Exp t 101 Grignard Synthesis of Benzoic Acid from K L Williamson Macroscale and Microscale Organic Experiments 2nd Ed 1994 Houghton Mifflin Boston d p364 Rev 9 22 03 Advanced Preparation It is imperative that all equipment and reagents be absolutely dry One lab period before you plan to run the reaction you should place the glassware to be usedtwo reaction tubes two shorty vials and a stirring rod can be dried in a 110 C oven to dry along with the magnesium turnings Prelab Exercise Prepare a flow sheet for the preparation of benzoic acid Through a knowledge of the physical properties of the solvents reactants and products show how the products are purified Indicate which layer in separations should contain the product Introduction In 1912 Victor Grignard received the Nobel Prize in chemistry for his work on the reaction that bears his name a carbon carbon bond forming reaction by which almost any alcohol may be formed from appropriate alkyl halides and carbonyl compounds The Grignard reagent is easily formed by reaction of an alkyl halide in particular a bromide with magnesium metal in anhydrous diethyl ether Although the reaction can be written and thought of as simply RMgBr RBr Mg it appears that the structure of the material in solution is rather more complex There is evidence that dialkylmagnesium is present that is R Mg R MgBr2 2 R MgBr and that the magnesium atoms which have the capacity to accept two electron pairs from donor molecules to achieve a four coordinated state are solvated by the unshared pairs of electrons on ether Et Et O R MgBr O Et Et The Grignard reagent is a strong base and a strong nucleophile As a base it will react with all protons that are more acidic than those found on alkenes and alkanes Thus Grignard reagents react readily with water alcohols amines thiols etc to regenerate the alkane R MgBr H20 R H MgBrOH R MgBr R OH R H MgBrOR R MgBr R NH2 R H MgBrNHR The starting material for preparing the Grignard reagent can contain no acidic protons The reactants and apparatus must all be completely dry otherwise the reaction will not start If proper precautions are taken however the reaction proceeds smoothly The magnesium metal in the form of metallic turnings has a coat of oxide on the outside A fresh surface can be exposed by gently bending or crimping a the turnings under the absolutely dry ether in the presence of the organic halide with a glass stirring rod Be careful not to break the stirring rod Reaction will begin at the exposed surface as evidenced by a slight turbidity in the solution and evolution of bubbles Once the exothermic reaction starts it proceeds easily the magnesium dissolves and a solution of the Grignard reagent is formed The solution is often turbid and gray due to impurities in the magnesium The reagent is not isolated but is reacted immediately with most often an appropriate carbonyl compound such as R O MgBr O R MgBr R R C R R to give in another exothermic reaction the magnesium alkoxide In a simple acid base reaction this alkoxide is reacted with acidified ice water to give the covalent ether soluble alcohol and the ionic water soluble magnesium salt O R C R MgBr R H Cl OH R C MgBrCl R R The great versatility of this reaction lies in the wide range of reactants that undergo reaction with the Grignard reagent In the present microscale experiment we shall carry out another common type of Grignard reaction the formation of a carboxylic acid from 1 mole of the reagent and 1 mol of carbon dioxide The primary impurity in the present experiment is biphenyl formed by the reaction of phenylmagnesium bromide with unreacted bromobenzene The most effective way to lessen this side reaction is said to be to add the bromobenzene slowly to the reaction mixture so that it will react with the magnesium and not be present in high concentration to react with previously formed Grignard reagent The impurity is easily eliminated since it is much more soluble in hydrocarbon solvents than benzoic acid The synthesis of benzoic acid must be accomplished in one lab period So plan accordingly Exp t 101 Cautions Ether is extremely flammable do not work with this solvent near flames or strongly heated hot plates or sand baths Some reagents are added slowly and dropwise to keep exothermic reactions from generating too much heat and boiling over Ether boils readily at 37 C the same temperature as your body Step 1 Preparation of Phenylmagnesium Bromide Phenyl Grignard Reagent Br MgBr anhydrous Mg Bromobenzene MW 157 02 bp 156 oC den 1 491 n20D 1 5590 Magnesium At wt 24 31 ether Phenylmagnesium bromide not isolated used in situ Advance Preparation It is imperative that all equipment and reagents be absolutely dry One lab period before you plan to run the reaction you should place the glassware to be used two reaction tubes two shorty vials and a stirring rod can be dried in a 110 C oven to dry along with the magnesium turnings Use anhydrous ether as solvent Rinse syringe and needle in dry anhydrous ether If it appears to be dirty obtain an unused syringe needle and syringe from stockroom Alternatively if the glassware syringe septa and magnesium appear to be perfectly dry they can be used without special drying The plastic and rubber ware should be rinsed with anhydrous ether prior to use Do not place plastic ware in the oven New sealed packages of syringes can be used without prior drying The ether used throughout this reaction must be absolutely dry use the anhydrous ether on the hooded shelf in the lab Don t use the ether under your hood However ether extractions of aqueous solutions can be carried out with the wet ether in your hood The container already on the hooded shelf is a 50 mL septum capped bottle This method of dispensing the solvent has three advantages The ether is kept anhydrous the exposure to oxygen is minimized and there is little possibility of its catching fire Ether is extremely flammable do not work with this solvent near flames To remove ether from a septum capped bottle inject a volume of air into the bottle equal to the amount of ether being removed Pull more ether than needed into the syringe and then push the excess back into the bottle before removing the syringe Try not to hold the syringe barrel in your warm hands as this will cause the ether to vaporize and squirt out of the syringe As the fluid gets lower in the bottle the short needle may not reach the fluid In this case turn the bottle upside down Remove a reaction tube from