Chem 333L Experiment 6 Pre-lab – Reactions of Carbocations Objectives 1. Synthesize triphenylmethylbromide from triphenylmethanol via a trityl carbocation intermediate. 2. Record mass, melting point, and calculate percent yield of triphenylmethylbromide. Theory 1. General properties of carbocations a. Orbital hybridization The central carbon atom is sp2 hybridized, which means the overall geometry of the molecule is trigonal planar. b. Relative stabilities The stability of carbocations is shown below: Allylic > 3° > 2° > 1° > methyl The allylic carbocation gains stability by spreading the positive charge to other atoms through resonance. For other groups such as the 3°, the positively charged carbon atom is stabilized by the inductive effects of the alkyl groups around it. For this reason, a methyl carbocation which has only hydrogen atoms around it is the least stable. The triphenylmethyl carbocation has both the resonance effects of an allylic compound as well as the inductive effects of the 3° compound. For this reason it is extremely stable and often referred to as a “super” allylic carbocation. Instead of the standard 2 resonance structures of an allylic compound, it has 10. 2. Reaction (SN1 Substitution) a. Mechanism As shown later in this report, the triphenylmethanol is first dissolved in the acetic acid (an ionizing, polar protic solvent), and then reacted with hydrobromic acid (a strong acid and good nucleophile). Both steps are designed for an efficient, quick SN1 reaction. b. SN1 and SN2 type reaction mechanisms Triphenylmethanol is a tertiary alcohol and thus can only undergo an SN1 reaction. S=represents “substitution” referring to the fact that one atom or group of atoms is substituted for another. N=represents the nucleophile to provide the pair of electrons needed to form the new covalent bond. 1 or 2= represents the number of molecules in the rate determining step of the reaction. SN2 reactions occur with two reagents that undergo substitution simultaneously. SN1 reactions only involve a single compound that reacts with the solvent in a multiple step process. SN2 reactions involve direct displacement of –OH. Primary alcohols react by direct displacement of –OH. Tertiary alcohols undergo SN1 reactions and in order for thealcohol to react it must first form a carbocation and then covalent bond can form. Unlike SN1, SN2 reactions are simultaneous and therefore seldom require the formation of a carbocation intermediate. SN1 reactions require three steps while SN2 reactions only require two. Why choose a 3° alcohol? Tertiary and primary alcohols undergo different reactions due to differences in steric hindrances and stability. Tertiary carbocations are very stable and require a low amount of energy for their formation. Primary carbocations are much less stable and therefore require a considerably greater amount of energy to form. • Theoretical Yield: Some compounds are regenerated during a reactions and thus do not act as limiting reagents regardless of the amount present. • Percent Yield: calculated by multiplying the ratio of actual yield to theoretical yield by 100%. MechanismProcedure 1. Place 100mg of triphenylmethanol into a small test tube. 2. Add 2mL of acetic acid and heat in a hot water bath until the solid dissolves. Stir with glass rod. 3. Have your TA add 0.2 mL of concentrated HBr to your test tube. 4. Heat solution for 5-10 minutes on a steam bath or in boiling water. 5. Let tube cool to room temperature, then use ice bath to cool further. Allow 10-15 minutes for compound to crystallize. 6. Collect the cooled crystalline product by suction filtration with a Buchner funnel then wash the solid on the Buchner funnel with 2-4 drops of water followed by 3-4 drops of ice cold hexane. 7. Drain the product well and air dry a few minutes on the Buchner funnel. 8. Transfer the product onto a piece of filter paper. Cover with a beaker and allow time to dry. 9. Record mass of the product and calculate percent yield. 10. Record the melting point of the product. ApparatusReagent Table Name, Structure, MW (g/mol) Melting °C Boiling °C Density (g/mL) Properties Acetic Acid CH3COOH 60.0524 16.6 117.9 1.05 Corrosive. Colorless liquid or solid with strong vinegar-like odor. Hydrobromic Acid HBr 80.9119 -87 -66.38 1.49 Corrosive Colorless gas Irritating, sharp odor Triphenylmethanol C19H16O 260.3348 163 360 - Crystalline white powder Hexane C6H14 -95` 69 .659 Colorless, flammable liquid. With a mild gasoline-like odor Triphenylmethylbromide C19H15Br 323.2315 152-154 230 at 15 mmHg - White solid Disposal • Wash excess aqueous solutions down drain with water. • Organic solvents are to be placed in the organic liquid waste container. • Dispose of soiled gloves, paper towels, and broken glass in the appropriate containers. References Chemistry 333L&334L: Microscale Experiments in Organic Chemistry Organic Chemistry 10th