Expt 6. Preparation of 1-(2-bromoethenyl)-4-methoxybenzene

revised 120418

Relevant textbook readings – Mohrig, Chapter 21.9. Klein, Chapter 9

Literature Reference - Evans, T. A. J. Chem. Educ. 2006, 83, 1062.

Overview

In this lab 4-methoxycinnamic acid will be reacted with N-bromosuccinimide (NBS) in the presence of catalytic triethylamine. The cinnamic acid undergoes a substitution reaction of Br for CO₂H under these conditions and thus gives 1-(2-bromoethenyl)-4-methoxybenzene (eq 1). Triethylamine catalyzes the reaction by deprotonating the carboxylic acid forming an alkenylcarboxylate, which reacts quickly with NBS to form a monobrominated neutral intermediate. (NBS functions as a source of positive bromine (Br⁺) in this reaction.) The intermediate then loses CO₂ to form the final product, which can exist as both an E and a Z isomer..

Procedure

Weigh 0.18 g  4-methoxycinnamic acid and 0.21 g NBS into a 5 or 6-mL 14/10 reaction vial equipped with a magnetic spin vane.

Add 3 mL CH₂Cl₂ followed by 10 μL of triethylamine.

Stir vigorously until the solids dissolve, approx  5–10  min.

Continue stirring while monitoring reaction progress both by TLC (CH₂Cl₂) as well as by paying attention to evidence of CO₂ evolution.

After the reaction is complete as indicated by TLC, use a chromatography column dry packed with silica gel (height of silica gel in column = approx 10 cm)  to remove succinimide, NBS and triethylamine from the product solution. First run hexane (15 mL) through the column so as to wash off any impurities present in the silica gel. Then add the reaction solution followed by 15 mL CH₂Cl₂.

Dry the CH₂Cl₂ solution over Na₂SO₄ and remove the solvent on the rotary evaporator.

Determine the mp and obtain a ¹H NMR spectrum of the product. One group will asked to be obtain a C-13 NMR spectrum.

Questions

1. Based on the observed mp as well as the coupling constants observed for the alkene doublets as compared to literature values, which stereoisomer of 1-(2-bromoethenyl)-4-methoxybenzene was formed?
2. Propose a mechanism for the reaction that accounts for the observed stereoselectivity.
3. The proton NMR of the product shows clear evidence for the presence of an impurity in the form of small doublets at 5.4, 6.0, 6.9, and 7.4 ppm. Propose an identity for this impurity and suggest a mechanism for its formation.