Why does the alkyl halide layer switch from a top layer to a bottom when #H_2O# is used to extract organic layer?

Most of the time, solvent extraction is performed with diethyl ether, for which the density, #rho#, #=# #0.71*g*mL^-1#. Because, the diethyl ether is less dense than water, the ethereal layer will be the top layer in the separating funnel, often even the ethereal layer is loaded with organic product.

On the other hand, consider the densities of methylene chloride, #CH_2Cl_2#, for which #rho#, #=# #1.33*g*mL^-1#, or chloroform, #CHCl_3#, #rho#, #=# #1.49*g*mL^-1#. Both of these solvents will collect at the BOTTOM of the separating funnel, even when they are NOT loaded with organic product.

Both halogenated solvents are considerably more expensive than say ether, which is why I suppose they are not habitually used in solvent extractions. The halogenated solvents are also much harder on the hands. If you get methylene chloride on the webbing between your fingers it stings. Also, there are special precautions necessary for the disposal of waste halogenated solvent.

At the end of the day, of course, you use the solvent that works best for a specific purpose. When you do your next solvent extraction, though, note the densities of the organic solvent, and make a prediction with respect to the top and bottom layers.