OCHEM 2 The Grignard Synthesis of Triphenylmethanol

OCHEM 2 The Grignard Synthesis of Triphenylmethanol - The...

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The Grignard Synthesis of Triphenylmethanol Lindy Hilpert Chemistry 340 Aaron Crandall March 19, 2008
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Introduction The purpose of this experiment was to synthesize a Grignard reagent, phenyl magnesium bromide, then use it to synthesize the alcohol, triphenylmethanol. After the alcohol was synthesized, the goal was to purify it through recrystallization, then characterize it based on its melting point, IR spectrum, C13-NMR, and H-NMR spectra. Theory A Grignard reagent is a type of organometallic, which consists of a bond between a metal and a carbon. There are three types of carbon-metal bonds: ionic, polar covalent, and covalent. The ionic bonded compounds (example: RNa) have a weak bond between the carbon and the metal, and are therefore not useful because they are so volatile, and they will react with nearly anything. The compounds that are polar covalent bonded are Grignard reagents (example: RMgBr), and are useful in making carbon-carbon bonds and reducing carbonyls. The covalent bonded compounds (example: R2Pb) are toxic. Anhydrous ether is often used as a solvent in creating Grignard reagents because it keeps out water and oxygen, makes the complex soluble, and is non-reactive. Water and oxygen cause undesired side reactions. The oxygen in ether has a lone pair of electrons which is attracted to the partially positive metal. Grignard reagents are made through single electron transfers with magnesium and an alkyl halide. In reality, the Grignard reagent forms a Schlenk equilibrium when being formed. A Schlenk equilibrium is an equilibrium that is obtained when a Grignard reagent is formed. It is an equilibrium between 2 equivalents of RMgX and R2Mg + MgX2. The Grignard reagent oxidizes a carbonyl into a hydroxyl group. The metal is less electronegative than the carbon, so the carbon bears a partial negative charge. This partial negative charge attacks the carbonyl at the partially positive carbon, forms a new carbon-carbon bond, and pushes an electron pair out of the double bond into the lone pair position. The metal then attaches itself at the now negatively charged oxygen. This compound is then treated with an aqueous acid to protonate the oxygen and form the hydroxyl group. Recrystallization is a way of purifying non-volatile organic substances. It consists of dissolving the substance to be purified in an appropriate hot solvent. An appropriate solvent will dissolve the substance when it is hot, but not when it is cool. Also, when the
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mixture of the solvent and substance are cooled, the crystals should reform quickly and neatly. It must also dissolve impurities readily so that they do not become trapped in the purified substance when it is cooling and recrystallizing. The solvent cannot react with the solute by breaking bonds. It cannot chemically alter the solute, only dissolve it, by breaking the intermolecular forces. Its boiling point must be lower than the solute’s melting point so that the solute does not melt and oil out in the solvent. Lastly, it should
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This lab report was uploaded on 04/07/2008 for the course CHEM 0340 taught by Professor Crandall during the Fall '07 term at Pittsburgh.

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OCHEM 2 The Grignard Synthesis of Triphenylmethanol - The...

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