Lab8 - L ab 8 Preparation of Benzoic Acid using a Grignard Reagent Chen Zhou(4857694 Partner Haotian Wu Demonstrator Craig Mckay March 26th 2008

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Lab # 8 Preparation of Benzoic Acid using a Grignard Reagent Chen Zhou (4857694) Partner: Haotian Wu Demonstrator: Craig Mckay March 26 th , 2008 Purpose: To synthesize benzoic acid using a Grignard reagent. Introduction: Grignard reagent are organometallic compounds that have an organic component plus a metal (usually a transition element), and these are used to form bonds between carbon atoms. Grignard reagents have the structure of RMgX, where R is either alkyl, vinyl or aryl; X is a halogen (usually bromine or chlorine). In this experiment, the reagent is made by reacting an organic compound, R-X in the form of bromobenzene, with magnesium in the diethyl ether solution to make R-MgX, where the metal becomes oxidized to Mg 2+ . A formal negative charge is placed on the alpha carbon of the Grignard reagent, making it a strong nucleophile. The mechanism for the reaction is shown below. Grignard reagents are very important to nucleophilic displacements with compounds containing polarized double bonds. For example, the addition of Grignards to aldehydes or ketones generates secondary and tertiary alcohols respectively. From the reaction, magnesium alkoxides are produced, and they have to be hydrolyzed during the workup phrase. This is usually done with a mild acid to provide the free hydroxyls and magnesium
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salts. A nucleophilic displacement describes the collapse of the intermediate, and H + and H 2 O are used in the workup phase to isolate the desired product. Grignards do not normally react with alkyl halides, as they only react with active electrophiles such as the carbonyl groups, and epoxies at the least substituted position. Grignards can also undergo acid-base reactions, as they are strong bases with a pK a over 30. Grignards will react with any acids functionality such as alcohols, acids and phenols, and they are converted to an unreactive related functional group. One critical experimental consideration is the presence of water. Any water in the reaction will protonate any Grignard reagent, and strongly interferes with the formation of the magnesium intermediate. To remove water from the reaction and equipment, the glassware needs to be heated in the oven. Since the glass absorbs water from the air after it cools, the apparatus must be assembled quickly while the parts are still hot. Dry ether will be used for the reaction, because hydrous ether will absorb small quantities of water. Procedure and observations:
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This note was uploaded on 01/30/2011 for the course CHEM 1321 taught by Professor Rashmi during the Spring '10 term at Carleton CA.

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Lab8 - L ab 8 Preparation of Benzoic Acid using a Grignard Reagent Chen Zhou(4857694 Partner Haotian Wu Demonstrator Craig Mckay March 26th 2008

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