I changed the summary image to have a more concrete example up top. Fast (reversible) formation of an anion (the conjugate base) followed by slow elimination.

If there were no neighboring electron withdrawing group would the carbanion be sp3 — suggesting it requires an anti position to kick out the leaving group? Or do carbanions (like secondary/tertiary amines) tend not to be chiral because they can spontaneously invert? Or do almost all E1cb mechanisms require neighboring electron withdrawing groups (like carbonyls or NO2) which means they will all have sp2 character like the aldol reaction?

Lastly, where you write: “Note – apparently, when F or Cl is the leaving group, deprotonation is the slow step. However, when the leaving group is Br, or I, deprotonation is rate-determining and breakage of C-X is fast.” I think for F or Cl you might have meant to say the deprotonation is a “fast” (not slow) step.

Thanks!