CHEMISTRY Organic Chemistry Laboratory II Spring 2016 Lab #2: Grignard Reaction: Preparation of Triphenylmethanol

CHEMISTRY 344 - Organic Chemistry Laboratory II – Spring 2016 Lab #2: Grignard Reaction: Preparation of Triphenylmethanol Purpose. In this lab you wil...
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CHEMISTRY 344 - Organic Chemistry Laboratory II – Spring 2016 Lab #2: Grignard Reaction: Preparation of Triphenylmethanol Purpose. In this lab you will use the Grignard Reaction, a classic reaction in organic chemistry, for the synthesis of triphenylmethanol from an alkyl halide and a ketone. You will use the techniques of running reactions under anhydrous conditions using a drying tube, extractions, and purification. Important Notes: 1) You must have lab goggles and your Hayden-McNeil Lab Notebook as specified in the lab syllabus. 2) The last page of the handout is an outline of how you should set up your notebook. 3) This is a two-week lab and although your notebook pages will be submitted at the conclusion of week # 2, your lab instructor will briefly examine at your notebook at the end of lab # 1 before you leave. 4) Complete your lab notebook prelab work as outlined in the lab syllabus and below: (notebook items I – IV). I. Your name, the name of your lab instructor, the name of your lab partner. II. Title of the experiment and date. III. Table of Reagents. Synthesis experiments require a table of all reagents, including the chemical name, formula, molecular weight, molecular structure, and physical properties (melting point, boiling point, density). You should record information for magnesium, bromobenzene, diethyl ether, benzophenone, and triphenylmethanol. IV. Background information and notes from the pre-lab assignment. You are expected to read this lab handout, prepare this notebook section with the following content present, and review this information to prepare for a prelab quiz. 1) Sketch the full reaction from the handout in your notebook (a mechanism is not required). This reaction scheme represents the actual physical work you will do in the lab. 2) In week one, you will synthesize the Grignard reagent and the alkoxide salt using this handout. Even though, the procedure lists the reagent amounts in moles, you still have to conduct the following calculations before lab: a) For the solid reagents (magnesium and benzophenone) - convert the values given in moles to values in grams. Record the calculation(s) and value(s) - Think about what physical constant to use. b) For the liquid reagent (bromobenzene) convert the value given in moles to values in grams and mL. Record the calculation and value - Think about what physical constants to use. 3) Water is detrimental to the success of this experiment. Write several sentences explaining why this is so. 4) A critical point in today’s experiment is the addition of the benzophenone / ether solution to your newly synthesized Grignard reagent. Explain the safety concerns that you should be aware of at this step. 5) Your notebook is now prepared to start the lab period. Leave space in your notebook at this point to record the pre-lab Quiz and any recitation notes on the day of your laboratory experiment.

