DUE DATES Today –At beginning of lab – Bromination of Toluene Report –Spectroscopy Problem Set, Part II, 1. –Notebook copies of Nitration of Methyl Benzoate Next Week –Spectroscopy Problem Set, Part II, 2. –At beginning of lab – Separation Scheme for Nitration of Methyl Benzoate

Today You can look at your Bromination notebook copy, but you cannot keep it. Please return it to me today. –Make a note of errors you made on your own notebook copy. The point total for the notebook copy is 25 pts. –There is a grading scheme for part I on my bench. Please look at it so you will understand how it was graded. Some helpful hints for future quizzes/notebooks: –Always make a Data Table – you will need the numbers –Use more than one page in your notebook! –Know the reaction equation and mechanism –Look for obvious questions (Fe metal catalyst in staple) –Keep studying spectroscopy

Spectroscopy on Last Quiz How many different (non-equivalent) carbon atoms are apparently in the molecule? Next to each peak, state how many hydrogens the carbon is bonded to. Are there any aromatic C’s or H’s present? Indicate on each spectrum where you could find absorptions due to aromatic substructure. ppmsplitting 14.2q 26.5d 74.6t 90.9s

Spectroscopy on Last Quiz quartet singlet triplet How many different (non- equivalent) hydrogen atoms are apparently in the molecule? Are there any aromatic C’s or H’s present? Indicate on each spectrum where you could find absorptions due to aromatic substructure. ____ How many neighboring hydrogens are there for those which absorb at ~1.4 d? ____ How many neighboring hydrogens are there for those which absorb at ~1.6 d? ____ How many neighboring hydrogens are there for those which absorb at ~4.2 d? quartet singlet triplet

Spectroscopy on Last Quiz How many non-equivalent hydrogens in para-xylene? How many non-equivalent carbons in the isomeric bromotoluenes? What is the splitting pattern for the indicated hydrogens in H-NMR? quartet singlet triplet

Nitration of Methyl Benzoate This reaction is another example of Electrophilic Aromatic Substitution. In today’s experiment, the reactant is the ester, methyl benzoate and the electrophile is the nitronium ion. The ester is a meta-director and causes the aromatic ring to react more slowly than benzene itself.

Nitration of Methyl Benzoate The nitronium ion is generated from a mixture of H 2 SO 4 and HNO 3 –In the mixture of acids, sulfuric acid is stronger and so nitric acid acts as the proton acceptor, the base. –The protonated nitric acid then loses water to form the nitronium ion, NO2+

Nitration of Methyl Benzoate The NO 2 + reacts with the pi electrons in the ring to form the arenium ion. The more stable arenium ion is the one from meta attack. The ester group is a meta-director. In none of the resonance contributors is the positive charge on the carbon bonded to the electron-withdrawing –CO 2 CH 3 group.

Nitration of Methyl Benzoate The arenium ion loses a proton to some base (HSO 4 - )in solution to regenerate aromaticity and result in overall substitution. Minor products might be the ortho and para isomers, and the di-nitrated ester.

In Lab Today Use the procedure in the Manual, not Pavia Safety Warnings H 2 SO 4 and HNO 3 are strong and corrosive acids. Do not allow contact with skin or clothing. The reacting mixtures must be maintained at ice cold temperatures!

Experiment Notes Mass of methyl benzoate: weigh the vial + contents (±.001 g); add contents to the cold H 2 SO 4 ; weigh empty vial. Volumes of acids: measure with small graduated cylinder; record measurement precisely. (Conversion to mass/mole explained later.) Use a dry empty vial to contain the HNO 3 and H 2 SO 4 mixture.

Experiment Notes Be sure to cool all mixtures as specified in the instructions. Don’t use thermometer – keep on ice for at least 10 min. Add acid mixture drop-wise. Do not allow water from the ice to drip into any reagent mixture.

Experiment Notes Vacuum filtration: –During wait time, set up filtration; cool distilled water and methanol to use as washing liquid. Use small Erlenmeyers or beakers, not graduated cylinders. –Continue to draw air through the filter to air- dry the crude product. –Weigh the crude product on the filter paper on a watch glass.

Experiment Notes During the vacuum filtration wait time, clean the small Erlenmeyer flask. Rinse with a few milliliters of methanol. Transfer the crude product to the flask. Add an equal weight of methanol (density = 0.79 g/ml)

Experiment Notes Recrystallization: See Pavia, p. 671, Table 11.1 A.Use the amount of MeOH as determined by mass. May need to use a bit more. Heat the solution in a beaker of hot water (b.pt. MeOH = 65 o ). Do not heat directly on hot plate. Bring just to a boil. Then remove from heat. Do not use decolorizing charcoal. B. Not applicable.

Experiment Notes Recrystallization, continued: C. 1. Allow the solution to cool to r.t.; do not cool rapidly in an ice bath. D. Air-dry the product in a place designated by the prep room.

Experiment Notes Clean-up duty this week: Next week: weigh dry product and take melting point.

Notebook Keeping Make sure all volumes and masses are recorded legibly in your notebook. Label all measurements. Be sure to measure precisely. Calculation of mass/moles of concentrated acids: See Manual under Synthesis Reports and also my lab web site.

Separation Schemes The purpose of a separation scheme is to show concisely how a product is separated from a reaction mixture. See the lab Manual p. 15; see Pavia The separation for this experiment begins at the point where the reaction mixture is added to ice water. The separation scheme for this experiment is due next week.