PROBLEM 7.48 - Organic Chemistry

Question

Question: Rank the species in each group in order of increasing leaving group ability.

a. Br^-,Cl^-,I^-

b. NH₃,H₂S,H₂O

Step-by-Step Solution

Answer

Answer

a. Cl^-< Br^- <I^-

b. NH₃ < H₂O <H₂S

Leaving group ability

When the size of the leaving group atom increases, it attains stable and independent existence. 

Another measure to identify the better leaving group is its  pK_a value.

Lesser the value of pK_a , the greater the ability it has to act as a leaving group. 

The given molecules

a. In this set of ions, the size of  I^- is the largest and next  Br^- is larger than Cl^-.

Thus, the leaving group ability is as shown below:

Cl^- < Br^- < I^-

 

b. The values of the given compounds are as follows:

NH₃        35

H₂O        15.7

H₂S         7

The best leaving group is the one that has less pK_a (acid dissociation constant) value, The order is shown below:

NH₃ < H₂O < H₂S