Page 42 - Q & A STPM 2022 Chemistry

Chemistry Term 3 STPM Chapter 14 Introduction to Organic Chemistry

O – O –

This delocalisation decreases the negative charge of the oxygen atom thus
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stabilising it.
On the other hand, the negative charge of the oxygen atom in the ethoxide
ion cannot undergo such delocalisation making it less stable than the
phenoxide ion.
As a result, phenol is a stronger acid than ethanol.
The delocalisation of the negative charge in the phenoxide ion occurs via
the following resonance structures:
:O – O O O

– –

–
(b) Methylamine and phenylamine are proton acceptors. They are Bronsted-
Lowry bases.
..
+
CH 3 NH 2 + H L CH 3 NH 3 +
..
!NH 2 + H !: !NH 3 +
+
The molecules make use of their lone pair electrons on the nitrogen atom
to form a coordinate bond with H . The strength of the base depends on
+
its readiness to combine with H . The higher the tendency, the stronger
+
the base.
The methyl group in methylamine is an electron-donating group. It
increases the electron density of the lone pair electrons on the nitrogen
atom making them more ready to combine with H . On the other hand,
+
the lone pair electrons in phenylamine can delocalise into the benzene
ring, via resonance, and make them less available to combine with H .
+
H H
N
H H
As a result, methylamine is a stronger base than phenylamine.

Term
3

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14 Q&A STPM Chem T3 Layout.indd 358 4/19/19 5:29 PM