Pre-lab

The starting aryl bromides, bromobenzene and para-bromoanisole, are stored in different reagent hoods in marked bottles. Use the starting material in the hood that is located closest to your fume hood. All of the solvents that you need are also located in this fume hood.

Because you won't know which aryl bromide you are going to use until you arrive in lab, prepare a Table of Physical Constants, and a table of hazard/disposal information for the following compounds:

• bromobenzene, para-bromoanisole (aka 4-bromoanisole), benzoic acid, para-anisic acid (aka para-methoxybenzoic acid, 4-methoxybenzoic acid)
• diethyl ether, hexane, toluene
• magnesium metal
• CO₂ "dry ice" (Frostbite hazard - handle with THICK gloves, no disposal required)
• 10% aq. HCl, 5% aq. NaOH
• CaSO₄ or Drierite (spent reagent looks pink or purple; place in labeled waste jar)

You do not have to look up the physical properties of the Grignard reagents because you will not be isolating them, but you should still acquaint yourself (and make notes in your notebook) about their chemical reactivity and handling. For your information, all Grignard reagents are strong bases that react rapidly with water (corrosive, avoid skin and eye contact). They can be disposed of (when necessary) by carefully combining them with a weak acid, like water. This gives an organic compound that can be disposed of in the usual way.

Two reagents, diethyl ether and CO₂, will be combined with the Grignard reagents. These reagents must be kept as water-free as possible. Both reagents absorb water from the air (this is especially true of "dry" ice). Therefore, use the special anhydrous ether in the reagent hood and keep this bottle tightly capped when not in use. Likewise, do not obtain dry ice until you are ready to use it, and use FRESH dry ice.

Reaction

Conduct all operations in a fume hood.

Assemble a 100 mL round bottom flask, magnetic stir bar, Claisen head, addition funnel, and reflux condenser [NOTE 1]. Prepare two drying tube with fresh drying agent , but do not attach them to the apparatus yet. (The drying agent contains white crystals and colored crystals. The latter should be blue. Purple crystals indicate a water-saturated drying agent and these should be discarded in the appropriate container).

Remove the hoses from your condenser, and remove the stopcock and cap from your addition funnel. Carefully dry the apparatus using a heat gun (FIRE/BURN hazard [NOTE 2]). Start heating at the bottom and work your way towards the top. After you finish and the apparatus is still somewhat warm to the touch, place the drying tubes on top of the reflux condenser and addition funnel, and insert the stopcock in the addition funnel.

After the apparatus is cool to the touch, charge the flask with magnesium metal (310 mg) and anhydrous ether (8 mL) and then some iodine flakes. Place a solution of the aryl bromide (1.25 mL) and anhydrous ether (8 mL) in the addition funnel and replace the drying tube on top of the funnel. Add approximately 1/3 of the aryl bromide solution to the flask while stirring. Place a ceramic heater under the flask and reflux the solution. As soon as the orange color of the iodine begins to fade, add the rest of the aryl bromide solution to the flask [NOTE 3]. After all of the aryl bromide solution has been added, gently reflux the mixture for another 30 minutes, then let it cool.

When the reaction mixture is cool, obtain freshly crushed dry ice (3 g) and place it in a large dry beaker. Pour the reaction mixture slowly over the entire surface of the dry ice, then stir the mixture with a dry spatula or glass rod until no dry ice can be observed.

Workup

Add 10% aqueous HCl (7 mL) to the mixture and stir. Add wash-grade ether (30 mL) to the mixture and continue stirring until little or no solids are visible. Transfer the mixture to a separatory funnel [NOTE 4]. Discard the aqueous layer and wash the organic layer once with water (10 mL). Extract the organic layer with 5% aqueous NaOH (2 x 15 mL) and combine the basic extracts in a beaker.

Gently heat the basic extracts to remove any dissolved ether. Precipitate the aromatic acid with a minimum amount of 10% aqueous HCl [NOTE 5]. Cool the mixture for 10-15 minutes with an ice bath, then collect the precipitate by vacuum filtration. Transfer the precipitate to a watch glass and let it dry for one week.

Recrystallize your product from a boiling mixture of hexane and toluene [NOTE 6]. Collect your product by filtration and let it air-dry.

Melting Point Analysis

Weigh your dry recrystallized product and measure its melting temperature.

Prepare two mixtures: product+authentic benzoic acid and product+authentic para-anisic acid. Mix equal amounts of each compound using roughly the amounts that you would typically insert in a melting point capillary tube.

Measure the melting temperature of each mixture by the following procedure: transfer the mixture to a capillary tube, melt the entire mixture, let it cool until all of it solidifies, then measure the melting temperature.