Chemistry 222 Fall 2012

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1 Experiment 2: Grignard Reaction Report Sheet Last Name: Siva First Name: Krishnaa Date: September 27, 2012 Section Number: 005 Objective: 1 mark (What is the purpose of this experiment?) The purpose of this experiment is to study the formation of the carboncarbon single bond which is largely found in organic chemistry. This is studied with the help of the Grignard reaction. In a Grignard reaction the carbon of an organic group of the Grignard reagent bonds with the carbonyl carbon of a ketone to form an alcohol. The method to produce a Grignard reagent is also studied in this experiment. Introduction: 2 marks (Brief description of the concept/reaction studied Grignard reactions) This lab begins with the formation of the Grignard reagent. Magnesium turnings were mixed with bromobenzene and absolute ether in a round bottom flask and was heated on a steam bath to produce a brown substance called phenyl magnesium bromide (the Grignard reagent). The focal point of this experiment involves the reaction of phenyl magnesium bromide, the Grignard reagent, and benzophenone, a ketone, to produce the crystalline alcohol triphenylcarbinol and MgBr(OH). This overall reaction is known as the Grignard reaction. It is important to note that this reaction also involves the production of an intermediate product which is an organomagnesium salt. This salt forms when the reaction mixture with the phenyl magnesium bromide and benzophenone, is mixed with the 10% sulfuric acid, ethyl ether and ice. Following this step, the organic layer is separated from the aqueous layer, and then filtered through a cone of anyhydrous sodium sulfate. Following some evaporation processes and cooling, the crystalline alcohol triphenylcarbinol is finally obtained through a suction filtration process. At this point, a carbon-carbon single bond gets formed between the carbon of the Grignard reagent's organic group and the carbonyl carbon of the ketone. To verify that it is indeed the crystalline alcohol triphenylcarbinol, the melting point of the product is taken at the end. Reaction Equation: 2 marks (Include proper structures and data [molar mass, concentration, density, volume, mass, moles, etc as appropriate including theoretical yield] for reactants and products) (The reaction equation with the table containing the proper information is on the following pages.) 1

2 Molar mass (g/mol) Melting Point C) Boiling Point ( C) Density g cm 3) Volume/ Mass Moles Bromobenzene ml Magnesium g Benzophenone g Crystalline Alcohol C Experimental: g Experimental: Experimental: Triphenylcarbinol C g Table 1. Results Yield and Physical Properties of Triphenylcarbinol 5 marks Triphenylcarbinol Yield (g) 0.25 g Yield (%) 8.74 % Appearance White crystalline structures Melting Point 151 C - C Discussion Questions: 10 marks 1. Suggest at least one chemical reason why your percent yield is less than 100%. (Incomplete reactions? Side reactions? Stability of reagents?) To begin with, the formation of the impurity, biphenyl, is one reason the percent yield is less than 100%. Biphenyl gets produced when the phenyl magnesium bromide reacts with the unreacted bromobenzene (Williamson and Masters, 2011). Benzophenone should have been added gradually in this experiment to keep the formation of the primary impurity of biphenyl to a minimal. With the slow addition, the bromobenzene concentration would be kept to a minimal and the bromobenzene would more likely react with the magnesium instead of the formerly made phenyl magnesium bromide. This impurity is somewhat removed near the end of the experiment with the help of the hexanes (solubility of biphenyl is higher in the hydrocarbon solvent compared to the triphenylcarbinol), but in the end, the percent yield is still affected to a certain degree. 2

3 Another reason why the percent yield was less than 100% is due to the fact that some of the solute was lost when pouring the solution (which had some crystals) into the Erlenmeyer flask (that was used for the suction filtration). If we had more time, we could have taken that Erlenmeyer flask and poured that solution into another Erlenmeyer flask and did the suction filtration again. This would have certainly increased the percent yield. 2. Clean and dry glassware is very important for performing a high yielding Grignard reaction. What effects would the presence of water or acetone would have on your reaction? With water, the Grignard reaction would not work out as well. The presence of water or acetone would certainly reduce the yielding of the Grignard reaction since the water causes the reagent to rapidly get decomposed and lose its nucleophilic property. With water, instead of forming numerous carbon-carbon bonds, some carbon-hydrogen bonds will be produced and that is not something we want to have in this experiment.. In reactions, the "R" part of the Grignard reagent is said to be polarized as negative and the "Mg" is polarized to be positive:. In the presence of water, the negative "R"s would react with the positive hydrogen of the water. If Grignard reagents were to react with just water, alkanes would be produced instead of alcohols (Clark, 2003) In addition to making sure that we have clean and dry glassware, a drying tube with calcium chloride on top of the condenser was used to absorb and remove any moisture that was present in the air (Yourspigs, 1999). 3. What would happen if you added the benzophenone all at once as opposed to drop wise? The benzophenone was added drop by drop to make sure the exothermic reaction that was taking place does not produce too much heat and boil over. With benzophenone, the reaction is very fast. Putting the benzophenone solution too quickly can result in the fierce release of everything in the flask (Williamson, 1999). It is important to make sure when adding the benzophenone, the reaction mixture does not get too hot and no bubbles should be seen or else it can get dangerous. For added safety, it is also indicated in the lab procedure to cool the reaction mixture when necessary. 4. Analyze the MP of your product. What evidence is present that supports the formation of triphenylcarbinol? One way to identify or confirm the product that was produced in the lab is by comparing it's melting point found in the lab to the theoretical melting point value 3

4 of substance. Since the theoretical melting point value of triphenylcarbinol, which is C corresponds with melting point range of our product, which - C, it is safe to say that the product that was produced in this lab is also triphenylcarbinol. Non of the theoretical melting points of the other substances that take part in this experiment matching with the melting point range found at the end of the experiment further supports our conclusion. 5. Draw a detailed mechanism for the formation of triphenylcarbinol from benzophenone using phenyl magnesium bromide (your Grignard reagent). The mechanism for the formation of triphenylcarbinol from benzophenone using phenyl magnesium bromide is on the next page. 4

5 Bibliography Clark, Jim. "Grignard Reagents. " Chemguide Web. 23 Sept Solomons, T.W.G. Graham, Fryhle, Craig and Johnson, Robert. Organic Chemistry. 10 th ed. New York: John Wiley & Sons, Print. Williamson, Kenneth and Masters, Katherine. Macroscale and Microscale Organic Experiments. 6th Ed. Belmont: Brooks/Cole, Print. Williamson, Kenneth. Macroscale and Microscale Organic Experiments. 3rd ed. Boston: Houghton Mifflin, Print. Yourspigs, U.P. "Chapter 5 - The Main Precursors." The Vaults of Erowid Web. 24 Sept

CHEM254 #4 The Synthesis of a Tertiary Alcohol Using a Pre-Made Grignard Reagent 1

CHEM254 #4 The Synthesis of a Tertiary Alcohol Using a Pre-Made Grignard Reagent 1 Background In this project, we will perform a Grignard reaction using a pre-made Grignard reagent. Grignard reagents can