grign - ORGANIC II LABORATORY(Major's) Grignard Reaction...

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ORGANIC II LABORATORY(Major's) KELLY Grignard Reaction READ: Landgrebe - Experiment 21 pp 427 to 431 We will follow an alternate experimental procedure from the one in the lab manual, however you should read these pages of text for a better understanding of the experiment. I. Introduction Grignard reagents play a commanding role in organic synthesis. These compounds can be adapted to the preparation of a large variety of functional groups, and the formation and reaction of organomagnesium derivatives are one of the major uses of alkyl halides in organic synthesis. Formation of the Grignard Reagent 1. RCl + Mg → dry ether RMgCl Types of Grignard Reactions 1. H 2 O + RMgCl → dry ether R-H + Mg(OH)Cl 2. CO 2 + RMgCl → dry ether RCO 2 - + MgCl → water RCO 2 H + Mg(OH)Cl 3. Formaldehyde + RMgCl → dry ether RCO - + MgCl 4. Aldehyde + RMgCl → dry ether salt of a secondary alcohol 5. Ketone + RMgCl → dry ether salt of a tertiary alcohol 6. Ester + 2 RMgCl → dry ether salt of a tertiary alcohol The reaction of a halide and magnesium occurs on the surface of the metal and is formally an oxidation of the metal. The reaction is usually carried out in dry ether solvent, the ether functioning as a Lewis base by solvating the Grignard reagent and permitting it to diffuse away from the metal. R-X + Mg Et 2 O R-Mg-X OEt 2 OEt 2 Solvated Grignard Reagent The formation of the organometallic reagent requires an active surface on the metal, and some difficulty may be had in getting the reaction started because of metal oxides on the metal surface. Grinding the magnesium in a mortar for a few minutes prior to use is usually effective in providing a clean surface. Another useful trick in starting a reaction is the addition of a small crystal of iodine to the mixture, which reacts with the magnesium to form the very reactive magnesium iodide (MgI 2 ) salt. A major pit-fall in preparing a Grignard reagent is that the presence of water, alcohol or any other "active hydrogen" compound can react rapidly with the organomagnesium compound as an acid. This reaction, shown below, consumes and wastes the Grignard compound by formation of hydrocarbon, and it may seriously complicate the preparation, since the other product (MgXOR) can coat the surface of the metal and prevent further formation of the organometallic compound. RO-H + R'MgX R'-H + MgXOR For this reason, the apparatus and all reagents and solvents must be scrupulously dry . Potential difficulties in running a Grignard Reaction. Apparatus and chemicals must be dry 1. Bake out apparatus in oven for at least three hours
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TITLE Page 2 2. Dry chemicals over anhydrous CaCl 2 3. The only air outlet to the apparatus should be covered with a drying tube. 4.
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grign - ORGANIC II LABORATORY(Major's) Grignard Reaction...

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