CHEMISTRY 344 - Organic Chemistry Laboratory II – Spring 2016 Lab #2: Grignard Reaction: Preparation of Triphenylmethanol (Week 1) Experimental Procedure – Week I. The following oven dried glassware is stored in the desiccated cabinet: drying tube, air condenser, 20 mL screw top vial, 25 ml Erlenmeyer flask, glass syringe, and a Pasteur pipette. DO NOT wash this glassware as traces of water on the glass surface will react with the Grignard reagent. Prepare a calcium chloride drying tube using cotton plugs. Assemble the drying tube, air condenser, and 20 mL vial as shown in the picture below. You are NOT required to re-write the experimental procedure. Immediately after the week 1 procedure is a description of the content that you must record for part V / Week 1. (Look ahead) Formation of the Grignard Reagent. Keep all glassware dry! Obtain 8.5x10-3 moles of Mg ribbon, and scrape off any white, oxidized MgO from the surface with a spatula to reveal the shiny metal underneath. Wipe the strip(s) with a paper towel to remove oil and dirt from your fingers or gloves, and cut the strip(s) into very small pieces (0.5 cm or less). Add the Mg to your pre-weighed, drying tube with CaCl2 dry, 20 mL reaction vial, and determine the exact mass of the Mg pieces. Record the mass of the vial + Mg. Using the dry glass syringe carefully add 7.7x10-3 moles of bromobenzne to the vial. Using the dry Pasteur pipette add cotton plugs about 5 mL of diethyl ether to the vial. The ultrasonic bath (sonicator) is in a hood and should contain tap water at air condenser approximately 300C. Place your vial in the test tube rack in the sonicator. The reaction should start within five minutes as the mixture will begin bubbling and take on a gray color. If no reaction begins in 10 minutes, your reaction has become contaminated with water and you need to start over. As the bubbling becomes more vigorous the reaction mixture eventually turns brown. The reaction is complete in 20 mL vial 45 minutes. Remove your vial containing the Grignard reagent from the sonicator bath and cool to room temperature. DO NOT remove the drying tube until you are ready to add the next reagent. Addition of benzophenone. This step must be done quickly to avoid contamination by water! Obtain 7.5x10-3 moles of benzophenone in a dry 3 mL reaction vial, and dissolve in 2 mL of diethyl ether. Remove the air condenser from the reaction vial and slowly add the benzophenone dropwise to the Grignard reagent over a 5 minute time period, while stirring with a spatula. Do not add the benzophenone too quickly or the heat from this exothermic reaction may cause the mixture to boil. The mixture will turn a pink / red color and then solidify as the alkoxide salt of triphenylmethanol. It is important that you continue stirring until all of the ether has evaporated from the reaction mixture. You may want to do this step near a hood to avoid the ether vapors. Keep the crude product vial with the stirrer inside stored in your kit for week 2. Then fully complete the information needed for part V of week 1 and ask your instructor to briefly look at your notebook.

V. Week 1 - Observations, Results and Data. The following information must be recorded in a table in your notebook labeled section V Week 1:  Amount of Mg (moles and grams) used in the reaction  Amount of bromobenzene (moles and grams) used in the reaction  Amount of benzophenone (moles and grams) used in the reaction  Amount of ether (mL) used in the reaction - no calculation needed for the ether Immediately after the table, show all of the clearly conducted and labeled calculations  All data in the table should be supported with a calculation – except the ether solvent Also in the table, write a sentence to describe the significant observations or list the observations you have made at each of the following steps:  When you prepared the Grignard reagent from magnesium and bromobenzene  When you added the benzophenone reagent to the Grignard reagent  When the ether evaporated and yielded a crude alkoxide salt.

CHEMISTRY 344 - Organic Chemistry Laboratory II – Spring 2016 Lab #2: Grignard Reaction: Preparation of Triphenylmethanol (Week 2) Important Notes: 1) You must have lab goggles and your required Hayden-McNeil Lab Notebook as specified in the lab syllabus. You will not be allowed in the lab without both of these items. 2) This is a two-week lab and your notebook pages will be submitted at the conclusion of week # 2. 3) Your notebook for week 2 should begin with a brief description of safety information and notes that you record during the prelab recitation. The next section should be part V / Week 2 4) You are NOT required to re-write the experimental procedure. Immediately after the week 2 procedure is a description of the content that you must record for part V / Week 2. (Look ahead) Experimental Procedure – Week 2. You will neutralize the alkoxide salt to yield the triphenylmethanol as a crude product for purification by a series of extractions followed by solvent removal. Your overall success and the purity of the final product will depend on the melting point, percent yield, and product appearance. Isolation of crude triphenylmethanol. Cool the reaction vial containing the alkoxide salt in an ice bath. Obtain 5 mL of 6 M HCl (CAUTION – add slowly!) and add this solution dropwise to the cold reaction mixture, while stirring with a spatula. Two reactions are occurring, protonation of the alkoxide salt (which is exothermic), and H2 gas being released as excess Mg is oxidized to Mg+2 with reduction of H+. Now add 5 mL of ethylacetate, and stir with a spatula. You should have two distinct layers with the triphenylmethanol dissolved in the ethylacetate layer. If you do not see two layers, pipet the entire mixture into a centrifuge tube and centrifuge for 1 minute. Carefully transfer the organic layer into a test tube. Add 2 mL of ethylacetate to the remaining aqueous solution, stir for 1 minute, and combine this organic layer with the first organic extract. Now add two micro

