Question

What was the reason for the addition of H2O (10 ml), followed by gentle heating? Draw a mechanism equation for the reaction taking place. Why was the water added slowly and through the condenser?

Here is my experiment...

Experimental procedure

Warning: acetic anhydride is corrosive and a lachrymator. When carrying it across the lab, when it is inside a flask, please temporarily stopper the flask.
Reaction

Place salicylic acid (200 mg = 0.200g) into a dry 25ml round bottom flask (RBF). Temporarily close the flask with a cork stopper.

Using a pluringe, measure acetic anhydride (2 ml) and add it to the reaction flask. Stopper the flask

again. One pluringe will be shared by all students for this purpose.

Securely clamp the RBF so it sits about 1 inch off of a sand bath. For now, keep the sand bath off.

Remove the stopper, and quickly attach a water condenser to the RBF (flowing water is unnecessary)

Turn the sand bath/Variac onto medium power (~75% of the maximum setting), let it warm up for about

2 minutes.

Carefully lower the reaction apparatus so the flask is heated in the sand bath. The reaction should be

heated for 7 minutes. This heating will cause the reaction temperature to be 80-100 0C.

Turn off the sand bath/Variac, raise the reaction mixture to 1 inch above the sand bath, allow the

reaction mixture to cool for ~5 min.

Take a TLC of your starting material (salicyclic acid) versus the reaction product.

Dilute a small amount (~5mg) starting material in acetone for the purpose of

spotting onto the TLC plate. The reaction mixture can be spotted directly without

dilution.

Obtain a Pasteur-pipette TLC spotter attached to a 3 L microcapillary (The

instructor will discuss this, and may show you how to prepare such a spotter).

We recommend pre-labeling the TLC plate as shown to the right. Note the “co-

spot” in the middle which serves as a scientific control experiment.

The starting material can be spotted from the sample you prepared (step 8a); the

reaction mixture can be directly spotted from the RBF (step 1) without dilution.

Develop the plate with 10:1 hexanes:ethyl acetate (H:E) and visualize with a UV lamp and also

by staining with FeCl3 (1% in MeOH:H2O).

If TLC indicates the reaction is incomplete, then return to step 5; your instructor may also suggest

adding additional acetic anhydride.

Add de-ionized H2O (10 ml) to the reaction mixture, drop-wise (slowly!) through the top of the

condenser, with a plastic pipette. Return the flask to the sand bath, apply heat to maintain gentle boil for 15 minutes, confirm the solution is homogenous (one layer) before stopping. You may need to pulse the heat power on and off to maintain the gentle boiling.

Turn off the sand bath and raise the flask once again; allow it to cool for only about 5 minutes.

Workup and purification

Pour the reaction mixture to a 50 ml Erlenmeyer flask (ERLF). Scratch sides of flask with a glass stir rod. Let the flask cool to ambient temperature, occasionally scratching the sides to encourage crystal formation. We may consider “seeding” your flask with authentic crystals at this point. Be patient!

Cool the reaction mixture in an ice bath until crystals form. In some cases this process has taken 20-30 minutes, be patient! Once a small amount of crystals form, additional crystals will form shortly!

Vacuum filter your product with a Buchner funnel onto a pre-tared filter paper. Allow the product to dry with vacuum turned on, for about 10 minutes while you commence cleanup.

If your product appears as an oily mass (non-crystalline): (a) heat the ERLF on a sand bath at high temperature (variac 8-9) until the oil dissolves; (b) grind the ERLF again, (c) cool to ambient temperature, (d) seed with authentic product, (e) cool in an ice bath to induce crystal formation. Be patient in each step of this process!

Characterization

16. Record the mass, physical appearance, and melting point range of your isolated product.

Cleanup and waste disposal

Submit your dry product in to a 20 ml vial labeled for your class section. Several samples will be analyzed by NMR spectroscopy. The NMR data will be discussed later after you’ve learned about NMR.

Dispose of the filtrate into the aqueous/inorganic waste container.

Dispose of the microcapillaries into glass waste; TLC plate into solid waste.

Answer 1

After the reaction is complete, water is added to quench (destroy) any remaining acetic acid and turn it into acetic acid.

The water adds across the anhydride bridge, cleaving it, and releasing two acetic acid molecules. Acetic acid is much less reactive than acetic anhydride.

Water was added through condenser as no other inlet is there.

Mechanism: