Experiment 17: NITRATION OF p-methylacetanilide
Objectives: • To synthesize methylnitroacetanilide isomers using an electrophilic aromatic substitution. • To purify product by recrystallization. • To identify and determine purity of product by melting point, TLC and HPLC analysis. • To characterize product by analysis of 1H-NMR and 13C-NMR spectra.
Before coming to lab… • Review these techniques: • TLC analysis • Recrystallization • Suction filtration • HPLC analysis • Melting Point Analysis
ELECTROPHILIC AROMATIC SUBSTITUTION This reaction takes place in two steps: 1. Initial reaction of an electrophile, E+ with the aromatic ring. 2. Loss of H+ from the resonance stabilized carbocation intermediate to regenerate the aromatic ring.
MECHANISM(Preliminary steps…) • Reaction of the aromatic compound with nitric acid alone is slow and potentially hazardous. • It is safer and faster to use a catalytic amount of sulfuric acid. • Sulfuric acid reacts with nitric acid to form the nitronium ion, a powerful electrophile. • Sulfuric acid protonates the hydroxyl group of nitric acid, allowing it to leave as water and form a nitronium ion.
MECHANISM(Nitration of Aromatic Ring…) • Electrophilic aromatic substitution by the nitronium ion gives the nitro product. • Attack on the electrophile forms the sigma complex… • …loss of proton to water… • …gives the aromatic nitro product.
REACTIVITY OF AROMATIC RINGS • The substituents ALREADY ON the aromatic ringaffect the reactivity of the aromatic ring, or how FAST the substitution will occur. • The substituent ALREADY ON the aromatic ring determines orientation of the substitution, or the position of the second (INCOMING) substituent. • Both the methyl group (-CH3) and the acetamide group (-NHCOCH3) are o, p activators. • The acetamide group is a STRONGERo, p director than the methyl group.
Week 1: Synthesis • Mix p-methylacetanilide and acetic acid in 50 mL Erlenmeyer flask. • Prepare nitrating mixture in small test tube (CAUTION!). • *IMPORTANT!* Cool this tube in ice water! • Add nitrating mixture drop wise while swirling. • Allow to react at room temp for 15 minutes. • Add cold deionized water.
Week 1: Crude Product Isolation • Prepare suction filtration apparatus, seating filter with cold deionized water. • Pour crude product slowly into center of funnel. Rinse with cold deionized water. • Prepare CRUDE PRODUCT TLC and HPLC samples in small sample vials. • Set crude solid aside to purify by recrystallization.
Week 1: Preparation of Filtrate Samples • Transfer 2 mL filtrate to a small test tube. • Add 10% NaOH drop wise until neutral (check every 5 drops with pH Hydrion paper). • Add 3 mL ethyl acetate. Cork tube and shake to mix. • Allow layers to separate. • Prepare HPLC and TLC samples of FILTRATE using 5 drops of this solution for each. Remember to add appropriate sample solvent for each! Filtrate
Week 1: Purification • Transfer CRUDE PRODUCT to 50 mL flask. • Dissolve in hot 90:10 ethanol/water. • Cool to room temp, then to 0oC. • Suction filter. Seat filter paper w/90:10 ethanol/water. • Prepare RECRYSTALLIZED PRODUCT TLC and HPLC samples.
Week 1: Sample Submission • Transfer small filter paper and product to a preweighed large filter paper and submit to instructor to dry until next lab period! Also, submit all TLC samples to instructor. • At the end of lab, you should have submitted: • To instructor… • 3 TLC sample vials • 1 packet of pure solid product • For HPLC analysis… • 3 auto sampler vials
Week 2: Product Analysis • TLC Analysis • Prepare TLC plate with 6 lanes. • Apply 3 provided standards along with samples of crude product, filtrate, and recrystallized product. • Develop in 2:1 ethyl acetate/hexane. • Visualize with UV lamp. • Identify compounds and calculate Rf value. • HPLC Analysis • Compare sample chromatograms to provided standard to identify compounds in each sample submitted. • Melting Point Analysis • Obtain the experimental melting point of your purified product. • Compare to the literature value to determine degree of purity.
Table 17.4 • All samples are submitted for analysis during 1st lab period. Samples will be run, and chromatograms returned during 2nd lab period!
Table 17.5 • Enter chemical shifts ONLY based on the spectra on pages 139 and 146. • Notice that the reactant is the product from Experiment 16. You should have already analyzed the NMR spectra!
SAFETY CONCERNS • Nitric acid and sulfuric acid are very corrosive! Use EXTREME caution when in use! • Ethanol is flammable! Be careful when heating!
WASTE MANAGEMENT • Place all liquid waste from experiment into the bottle labeled “LIQUID WASTE”. • Place solid waste in bottle labeled “SOLID ORGANIC WASTE”. • Place all used TLC spotters and melting point capillaries in broken glass container, NOT trashcan!
CLEANING • Clean all glassware with soap, water, and brush if necessary. • Rinse all glassware with wash acetone before returning to lab drawer. • DO NOT return any glassware dirty or wet to lab drawer! • All hotplates and MelTemps must be unplugged from outlets, with cords wrapped neatly.