An exceptional splitting of Khovanov’s arc algebras in characteristic 2

We show that there is an associative algebra $\widetilde H_n$ such that, over a base ring $R$ of characteristic 2, Khovanov’s arc algebra $H_n$ is isomorphic to the algebra $\widetilde H_n[x]/(x^2)$. We also show a similar result for bimodules associated to planar tangles and prove that there is no such isomorphism over $\mathbb Z$.