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Nitration of Bromobenzene by Electrophilic Aromatic Substitution

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Nitration of Bromobenzene by Electrophilic Aromatic Substitution Part A, p. 515: Nitration procedure. (p. 504, 4th Ed) Part B, p. 517: TLC. (p. 506, 4th Ed) Part C, p. 518: Column chromatography. (p. 507 4th Ed) Important Concepts • Electrophilic aromatic substitution reactions • Nitronium ion as an electrophile • Activating vs Deactivating groups • o,p-directors vs meta directors • Using resonance structures to predict substitution pattern Chem 343 - Nitration of bromobenzene, Copyright - D.J. Dyer & SIUC 1 Introduction to Electrophilic Aromatic Substitution 1. Friedel-Crafts Alkylation General Reaction Mechanism R X R E Catalyst E R.D.S H R = alkyl; X = Cl,Br, Catalyst = AlCl3, etc. (1) 2. Friedel-Crafts Acylation E E O O H (2) + H+ R Cl R AlCl3 R = alkyl, aryl Nitration Mechanism O 3. Halogenation HO N O (1) H O N Cl O 2 O N O + H2O FeCl3/Cl2 O H OSO3H Nitronium ion Br2 Br O N O NO2 H+ (2) H 4. Nitration NO2 NO2 HNO3 NO2 H OH2 + H O+ H2SO4 (3) 3 Chem 343 - Nitration of bromobenzene, Copyright - D.J. Dyer & SIUC 2 Our Reaction Major product Minor product Trace amounts Br Br Br Br HNO3 Br + + + O N H2SO4 2 O2N NO2 NO2 NO2 4-Bromo-nitrobenzene 2-Bromo-nitrobenzene 3-Bromo-nitrobenzene Multiple nitrations (para Substitution) (ortho Substitution) (meta Substitution) Which is more polar? Which product has the smallest RF value? Br Br O N 2 NO2 Less polar More polar Chem 343 - Nitration of bromobenzene, Copyright - D.J. Dyer & SIUC 3 Part A - Nitration Procedure Add bromobenzene 1. Assemble apparatus as described. We will through condenser essentially follow the procedure in the book. We will use a digital 2. Control the rate of addition and add ice to water thermometer bath in order to keep temperature below 55 oC. 3. After addition, heat the solution for 15 minutes (keep temp. below 60 oC ), then let cool to room Cork temperature. Claisen 4. Pour into ice water and recrystallize the crude Adapter solid from 95% ethanol. Add ice to 5. Concentrate the mother liquor to a slurry on the maintain temp. Two clamps rotovap. Do not evaporate to dryness. 6. Filter the slurry from the rotovap and rinse the crystals before drying. Do not combine with the HNO3/H2SO4 1st crop of crystals until you have confirmed purity by TLC. 7. Concentrate mother liquor prior to column chromatography. Chem 343 - Nitration of bromobenzene, Copyright - D.J. Dyer & SIUC 4 Part C - Column Chromatography Use wet pack 1. Fill column 25% with solvent (95/5 - Hex/EtOAc). (slurry) method 2. Let a little solvent flow through the frit and stopcock. 3. Place silica in beaker and add solvent, stir and swirl vigorously while you pour it into the column. Use more solvent to transfer as much silica as needed. Rinse sides ~ 1 cm Sand of column with solvent to ensure all silica is immersed below the solvent level. Silica gel 4. Fill the column with solvent and let solvent flow through column until the silica is packed tightly. Make Glass Frit sure that the solvent does not reach the level of the silica. It is OK to re-use the solvent that is in the collection beaker while packing the column. 5. After the column is packed tightly, add ~ 1 cm of sand above the silica gel. 6. Allow solvent to drain until it reaches the level of the sand, then load the column with your crude sample. 7. Allow a small amount of solvent to flow through the sand until you are confident that your compound has Which compound been transferred to the silica layer. Then fill the column will come off the with eluant and open the stopcock so that there is a column first? moderate flow of solvent. Start eluting with 95/5 and ramp up to 90/10 - Chem 343 - Nitration of bromobenzene, Copyright - D.J. Dyer & SIUC Hex/EtOAc) 5 Why do we get more para than ortho? (but if any organic chemist ask in lecture for product to reaction MUST give both ortho and para products) Br Predicted Observed ortho para ortho para + 2 : 1 1 : 2 E tBu tBu OMe OMe OMe E E+ E+ + E E NO2 NO2 E+ Slow E Chem 343 - Nitration of bromobenzene, Copyright - D.J. Dyer & SIUC 6 Required Data - m.p. of purified products. - TLC with crude, pure para and pure ortho on same plate. - Yield, %Yield for each product. - IR and 1H NMR of organic starting materials and final products. Chem 343 - Nitration of bromobenzene, Copyright - D.J. Dyer & SIUC 7.
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