page 23

Question 1

What Makes a Good Leaving Group?

Answer 1

A good leaving group is a weak base, meaning it can stabilize the negative charge effectively after leaving.
The lower the pKa of the conjugate acid, the better the leaving group.

Question 2

Trends in Leaving Group Ability:

Answer 2

Halides (
C
l
−
Cl
−
,
B
r
−
Br
−
,
I
−
I
−
) are good leaving groups because they are weak bases.
Water (
H
2
O
H
2
​
O) is a good leaving group when formed from protonated alcohols (
R
−
O
H
2
+
R−OH
2
+
​
).

Question 3

Order of Leaving Group Ability:

Answer 3

I
−
>
B
r
−
>
C
l
−
>
H
2
O
I
−
>Br
−
>Cl
−
>H
2
​
O
This trend corresponds to increasing stability of the leaving group.

Question 4

Why is
I
−
I
−
a better leaving group than
C
l
−
Cl
−
?

Answer 4

Answer:
I
−
I
−
is a weaker base and more stable due to its larger size and polarizability.

Question 5

What is the leaving group when
R
−
C
l
R−Cl undergoes nucleophilic substitution?

Answer 5

Answer:
C
l
−
Cl
−

Question 6

Arrange
H
2
O
H
2
​
O,
C
l
−
Cl
−
,
B
r
−
Br
−
, and
I
−
I
−
in order of increasing leaving group ability.

Answer 6

Answer:
H
2
O
<
C
l
−
<
B
r
−
<
I
−
H
2
​
O<Cl
−
<Br
−
<I
−

Question 7

Which leaving group has the strongest conjugate acid,
C
l
−
Cl
−
or
H
2
O
H
2
​

Answer 7

O?
Answer:
H
2
O
H
2
​
O has the strongest conjugate acid (
H
3
O
+
H
3
​
O
+
, pKa = -1.7).