SC CHEM 333 - Reactions of Carbocations Pre-Lab (6 pages)

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Reactions of Carbocations Pre-Lab



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Organic Chemistry 333L section 03M Laboratory Pre Lab Reactions of Carbocations By Cameron Kahn 22 October 2013 TA James Collie Objectives In this lab one will synthesize triphenylmethyl bromide from triphenylmethanol through a tritylcarbocation intermediate This experiment will also teach one how to find the mass melting point and percent yield of triphenylmethyl bromide Theory 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 University of Missouri In this experiment it is very similar to what Victor Grignard tested Carbocations are reactive intermediates in many organic reactions Triphenylmethanol is a tertiary alcohol and is the reason why it undergoes an SN1 reaction It is important to note that tertiary alcohols undergo Sn1 reactions and in order for the alcohol to react it must first form a carbocation and then the covalent bond can form Handy It is because of this that a 3o alcohol is needed in the reaction The following table shows the criteria for an Sn1 and Sn2 reactions 1 Page Criteria Sn1 Sn2 Solvent Choice Polar protic solvents Paolar aprotic solvents Methanol ethanol water DMSO Acetone acetic acid etc dichloromethane diethyl ether Leaving Group 3o 2o Methyl 1o 2o Nucleophile Moderate to weak Moderate to strong CN N3 Nucleophiles H2O CH3OH CH3S HO Br INH3 CH3CO RSH Rearrangement of Final product is a racemic Final product is an inversion Stereocenter mixture of stereochemistry Forms Carbocation Yes Rate Limiting Step No concentrated reaction the table is courtesy of the Chemistry 333 Lab Manual with copyrights to George Handy and Salman Ali Another thing to mention is that in general carbocations have a center carbon atom that is sp2 hybridized and has an order of stability that goes like this Allylic 3o 2o 1o methyl The more alkyl groups attached to the carbocation the more stable it will be Handy Triphenylmethyl carbocation is extremely stable and believed to be a super allylic carbocation with 10 resonance structures In this experiment triphenylmethanol dissolves in acetic aid It will then react with hydrobromic acid The intermediate carbocation will react with the bromide ion immmediately Handy This is seen in the following mechanism Mechanism 2 Page Procedure Start by placing 100 mg of triphenylmethanol into a small test tube Next add 2 mL of acetic acid and then heat the test tube in a hot water bath until all the solids have dissolved Take great care and insure to stir the contents of the test tube with a glass rod The TA will now need to add 0 2 mL 7 to 8 drops of concentrated HBr to your test tube This will need to take place under the fume hood Now heat the solutions for 5 10 minutes on a steam bath Take the test tube out of the bath and let the test tube cool to room temperature Next place the test tube in an ice bath to allow crystallization to occur The compound crystallizes slowly so allow adequate time for crystals to form 10 15 minutes Use suction filtration with a Buchner funnel to collect the cooled crystalline product and then wash the solids with 3 to 4 drops of water followed by 3 to 4 drops of ice cold hexane Drain and air dry the product for a few minutes before one transfers the crystalline product onto a piece of filter paper Invert a beaker over the transferred product and 3 Page let air dry Weigh the product and record the measurement Calculate percent yield and melting point and record the findings An important reminder is that he she will be expected to calculate the theoretical yield of triphenylmethyl bromide in the results Apparatus More zoomed in version on the Steam bath Apparatus pictures are copyright of George Handy and Salman Ali from the Chemistry 333 Lab manual 4 Page Reagent Table Name Structure Molecular weight g mol Melting Point oC Boiling Point oC Density g ml 60 0524 16 6 117 9 1 05 80 9119 87 66 38 1 49 260 3348 163 360 86 1766 95 69 659 323 2315 152 154 230 at 15mm Hg CH3COOH Acetic Acid Hydrobromic Acid HBr C19H16O Triphenylmethanol C6H14 Hexane C19H15Br Triphenylmethyl bomide Disposal Dispose of all products in their proper container Liquids go in liquids container Gloves go in glove bucket Glass goes in glass bucket Organic solids and organic waste go in the organic waste containers Place all paper towels in the trash can If unsure about any disposal procedure consult the TA 5 Page References 1 Ali George Handy and Salman Essentials of Organic Chemistry Montgomery AL 36117 QDE Press 2012 2 Missouri University of Experiment 8 part 1 Grignard Synthesis of Triphenylmethanol 28 Feb 2013 02 03 2013 http www chem missouri edu chem2140 Chem333LGrignard pdf 6 Page


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