spatula scoops of magnesium sulfate to dry the organic layer and leave the mixture for 10 minutes (with occasional mixing) at room temperature. While waiting for the organic layer to dry, clean and dry your 25 mL Erlenmeyer flask, and obtain its empty mass. Decant the dry organic layer into the flask, being careful to leave the MgSO4 behind. Wash the remaining MgSO4 by adding 1-2 mL of fresh ethylacetate, stir, and allow the solid to settle for at least one minute. Transfer this portion to the original ethylacetate solution. Evaporate the organic solvents on the drying station, and obtain the mass and melting point of your crude triphenylmethanol. Crystallization from 2-propanol. Place a round spin bar in your 25 mL Erlenmeyer flask, and add 1 mL of 2propanol (isopropanol) for every 0.1 g of crude product. Heat the mixture, with stirring, to 80oC. If undissolved solid remains at 800C, add additional 2-propanol (5 drops at a time) until the solid just dissolves (no more than 20 extra drops). Record the total volume of 2-propanol added. Allow the hot, saturated solution to cool slowly at room temperature. If no solid forms in 10 minutes, scratch the inside bottom of the flask with a glass rod. Once crystallization has begun place the flask in an ice bath for 5 minutes. Isolate the product by filtration on a Hirsch funnel, and wash your crystallized product with 1-2 mL of ice-cold 2-propanol. Scrape the crystallized triphenylmethanol onto a watch glass, and dry the product in an 800C oven for 20 minutes. Obtain the mass, melting point of the dry, purified triphenylmethanol. V. Week 2 - Observations, Results and Data. The following information must be recorded in a table in your notebook labeled section V Week 2:  Amount of crude product (moles and grams)  Melting range of dry, crude product  % Yield of dry, crude product  Total volume of isopropanol needed/used for recrystallization  Mass of dry, crystallized product (moles and grams)  Melting range of dry, crystallized product  Literature melting range  % Yield of dry, crystallized product Immediately after the table, show all of these clearly conducted and labeled calculations  All data in the table should be supported with a calculation Also in the table, write a sentence to describe the significant observations or list the observations you have made at each of the following steps:  When you reacted the alkoxide salt with 6M HCl  When you performed the extractions  When you conducted the crystallization and isolated your final product

VI. Discussion. Write a paragraph that addresses the techniques you used and observations that you recorded during the multi-step synthesis. Address whether the experiment went according to the planned procedure or any deviations occurred.

VII. Conclusions. Write a paragraph that addresses your data. Was your experiment a success considering the purpose, goals, and results of the experiment? Directly address the % yield, IR, and melting point data. What can you state about the efficiency of this reaction and your experimental technique?

CHEMISTRY 344 - Organic Chemistry Laboratory II – Spring 2016 Lab #2: Grignard Reaction: Preparation of Triphenylmethanol

Overview of Notebook Structure: Your notebook should have the following structure and sequence of sections for this laboratory experiment. The headings also indicate the points assigned to those sections.

I.

Your name, the name of your lab instructor, the name of your lab partner.

II.

Title of the experiment and date.

III.

Table of reagents – Record the requested information

IV.

Background information – The prelab assignment, notes during the recitation, and the pre-lab quiz.

V.

Week 1 - Observations, Results and Data - table, calculations, observations

V.

Week 2 - Observations, Results and Data - table, calculations, observations

VI.

Discussion. Write a paragraph to addresses the techniques and observations

VII.

Conclusions. Write a paragraph to address your data. Was your experiment a success?