CHEMISTRY Organic Chemistry Laboratory II Spring 2019 Lab #2: Grignard Reaction: Preparation of Triphenylmethanol

Transcription

1 CHEMISTRY Organic Chemistry Laboratory II Spring 2019 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, extraction, and purification by recrystallization. Important Notes: 1) You must have lab goggles and your Hayden-McNeil Lab Notebook as specified in the lab syllabus. The format for writing in your notebook is outlined below. 2) This is a two-week lab and your notebook pages will be submitted at the conclusion of Part II. 3) Format your lab notebook items I - IV as outlined below. 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 (Parts I & II). Synthesis experiments require a table with the first column listing the Chemical Name of all reagents, roughly in the order in which they are used in the experimental procedure. That is followed by a column with the Chemical Formula, and a column of the Purpose of the reagent (i.e., reactant, drying agent, solvent, etc.). For everything you identified as Reactant, Product, or Solvent, you will also need columns for: structure, molecular weight, and physical properties (melting point, boiling point, density). Copy the sample table below into your notebook, and fill in the required information. Refer back to your previous reagent tables for assistance. Chemical Name Purpose chemical formula structure molecular (g/mole) melting point boiling point density (liquids) (g/ml) Magnesium N/A N/A N/A N/A Diethyl ether N/A N/A Benzophenone N/A N/A Hydrochloric acid N/A N/A N/A N/A N/A Ethyl acetate N/A N/A Magnesium sulfate N/A N/A N/A N/A N/A 2-propanol N/A N/A Triphenylmethanol N/A N/A 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, and review this information to prepare for a prelab quiz. 1) Goals. Based on your reading, what are the Experimental Goals for Parts I & II of this experiment? 2) Lab Safety Parts I & II. Copy the table outlined below into your lab notebook. Read over the experimental procedure for Parts I & II, then complete the table so that you have a thorough list of key lab safety topics specific to this lab. Parts of the table have been completed for you. N/A Common Risk factors in the organic chemistry lab (cross out those that are not applicable to this lab) What is the associated Hazard? (burn, sharp/cuts, solvent vapor, skin irritant, eye irritant, etc.) How will you limit Exposure to the hazard? SDS Section 2 summary SDS Section 8 summary Hot plate Hazardous chemicals Chemical exposure may result in irritation and or illness Strong acids and/or bases

2 Volatile solvents Glassware Melting point capillary tube Electrical equipment Vacuum filtration InfraRed Light Diethyl ether Health & Safety (look up SDS at Flinn website) Cuts from broken glass Glassware implodes under vacuum Keep solvent covered; use in hood Look for frayed power cords; avoid spilling water 3) Reaction Equation. Under your reagent table draw the overall reaction based on the scheme detailed in this handout. 4) Watch this video from Dr. Kenneth Overly of Providence College: This is a nice overview of the reaction, although we will use different apparatus. 5) In Part I, you will synthesize the Grignard reagent, alkoxide salt, and crude product. The procedure lists the reagent amounts in moles, so you need 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). b) For the liquid reagent (bromobenzene) convert the value given in moles to values in grams and ml. Record the calculation and value. 6) Water is detrimental to the success of this experiment. Write 1-2 sentences explaining why this is so. 7) A critical step in this experiment is the addition of the ether solution of benzophenone to your Grignard reagent. Explain the safety concerns that you should be aware of at this step. Your notebook is now prepared to start the lab period. Be sure to leave space in your notebook to take notes during the recitation period. Equation for the reaction (Parts I & II): Qualitative Observations. You are expected to record detailed observations for all steps in this experiment. You will be graded on the accuracy and completeness of these observations, which you will use to answer post-lab questions. Experimental Procedure Part I. The following oven-dried glassware are 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. Formation of the Grignard Reagent. 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

