Page 41 - Q & A STPM 2022 Chemistry

Chemistry Term 3 STPM Chapter 14 Introduction to Organic Chemistry

(a) Which is a stronger acid, I or II. Explain your answer.
(b) Arrange acids II, III and IV in order of increasing acid strength. Explain your
answer.
(c) How would the strength of acid IV change if the !NO 2 group is replaced
by !NH 2 group?
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Answer:
(a) Acid II > acid I
The benzene ring in acid II is an electron-withdrawing group that weakens
the O!H bond in !COOH.

Exam Tips
Exam Tips
Unsaturated acids or aromatic acids are stronger acids than aliphatic acids.
(b) III < II < IV
!OH is an electron-donating group that strengthens the O!H bond
in the !COOH group. On the other hand, !NO 2 is an electron-
withdrawing group that weakens the O!H bond.
(c) The acid strength will decrease. This is because —NH 2 is an electron-
donating group.

Section C Essay Questions

Question 4
Using the concept of resonance explain the differences in
(a) the acidity between C 2 H 5 OH and C 6 H 5 OH
(b) the basicity between CH 3 NH 2 and C 6 H 5 NH 2
Answer:
(a) Ethanol and phenol act as acids via the following dissociation process:
C 2 H 5 O!H L C 2 H 5 O + H +
–
Ethoxide ion
OH O –
& &
L + H +

Phenol Phenoxide ion Term
The strength of the acids depends on the degree of dissociation. This in
turn depends on the stability of the anions formed. The more stable the
anions, the higher the degree of dissociation. Due to the presence of the 3
benzene ring, the negative charge of the oxygen atom in the phenoxide
ion can delocalise into the benzene ring via resonance:

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