  The normalized singular chains of a path connected pointed space $X$ may be
considered as a connected $E_{\infty}$-coalgebra $\mathbf{C}_*(X)$ with the
property that the $0^{\text{th}}$ homology of its cobar construction, which is
naturally a cocommutative bialgebra, has an antipode, i.e. it is a
cocommutative Hopf algebra. We prove that a continuous map of path connected
pointed spaces $f: X\to Y$ is a weak homotopy equivalence if and only if
$\mathbf{C}_*(f): \mathbf{C}_*(X)\to \mathbf{C}_*(Y)$ is an
$\mathbf{\Omega}$-quasi-isomorphism, i.e. a quasi-isomorphism of dg algebras
after applying the cobar functor $\mathbf{\Omega}$ to the underlying dg
coassociative coalgebras. The proof is based on combining a classical theorem
of Whitehead together with the observation that the fundamental group functor
and the data of a local system over a space may be described functorially from
the algebraic structure of the singular chains.
