i.e. #RCH=CHR#...and you could add an acid, which we will represent as #D_3O^+#...the which contains electrophilic protons/deuterons...
#RCH=CHR + stackrel(+)D-OD_2rarr Rstackrel(+)CH-CH(D)R+D_2O#
The electrophilic carbocation should then react with the hydroxyl function....
#Rstackrel(+)CH-CH(D)R+OD_2 rarrRCH(OD)-CH(D)R+D^+#
And by way of example, we could bubble ethylene gas, #H_2C=CH_2#, thru a solution containing water, and ethanol, and halide salts, and cyanide salts....there would be NO reaction UNTIL we added a source of #H_3O^+#...and this would PROTONATE the olefin to give a formal carbocation, the which would then react with ANY nucleophile present in the solvent (including the solvent itself)......
#Rstackrel(+)CH-CH_2Rstackrel(H_2O,""^(-)C-=N, X^-)rarrRCH(OH)-CH_2R+RCH(C-=N)-CH_2R+RCH(X)-CH_2R#
