Which is good leaving Group OH- or Cl-?

[predent7]

PKa of HCl and H2O are -7, and 15.7.

What does this information gives about relative leaving group ability of OH- or Cl-?

Thanks.

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[MowgliR]

Cl- is a better LG than -OH

 

[qkchen]

Cl- is a better LG than -OH

i concur. in mechanisms, the -OH has to be protonated into a -OH2+ in order to make it a relatively good leaving group.

 

[UndergradGuy7]

PKa of HCl and H2O are -7, and 15.7.

What does this information gives about relative leaving group ability of OH- or Cl-?

Thanks.

HCl = strong acid (lower pKa, higher Ka) so strong acid gives a weak conjugate base (Cl-).

H2O is weak acid, gives a stronger conjugate base OH-.

Strong base = bad leaving group. Think, a strong base wants to react. So it will not want to leave. It cannot stay "alone."

 

[predent7]

HCl = strong acid (lower pKa, higher Ka) so strong acid gives a weak conjugate base (Cl-).

H2O is weak acid, gives a stronger conjugate base OH-.

Strong base = bad leaving group. Think, a strong base wants to react. So it will not want to leave. It cannot stay "alone."

Thanks guys, what a good explanation.. " Think, a strong base wants to react. So it will not want to leave." !

 

[predent7]

Cl- is a better LG than -OH

what a good blog site randy ..!

 

[orgohacks]

Understanding pKas is KEY to doing well in organic chem. This can't be overemphasized. Strong bases (like OH-, NH2-) are bad leaving groups.

 

[476515678234896]

^^pka's weren't really that important in rxns for my ochem class. we only emphasized it during peptides, proteins, amides, amines, CarboxAcids, ect....not a lot w/ lg's and rxns.....


normal?

 

[sonofastunna]

OH is a horrendous leaving group.

 

[orgohacks]

^^pka's weren't really that important in rxns for my ochem class. we only emphasized it during peptides, proteins, amides, amines, CarboxAcids, ect....not a lot w/ lg's and rxns.....


normal?

Seems unusual. To me, pKas are like the Rosetta stone of organic chemistry, and I'm only exaggerating slightly. The LG ability is directly proportional to the pKa of the conjugate acid... also, strong bases also tend to be extremely good nucleophiles.

 

[predent7]

Seems unusual. To me, pKas are like the Rosetta stone of organic chemistry, and I'm only exaggerating slightly. The LG ability is directly proportional to the pKa of the conjugate acid... also, strong bases also tend to be extremely good nucleophiles.

So are they parallel, LG ability is directly proportional to the pKa?
wOw, I knew new things !

 

[MowgliR]

what a good blog site randy ..!

thanks! haha follow the blog!