Advices For Studying Organic Chemistry

the transition state (lowers its free energy) ⇒ lowers the free energy of

activation ⇒ increases the rate of the reaction.

3. Relative reactivity of some leaving groups:

Leaving group TosO– I– Br– Cl– F– HO–, H 2 N–, RO–

Relative reactivity 60,000 30,000 10,000 200 1 ~0

4. Other good leaving groups:

−O S

O

O

R

−O S

O

O

OR

O S

O

O

− CH 3

An alkanesulfonate ion An alkyl sulfate ion p-Toluenesulfonate ion

1) These anions are all the conjugate bases of very strong acids.

2) The trifluoromethanesulfonate ion (CF 3 SO 3 – , triflate ion) is one of the best

leaving group known to chemists.

i) It is the anion of CF 3 SO 3 H, an exceedingly strong acid –– one that is much

stronger than sulfuric acid.

CF 3 SO 3 – , triflate ion

(a “super” leaving group)

5. Strongly basic ions rarely act as leaving groups.

X− R OH R X + OH−

This reaction doesn't take place because the

leaving group is a strongly basic hydroxide ion.

1) Very powerful bases such as hydride ions (H:–) and alkanide ions (R:–) virtually

never act as leaving groups.