Experiment Six - Experiment Six Reactions of Carbocations...

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Experiment Six Reactions of Carbocations O BJECTIVES 1. Synthesize triphenylmethyl bromide from triphenylmethanol via a trityl carbocation intermediate. 2. Record mass, melting point, and calculate percent yield of triphenylmethyl bromide. T HEORY 1. General properties of carbocations a. Orbital hybridization The center carbon atom in a carbocation is sp 2 hybridized, which means the other three substituents attached are coplanar and have 120 o angles. b. Relative stabilities The order of stability for carbocations is as follows: Allylic > 3 o > 2 o > 1 o > methyl The allylic carbocation is the most stable because the positive charge can be delocalized through resonance. Alkyl groups can help stabilize through inductive effects (the donation of electron density via sigma bonds). Therefore, the more alkyl groups attached to the carbocation, the more stable it will be. The methyl carbocation is so unstable, it cannot be isolated and is believed to never form. The triphenylmethyl carbocation is extremely stable. It can be viewed as a “super” allylic carbocation. Whereas a normal allylic carbocation only has two resonance structures, the triphenylmethyl carbocation has ten!! They are shown on the next page: 1 CHEM 331L Experiment 6: Reactions of Carbocations C C C + C
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C C C C C C C C C C 2. Reaction (substitution reaction) Overall reaction a. Reaction mechanisim In this experiment, triphenylmethanol is first dissolved in acetic acid (a good ionizing, polar protic solvent), and it is then reacted with hydrobromic acid (a strong acid and nucleophile). The intermediate carbocation will react with the bromide ion immediately. 2 CHEM 331L Experiment 6: Reactions of Carbocations H O C C Br HBr S N 1 Reaction: General: Triphenylmethanol Triphenylmethyl bromide
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Rate determining step: Generation of the high energy carbocation. b. S N 1 and S N 2 type reaction mechanisms Triphenylmethanol is a tertiary alcohol and for this reason it undergoes S N 1 reactions. S N 1: S = represents the fact that one atom or group of atoms is substituted for another atom or group of atoms. For example: Br for OH. N = represents the nucleophile, which is Br ion in this reaction, to provide the pair of electrons to form the new covalent bond. 1 = represents a unimolecular transition state, meaning that a single molecule is present in the transition state. One substituent must leave before another can attack. S
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This note was uploaded on 02/08/2012 for the course CHEM 331l taught by Professor Mr.handy during the Fall '11 term at South Carolina.

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Experiment Six - Experiment Six Reactions of Carbocations...

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