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 1 H-NMR and 13 C-NMR spectra.

Before coming to lab…  Review these techniques: TLC analysis Recrystallization Suction filtration HPLC analysis Melting Point Analysis

CHEMICAL EQUATION

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 ring affect 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 (-CH 3 ) and the acetamide group (-NHCOCH 3 ) are o, p activators.  The acetamide group is a STRONGER o, 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!).  MODIFICATION: Cool this tube in ice water!  Add nitrating mixture dropwise 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.  MODIFICATION: Transfer solid and filter paper to preweighed watch glass and place in oven for 15 minutes to dry.  During this time, proceed to preparation of filtrate for analysis, detailed on the next slide!  Remove from oven. Reweigh (1A) and calculate % yield (1B).  Prepare CRUDE PRODUCT TLC and HPLC samples in small sample vials.

Week 1: Preparation of Filtrate Samples  MODIFICATION: Transfer 2 mL filtrate to a small test tube.  Add 10% NaHCO 3 dropwise 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.  Transfer TOP LAYER to a clean test tube.  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 90:10 ethanol/water.  Cool to room temp, then to 0 o C.  Suction filter.  Prepare RECRYSTALLIZED PRODUCT TLC and HPLC samples.  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.

Table 17.1 Theoretical yield (g)  should have this calculated based on the limiting reagent prior to coming to lab. 1AMajor product mass from synthesis (g)  Obtain 1 st lab, after drying in oven. 1B% yield from synthesis  If this calculated value is >100%, you must return the crude product to the oven to dry for a longer period of time.  1A/theoretical yield. 2AMajor product mass after recrystallization (g)  Obtain 2 nd lab, at the beginning of the lab period. 2BOverall % yield  2A/theoretical yield. Experimental melting range ( o C)  Obtain 2 nd lab period. Product Appearance  Record the physical state and color of your product.

Table 17.2 Atom Economy  Can calculate 1 st lab.  Review calculation in Experiment 13. Experimental Atom Economy  Can calculate 1 st lab.  Review calculation in Experiment 13. E product  Can calculate 2 nd lab, after product has been purified and dried.  Review calculation in Experiment 13. Cost per Gram  Can calculate 2 nd lab, after final product mass has been obtained.

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 R f 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 17.3 Compound TLC R f values StandardsCrude sample Filtrate sample Recrystallized sample p-methylacetanilide  All R f values are unitless!  All R f values are 2 decimal places ONLY!  If reactant present in this lane, it explains product loss! 4-methyl-2- nitroacetanilide  If major product present in this lane, it explains product loss! 4-methyl-3- nitroacetanilide

Table 17.4 Compound Standards Crude Sample Filtrate Sample Recrystallized Sample Retention times (min) Retention times (min) Area % Retention times (min) Area % Retention times (min) Area % p-methylacetanilide 4-methyl- 2-nitroacetanilide 4-methyl- 3-nitroacetanilide All samples are submitted for analysis during 1 st lab period. Samples will be run, and chromatograms returned during 2 nd lab period!

Product Analysis (NMR Spectroscopy)

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 SYNTHESIS portion of experiment into the bottle labeled “Aqueous Waste (Nitration)”.  Place all liquid waste from RECRYSTALLIZATION/TLC portion of experiment into the bottle labeled “Organic Waste (Nitration)”.  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.