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Question 1

Poor Leaving Groups for Nucleophilic Substitution

Answer 1

Question 2

What Makes a Poor Leaving Group?

Answer 2

Poor leaving groups are strong bases, meaning they are unstable when separated from the substrate.
Conjugate bases of weak acids are poor leaving groups, indicated by their high pKa values.

Question 3

Trends in Leaving Group Ability:

Answer 3

Leaving group ability decreases with increasing basicity.
Example: F⁻ < ⁻OH < ⁻NH₂ < H⁻.

Question 4

Why Do These Molecules Not Undergo Substitution?

Answer 4

Poor leaving groups resist leaving the substrate, making nucleophilic substitution difficult or impossible.

Question 5

Why is ⁻OH a poor leaving group?

Answer 5

Answer: ⁻OH is a strong base with a high pKa of its conjugate acid, H₂O (15.7), making it unstable as a leaving group.

Question 6

Arrange F⁻, ⁻OH, ⁻NH₂, and H⁻ in order of increasing leaving group ability.

Answer 6

Answer: ⁻NH₂ < H⁻ < ⁻OH < F⁻.

Question 7

Which has a better leaving group: HF or H₂O?

Answer 7

Answer: HF has a better leaving group (F⁻) because it has a lower pKa (3.2) compared to H₂O (15.7).

Question 8

Why do alkyl groups (R⁻) not act as leaving groups?

Answer 8

Answer: Alkyl groups are extremely strong bases with very high pKa values (≈50), making them highly unstable as leaving groups.

Question 9

Answer 9

Part 1: Which is the better leaving group in each pair?

Cl⁻ or I⁻
Answer: I⁻ is the better leaving group because it is a weaker base and more stable as a free ion due to its larger size and lower charge density.
NH₃ or ⁻NH₂
Answer: NH₃ is the better leaving group because it is neutral and less basic compared to ⁻NH₂, which is highly basic and unstable as a free ion.
H₂O or H₂S
Answer: H₂O is the better leaving group because it is less basic than H₂S, and its conjugate acid (H₃O⁺) has a lower pKa than that of H₂S (indicating stronger leaving group ability).

Question 10

Answer 10

Which molecules contain good leaving groups?

CH₃CH₂CH₂Br
Answer: Yes, this molecule contains a good leaving group (Br⁻), as bromide is a weak base and an effective leaving group.
CH₃CH₂CH₂OH
Answer: No, hydroxide (⁻OH) is a poor leaving group because it is a strong base.
CH₃CH₂OH₂⁺
Answer: Yes, this molecule contains a good leaving group (H₂O). When the oxygen atom is protonated, water can leave easily as it is a neutral and stable molecule.
CH₃CH₃
Answer: No, this molecule does not have a leaving group.