  The relativistic semiclassical evolution of the position of an electron in
the presence of an external electromagnetic field is studied in terms of a
Newton equation that incorporates spin effects directly. This equation emerges
from the Dirac equation and allows the identification of scenarios where spin
effects are necessary to understand the main characteristics of the electron
trajectories. It involves the eigenvalues of the non-Hermitian operator
$\Sigma_{\mu\nu}F^{\mu\nu}$ with $\Sigma_{\mu\nu}$ and $F^{\mu\nu}$ as the spin
and electromagnetic tensors. The formalism allows a deeper understanding on the
physics behind known analytical solutions of the Dirac equation when
translational dynamics decouples from spin evolution. As an illustrative
example, it is applied to an electron immersed in an electromagnetic field
which exhibits chiral symmetry and optical vortices. It is shown that the
polarization of intense structured light beams can be used to suppress or
enhance spin effects on the electron semiclassical trajectory; the latter case
yields a realization of a Stern-Gerlach apparatus for an electron
