Page 43 - Q & A STPM 2022 Chemistry

Chemistry Term 3 STPM Chapter 14 Introduction to Organic Chemistry
The resonance structures of phenylamine is shown below:
+ + +
:NH 2 NH 2 NH 2 NH 2
– –
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Question 5

Given the following three acids:
Acid Structural formula
Propanoic acid CH 3 CH 2 COOH
2-Chloropropanoic acid CH 3 CH(Cl)COOH

3-Chloropropanoic acid CH 2 (Cl)CH 2 COOH
(a) Arrange the three acids in order of increasing acid strength. Explain your
answer using the concept of induction.
(b) Which of the above acids has the strongest conjugate base? Explain your
answer.

Answer:
(a) CH 3 CH(Cl)COOH > CH 2 (Cl)CH 2 COOH > CH 3 CH 2 COOH
The chloropropanoic acids are stronger than propanoic acid. This is
because chlorine is an electron-withdrawing group (negative inductive
effect). This helps to weaken the O!H bond in the molecule and increases
its degree of dissociation. 3-Chloropropanoic acid is a weaker acid than
2-chloropropanoic acid because the inductive effect of the chlorine atom
decreases when distance between the chlorine atom and the carboxyl
group increases.
CH 3 —CH—COOH Cl;CH 2 !CH 2 !COOH
p
Cl
(b) CH 3 CH 2 COOH would produce the strongest conjugate base.
A conjugate base is formed when an acid loses a hydrogen ion. For example:
HA L H + A –
+
Acid Conjugate base
If the acid is relatively strong, the above equilibrium will lie more to the Term
right-hand side. Then the tendency of the conjugate base to accept H is
+
low. The conjugate base is weak and vice versa. 3
Hence, the weakest acid would produce the strongest conjugate base.

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14 Q&A STPM Chem T3 Layout.indd 359 4/19/19 5:29 PM