Chemistry 223 Carboxylic Acid Deriwatives Grignard Reaction: Synthesis of Triphe

Question

Chemistry 223 Carboxylic Acid Deriwatives Grignard Reaction: Synthesis of Triphenylmethanol Objectives: a) To perform a Grignard reaction on a carboxylic acid derivative b) To make and purify triphenylmethanol Introduction: Grignard reagents are extremely useful synthetically because they can be used to create carbon- carbon bonds. Grignard reagents are formed by the free-radical reaction of an alkyl or aryl halide with magnesium metal in an ether solvent (typically ethyl ether or THF). The magnesium-carbon - bond is covalent, but very polar, making the alkyl or aryl group anionic-like in character. These groups are good bases and strong carbon nucleophiles. Grignard reagents react with a variety of functional groups, including carbonyl compounds, such as aldehydes, ketones and esters, and epoxides. After the attack of the nucleophile to the carbonyl or epoxide, the magnesium salt of an alcohol is produced. A second step, the addition of aqueous acid, protonates the alcohol Since Grignard reagents are strong bases, no proton sources can be present during the reaction. It is important that all glassware be dry for a successful reaction. Also note that Grignard reactions can be slow in starting. Magnesium metal becomes coated with magnesium oxide in the presence of air. This coating must be removed for the Grignard reagent to form. In this experiment, you will prepare a Grignard reagent, phenyl magnesium bromide, and react this with methyl benzoate to produce triphenylmethanol. OH CH Mg MgBr 1) Et20 2) H The following procedure was adapted from: Lehman, J.W., Multiscale Operational Organic Chemistry, Prentice-Hall, Inc., 2002 Safety and Important Notes: Ethyl ether is highly flammable. Be sure there are NO open flames in the laboratory. Ethers form dangerous peroxides after exposure to air. Check that the reagent bottle hasn't been opened more than 90 days previously. If so, it needs to be tested for these peroxides. Bromobenzene is an irritant. Avoid contact with skin (wear gloves) and inhalation of the vapors. . This is a water-sensitive experiment. Be sure all glassware is dry before beginning. Residual acetone can also interfere with the reaction. You may want to use the drying oven. Carboxylic Acid Derivatives 1 Page emistry 223 Organic Laboratory, "Synthesis of Triphenylmethanol,"

Explanation / Answer

Well you are performing grignard reaction. Making grignard using alkyl halide and magnesium. Once the grignard is formed, you react with the ester. Grignard reactions are too moisture senstive. You might have introduced moisture or your grignard has not formed at all.

First you activate the Mg, by washing it with 10% HCl twice, then with distilled water followed by acetone and dry it in oven.
Take flame dried RB and maintaine inert atmosphere by continuously purging N2 or Ar gas. Add Mg to it and then add a pinch of I2 and stirr it in inert atmosphere. You will observe the shiny color of Mg turned black. To this you add your alkyl halide solution in anhydrous diethyl ether. (Diethyl ether should be freshly distilled).

Once, you add the alkyl halide solution, your solution color will be dark brown, due to color of I2. Once, the grignard is forming, your dark brown color will start dissapearing and you will get a less colored (as the color of alkyl halide) turbid solution. This indicates your grignard has formed.

Now, to this solution you add your ester solution in diethyl ether.

Try this, procedure, it will work. I perform grignard reactions day in and out for my research. The same issue had with me in my initial days. Good luck.

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