Question

a) In the decarboxylative elimination, two different solvents are used, butanone and water. What types of solvents are these and why is that important for the ultimate course of reaction in each case? b) Starting with (R,R)-2-3-dibromo-3-phenylpropanoic acid, which alkene products you would expect to observe if doing the reaction in water? How about in butanone? Provide reaction mechanisms to support your answers. be different. c) which alkene product would be expected if (S,S)-2,3-dibromo-3-phenylpropanoic acid isomer was used as a substrate? How about (R,S) or (S,R) isomers? explain briefly. d) The final alkene beta-bromostyrene product was washed with brine solution. what is the function of the brine solution?

Answer 1

In both solvents (water and butanone) the first step is deprotonation of the acid by a base to give the corresponding carboxylate, which then eliminates Br^- and CO₂.

If the polarity of the solvent is low or intermediate (in the case for butanone), elimination of CO₂ and Br^- occurs in a single-step, concerted reaction (E2 type). In order to provide the most favorable geometry for the corresponding transition state (i.e., maximal overlap between the occupied σ orbital of the cleaving C-C bond and the empty σ* orbital of the C-Br bond) the bromine substituent and the carboxyl group must be in anti-position to each other, leading exclusively to the Z isomer.

In case of a highly polar solvent such as water, bromide loss occurs first (E1 type) to give the intermediate zwitterion, in which the positive charge is stabilized by delocalization on the aromatic ring. Free rotation around the C-C bond is allowed in this intermediate. In a second step carbon dioxide (CO₂) leaves perpendicularly to the plane of the cation (which provides the maximal overlap between the occupied σ orbital of the cleaving C-C bond and the empty p orbital of the positively charged carbon). Now, of the two possible transition state geometries the one in which the most bulky groups (phenyl and bromine) are opposite to each other, leading to the E isomer, is slightly more favored than the other. Therefore this mechanism leads mainly to the most stable E isomer.

(d) Brine is the saturated solution of sodium chloride. It is used during the workup process. It provides a good separation between organic and queous layers during workup process.