown starting materials until characterisation is complete, in case you need to analyse them.
Once you identify the product, back-calculate mmol, theoretical yield, and percent yield.
TLC: Run a TLC plate of your crude product alongside starting material spots. Use UV visualisation. A new spot at a different Rf from the starting material indicates product formation. Disappearance of the starting material spot indicates complete consumption.
Melting point: Measure the melting range. A sharp m.p. matching the literature value indicates a pure product. A broad or depressed m.p. suggests impurities.
IR spectroscopy: Look for diagnostic stretches. For 4'-acetyl-1,1'-biphenyl, a strong carbonyl (C=O) stretch around 1680 cm-1 is key. Compare your spectrum to the literature spectrum.
1H NMR spectroscopy: Look for the expected signals. For 4'-acetyl-1,1'-biphenyl: aromatic protons in the 7.4 to 8.1 ppm region, and the acetyl CH3 singlet around 2.6 ppm. Extra signals not in the literature spectrum suggest impurities or unreacted starting material.
Dispose of leftover reagents and solvents in appropriate waste containers.
Rinse glassware with acetone before washing with soap.
Wash NMR tubes with the NMR tube washer; dispose of soap solution in the waste container.
Students often forget to pre-weigh the flask before evaporating the ether. If you skip this, you cannot calculate the mass of the product. Always tare or record the empty flask mass.
A common error is assuming that any solid collected by filtration is pure product. The crude solid can contain unreacted starting material, catalyst residue, or by-products. That is why you characterise with TLC, m.p., IR, and NMR.
Students sometimes think a yield above 100% means the reaction went well. It does not. It means your product contains impurities (residual solvent, water, unreacted starting material) or there was a weighing error.
Some students forget that in Session 2 you use the same masses as Session 1, not the same mmol. Because the unknown has a different MW, the actual mmol differ, and you only discover this after identifying the product.
⚠️ Know the purpose of each step in the procedure. "Why do you add water after the precipitate forms?" (to help wash out salts and catalyst). "Why dry with MgSO4?" (to remove water from the ether layer so the product is dry).
⚠️ Be able to interpret a TLC plate: which spot is starting material, which is product, whether starting material is fully consumed.
⚠️ For Session 2, you must use characterisation data to identify your unknown. This means matching IR stretches, NMR signals, and m.p. to literature values for the possible products. Have a table of expected values for the possible unknowns ready.
⚠️ Know how to write a claim-evidence-warrant response. Part 2 Analysis Questions expect you to state your claim ("My product is X"), cite evidence ("The IR shows a C=O stretch at 1682 cm-1, matching the literature value"), and explain the warrant ("This stretch is diagnostic for an aryl ketone").
⚠️ Be prepared to explain yields below or above 100%. Low yield: incomplete reaction, loss during filtration or transfer, incomplete extraction. Above 100%: residual solvent, water, impurities, weighing error.
True or False: You should weigh the round-bottom flask before transferring the ether solution into it.
True. You need the empty flask mass to calculate the product mass by difference.
Fill in the blank: MgSO4 is added to the ether solution to remove ________.
Water.
True or False: In Session 2, you calculate mmol of unknowns before running the reaction.
False. You use the same masses as Session 1 and calculate mmol after identifying the unknowns.
Fill in the blank: A melting point that is lower and broader than the literature value suggests the product is ________.
Impure.
True or False: A single spot on TLC proves your product is pure.
False. A single spot on TLC means only one component is visible under those conditions. Other impurities may co-elute or not be UV-active.
(a) NMR match: Compare your NMR to the literature NMR for 4'-acetyl-1,1'-biphenyl. Cite one specific signal (e.g. "The CH3 singlet at 2.6 ppm matches the literature").
(b) Extra NMR signals: If you see peaks not in the literature spectrum, report them. Common culprits: residual solvent (ethanol at ~1.2 and 3.7 ppm, ether at ~1.2 and 3.5 ppm), water (~1.6 ppm in CDCl3), or starting material.
(c) IR match: Cite one key stretch (e.g. "C=O stretch at ~1680 cm-1 for the ketone").
(d) TLC new spots: Your crude product should show at least one spot that is not at the Rf of starting material.
(e) TLC consumption: If the starting material spot has disappeared, consumption is complete.
(f) Melting point match: Compare your measured m.p. to the literature value (4'-acetyl-1,1'-biphenyl: lit. m.p. ~121 degrees C).
Be honest about what was difficult. Common challenges: getting the product to precipitate, losing product during filtration, or low yield due to incomplete solvent evaporation.
(a) Product identity: State the product, name the unknown reagents, and cite the analytical data that supports your identification.
(b) Purity: Use TLC (single spot?), m.p. (sharp and matching literature?), and NMR (extra signals?) to argue purity.
(c) Yield explanation: If less than 100%, name specific losses (filtration, transfer, incomplete reaction). If more than 100%, name likely impurities.
Compare your IR, NMR, and m.p. to literature values for your proposed product.
Calculate mmol of aryl bromide = mass used / MW of identified aryl bromide.
Calculate mmol of boronic acid = mass used / MW of identified boronic acid.
Identify the limiting reagent (whichever has fewer mmol).
Theoretical yield = mmol of limiting reagent x MW of product.
Isolated yield (%) = (actual mass / theoretical yield) x 100%.
Q: You dissolve your crude solid in ether and add MgSO4. Why?
A: The ether dissolves the organic product while leaving behind inorganic salts and catalyst. MgSO4 is a drying agent that absorbs residual water from the ether solution, ensuring the product is dry when the solvent is evaporated.
Q: Your TLC plate shows two spots for your crude product, one at the same Rf as starting material and one at a new Rf. What does this tell you?
A: The reaction is incomplete. Some starting material remains unconsumed, and a new product has formed. The new spot at a different Rf is likely your Suzuki coupling product.
Q: Your measured melting point is 115 to 119 degrees C, but the literature value for 4'-acetyl-1,1'-biphenyl is 121 degrees C. What does this suggest?
A: The broad, depressed melting range suggests the product contains impurities. A pure sample would melt sharply at or very near 121 degrees C.
Q: In Session 2, you used 0.199 g of an unknown aryl bromide. After characterisation you determine it is 4-fluorobromobenzene (MW = 175.00 g/mol). How many mmol did you use?
A: mmol = mass / MW = 0.199 g / 175.00 g/mol = 0.00114 mol = 1.14 mmol.
Q: Why is it important to retain your unknown starting materials in Session 2?
A: If the reaction fails and you cannot isolate product, you may need to characterise the starting materials themselves (by m.p. or NMR) to determine their identity.
The workup techniques here (vacuum filtration, drying with MgSO4, solvent evaporation) are the same ones used throughout CHEM 216 labs. TLC, melting point, IR, and NMR are the core characterisation toolkit for organic chemistry, and comfort with interpreting each of these carries through every subsequent experiment. The structure-determination logic in Session 2 (comparing analytical data to literature values to identify an unknown) is exactly the kind of reasoning tested on practical exams.
Suzuki coupling procedure, CHEM 216 Experiment 4, vacuum filtration organic chemistry, TLC analysis, Rf value, melting point purity test, IR spectroscopy carbonyl stretch, 1H NMR aromatic protons, MgSO4 drying agent, percent yield calculation, 4'-acetyl-1,1'-biphenyl characterisation, unknown identification organic chemistry lab, claim evidence warrant chemistry, aryl bromide unknowns, boronic acid unknowns, biaryl product, University of Michigan organic chemistry