3 gloves, and cut the strip(s) into very small pieces (0.5 cm or less). Add the Mg to your pre-weighed, dry, 20 ml reaction vial, and determine the exact of the Mg pieces. Record the of the vial + Mg. Using the dry glass syringe carefully add 7.7x10-3 moles of bromobenzene to the vial. Using the dry Pasteur pipette add about 5 ml of diethyl ether to the vial. The ultrasonic bath (sonicator) is in a hood and should contain tap water at approximately 30 0 C. 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. The reaction is complete in 45 minutes or when bubbling stops. Allow the vial to 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 while stirring with a spatula, slowly add the benzophenone dropwise over a 5 minute time period. 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 forms. Stir for an additional 5-10 minutes. You may want to do this step near a hood to avoid the ether vapors. Aqueous work-up. You will neutralize the alkoxide salt to yield crude triphenylmethanol for purification next week. Reconnect the air condenser (no need for the drying tube), and cool the reaction vial in an ice bath. Obtain 5 ml of 6 M HCl (CAUTION strong acid!). Add the HCl solution dropwise to the cold reaction mixture, while swirling. Two reactions are occurring, protonation and neutralization of the triphenylmethanol alkoxide salt (which is exothermic), and H 2 gas being released as excess Mg is oxidized to Mg +2 with reduction of H +. The H 2 gas may cause foaming! After all HCl is added you can remove the air condenser and use a spatula to break up the pasty material. After all bubbling has stopped, cap the reaction vial and place in a small beaker to prevent tipping until next week. Then fully complete the information needed for Section V of Part I and ask your instructor to briefly look at your notebook. V. Quantitative Observations. Using the suggested format below, build Data, Result and IR tables in your lab notebook. Immediately after your tables show all calculations with clearly labeled headings. These tables will be completed in Parts I & II. Data Table for Lab #2 Parts I & II Grignard Reaction Sample Molecular (g/mole) Exact (grams) Magnesium Benzophenone Triphenylmethanol (crude) Triphenylmethanol (recrystallized) Volume (ml) Moles Limiting reagent? (place an X) Calculations: Results Table for Lab #2 Parts I & II Grignard Reaction Sample Theoretical yield (grams) Theoretical yield (moles) Triphenylmethanol (crude) Triphenylmethanol (recrystallized) % Recovery % Yield Melting point range Literature melting range Calculations:

4 IR Data Table for Lab #2 Part II Grignard Reaction (Complete after Part II of this lab). Sample Structure Key functional groups and IR bands Benzophenone Triphenylmethanol Recrystallized product Post-Lab Questions for Part I. (1) Which step in the Part I procedure gave you the most trouble and why? (2) Using complete sentences and your observations, data and results, discuss your evidence that you synthesized a new compound. (3) % Yield is calculated by taking the or moles of product, divided by the theoretical or moles of product (x100). What type of useful information can you get from the % Yield? Based on that answer, what is your % Yield from Part I telling you? (4) Read over Part II of this lab. You will not tear out your notebook pages for Part I. However, before you leave lab you must have your lab instructor check your notebook and initial the last page. CHEMISTRY Organic Chemistry Laboratory II Spring 2019 Lab #2 (Part II): 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, extraction, and purification by recrystallization. Important Notes: 1) You must have lab goggles and your Hayden-McNeil Lab Notebook as specified in the lab syllabus. Note that there is no pre-lab work for Part II. 2) This is a two-week lab and your notebook pages will be submitted at the conclusion of Part II. 3) Format your lab notebook items I - IV as outlined below. 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. (Just review your Part I table) IV. Background information and notes from the pre-lab assignment. You are expected to read this lab handout. There is no other pre-lab preparation for Part II.

5 Experimental Procedure Part II. At the end of the Part I lab you neutralized the alkoxide salt to yield crude triphenylmethanol that you will now purify by extraction and recrystallization. The purity of your final product will be based on the melting point, percent yield, and product appearance. Your overall success will be based on your lab technique and preparation. Qualitative Observations. You are expected to record detailed observations for all steps in this experiment. You will be graded on the accuracy and completeness of these observations, which you will use to answer post-lab questions. Isolation of crude triphenylmethanol. Examine your vial of crude triphenylmethanol in 6 M HCl (CAUTION strong acid). Record your observations, and note any changes from Part I. 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, allow the layers to separate, and combine this organic layer with the first organic extract. Now add 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. Pipette the dry organic layer into the flask, being careful to leave the MgSO 4 behind. Wash the remaining MgSO 4 by adding 1-2 ml of fresh ethylacetate, stir, and allow the solid to settle for at least one minute. Combine this portion with the initial ethylacetate solution. Evaporate the organic solvent at the drying station at 40 o C (15 minutes), and obtain the and melting point of your crude triphenylmethanol. Recrystallization from 2-propanol. Place a round spin bar in your 25 ml Erlenmeyer flask, and add 1 ml of 2-propanol (isopropanol) for every 0.1 g of crude product. Heat the mixture, with stirring, to 80 o C. If undissolved solid remains at 80 0 C, add an additional 5 drops of 2-propanol, until the solid just dissolves (you can add more, but no more than 20 total 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 80 0 C oven for 20 minutes. Obtain the, melting point and IR spectrum of the dry, purified triphenylmethanol. V. Quantitative Observations. Complete the Data, Result and IR tables you started in Part I of this lab. Don t forget to show all calculations with clearly labeled headings. Post-Lab Questions for Part II. (1) Using complete sentences and your observations, data and results, discuss your evidence that the product you synthesized is a pure compound. (2) % Recovery is calculated by taking the of pure product, divided by the of crude product (x100). What type of useful information can you get from the % Recovery? Based on that answer, what is your % Recovery telling you? (3) % Yield is calculated by taking the or moles of product, divided by the theoretical or moles of product (x100). What type of useful information can you get from the % Yield? Based on that answer, what is your % Yield telling you? (4) Discuss whether the IR data are consistent with your product. Be sure to staple a copy of your IR spectrum with your notebook pages.