# Reaction of cinnamic acid with Br2/Na2CO3

The first reaction produces benzaldehyde, and the next one (perkin's condensation)produces Cinnamic acid.(X)

Now the treatment of X with $$\ce{Br2/Na2CO3}$$ is whats troubling me. $$\ce{Na2CO3}$$ being a base, abstracts the hydrogen from the $$\ce{COOH}$$ group. $$\ce{Br2}$$ reacts with the alkene portion to yield a cyclic intermediate. What next?

The solution claims that somehow The $$\ce{CO2-}$$ group leaves and a bromoalkene forms. The addition Of moist KOH in the next step results in $$\ce{E2}$$ elimination to yield option (c).

I cant quite digest the $$\ce{CO2-}$$ group leaving. I thought of a mechanism, which is similar to the syn-elimination in esters: but that doesnt work out well..

Any hints/ insights will be appreciated. The correct answer is (c).

## 1 Answer

The sodium carbonate is just there to mop up any $$\ce{HBr}$$. The $$\ce{Br2}$$ adds across the double bond to give cinnamic acid dibromide.

The treatment of cinnamic acid dibromide with $$\ce{KOH}$$ at elevated temperature eliminates 2 eq. of $$\ce{HBr}$$ to give Phenylpropiolic acid. This is known to decarboxylate through the intermediate acetylene anion.

• I agree with answer (c) but, possibly, a different mechanism. The post says that a bromoalkene is formed first. Dibromocinnamic acid in base concertedly loses CO2 and bromide to give (Z)-(2-bromovinyl)benzene. This reaction is known and is stereospecific using amines as the base. [Note: Bromination of methacrylic acid followed by heating the dibromide in pyridine give 2-bromopropene]. KOH then effects E2 elimination of the vinyl bromide to give phenyl acetylene. I think this is a made-up problem and I think your mechanism, in the presence of KOH, would prevail in the flask! – user55119 Jul 30 '20 at 12:23
• ...continued. Reference for the stereospecific decarboxylation: DOI:10.1016/j.tet.2005.02.043 – user55119 Jul 30 '20 at 18:03
• Just a doubt, in our lectures the difference between alcoholic KOH and aqeuous KOH was taught. I guess it's aqueous KOH here with 200°c. It was taught to us that aqueous KOH can only do substitutions not E2. – vanshita rawat Sep 25 '20 at 3:32
• @vanshitarawat note the temperature at which the reaction is - 473K - there is no KOH in solution as there no liquid water present. – Waylander Sep 25 '20 at 6:45