Lab 2 - Friedel Crafts Reaction

Lab 2 - Friedel Crafts Reaction - Lab 2: The Friedel-Crafts...

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Lab 2: The Friedel-Crafts Reaction Discussion: Analysis of the 1 H and the 13 C[ 1 H] NMR spectra of the product leads to the conclusion that the final product is 4,4’-di- t -butylbiphenyl – please refer to Figure 1. In the 1 H NMR spectra, there are 3 peaks indicating 3 different types of hydrogen atoms; in the 13 C[ 1 H] NMR spectra, there are 6 peaks indicating 6 different types of carbon atoms. From the fact that the original biphenyl already contains 2 different hydrogens and 4 different carbons, it is reasonable to conclude that the final product must have symmetry. The single peak at 1.331 ppm with intensity 17.00 on the 1 H NMR spectra indicates that the tert -butyl groups must also be symmetrical because all 18 uncoupled methyl hydrogens are equal. Other possible asymmetric isomers – see Figure 2 – from this reaction would yield a greater number of 13 C and 1 H peaks at varying chemical shifts in the NMR spectra. For example, a meta-substituted isomer has at least 5 different hydrogens and 8 different carbons. Applying principles of EAS, the tert -butyl groups attach para relative to one another at both ends of the biphenyl molecule. The mechanism, referred to as an Electrophilic Aromatic Substitution, involves using a strong Lewis acid catalyst – AlCl 3 in this case – to polarize the carbon-halogen bond and cleave t -butyl chloride to form a carbocation. A double bond in the aromatic ring attacks the t -butyl carbocation to form a resonance stabilized intermediate. Aside from the NMR analysis, the major product in Figure 1 is favored because it is stabilized by
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This note was uploaded on 10/08/2011 for the course CHEM 3BL taught by Professor Chunmei during the Fall '08 term at University of California, Berkeley.

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Lab 2 - Friedel Crafts Reaction - Lab 2: The Friedel-Crafts...

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