  We consider in this paper modified fractional Korteweg-de Vries and related
equations (modified Burgers-Hilbert and Whitham). They have the advantage with
respect to the usual fractional KdV equation to have a defocusing case with a
different dynamics. We will distinguish the weakly dispersive case where the
phase velocity is unbounded for low frequencies and tends to zero at infinity
and the strongly dispersive case where the phase velocity vanishes at the
origin and goes to infinity at infinity. In the former case, the nonlinear
hyperbolic effects dominate for large data, leading to the possibility of shock
formation though the dispersive effects manifest for small initial data where
scattering is possible. In the latter case, finite time blow-up is possible in
the focusing case but not the shock formation. In the defocusing case global
existence and scattering is expected in the energy subcritical case, while
finite time blow-up is expected in the energy supercritical case.
  We establish rigorously the existence of shocks with blow-up time and
location being explicitly computed in the weakly dispersive case, while most of
the results on the strongly dispersive case are derived via numerical
simulations, for large solutions. Moreover, the shock formation result can be
extended to the weakly dispersive equation with some generalized nonlinearity.
  We will also comment briefly on the BBM versions of those equations.
