In: Chemistry
Rank the following functionalities from worst to best leaving group ability. Which favor an Sn1 mechanism and why?
F-, OH-,OTs-,Br-,H2O
The order worst to best leaving group is
OH- < F- < Br-< H2O < OTs-
The leaving group ability mainly depends on two factors
1) strength of C - X (X: leaving group) bond
and
2) Polarization of C-X bond
Or we can say
A good leaving group is a bulkier weak base/acid.
OTs- is a weak conjugate base of TsOH strong acid since it is a bulkier base, its overlap is weakest with C.
H2O is a weak conjugate acid of strong base OH- ion so H2O is a good leaving group.
Due to the protonation of OH with H+ ions, H2O is formed. C-O overlap will decrease and H2O becomes good leaving group.
Br- is weak conjugate base of strong acid HBr
F- and OH- have the same size but F- is more electronegative than O so OH is worst leaving group than F