1.2.4 Acid–base properties and pH
One of the adverse effects of aspirin is stomach bleeding, which is partly
due to its acidic nature. In the stomach, aspirin is hydrolysed to salicylic
acid. The carboxylic acid group (À ÀCOOH) and a phenolic hydroxyl group
(À ÀOH) present in salicylic acid make this molecule acidic. Thus, intake of
aspirin increases the acidity of the stomach significantly, and if this
increased acidic condition remains in the stomach for a long period, it
may cause stomach bleeding. Like aspirin, there are a number of drug
molecules that are acidic in nature. Similarly, there are basic and neutral
drugs as well. Now, let us see what these terms acid, base and neutral
compounds really mean, and how these parameters are measured.
O
OH
O
O
OH
OH
O
Hydrolysis in the stomach
Aspirin
Salicylic acid
Simply, an electron-deficient species that accepts an electron pair is called
an acid, e.g. hydrochloric acid (HCl), and a species with electrons to donate
is a base, e.g. sodium hydroxide (NaOH). A neutral species does not do
either of these. Most organic reactions are either acid–base reactions or
involve catalysis by an acid or base at some point.
Arrhenius acids and bases
According to Arrhenius’s definition, an acid is a substance that
produces hydronium ion (H 3 O
þ ), and a base produces hydroxide ion
(OH
À
) in aqueous solution. An acid reacts with a base to produce salt and
water.
HCl (Acid) þ NaOH (Base) ÀÀÀ*
)ÀÀÀNaCl (Salt) þ H 2 O (Water)
Brønsted–Lowry acids and bases
The Danish chemist Johannes Brønsted and the English chemist Thomas
Lowry defined an acid as a proton (H
þ ) donor, and a base as a proton (H
þ
)
acceptor.
HNO 2 (Acid)þH 2 O (Base) ÀÀÀ* )ÀÀÀNO
À
2 (Conjugate base)
þ H 3 O
þ (Conjugate acidÞ
6
CH1 INTRODUCTION
One of the adverse effects of aspirin is stomach bleeding, which is partly
due to its acidic nature. In the stomach, aspirin is hydrolysed to salicylic
acid. The carboxylic acid group (À ÀCOOH) and a phenolic hydroxyl group
(À ÀOH) present in salicylic acid make this molecule acidic. Thus, intake of
aspirin increases the acidity of the stomach significantly, and if this
increased acidic condition remains in the stomach for a long period, it
may cause stomach bleeding. Like aspirin, there are a number of drug
molecules that are acidic in nature. Similarly, there are basic and neutral
drugs as well. Now, let us see what these terms acid, base and neutral
compounds really mean, and how these parameters are measured.
O
OH
O
O
OH
OH
O
Hydrolysis in the stomach
Aspirin
Salicylic acid
Simply, an electron-deficient species that accepts an electron pair is called
an acid, e.g. hydrochloric acid (HCl), and a species with electrons to donate
is a base, e.g. sodium hydroxide (NaOH). A neutral species does not do
either of these. Most organic reactions are either acid–base reactions or
involve catalysis by an acid or base at some point.
Arrhenius acids and bases
According to Arrhenius’s definition, an acid is a substance that
produces hydronium ion (H 3 O
þ ), and a base produces hydroxide ion
(OH
À
) in aqueous solution. An acid reacts with a base to produce salt and
water.
HCl (Acid) þ NaOH (Base) ÀÀÀ*
)ÀÀÀNaCl (Salt) þ H 2 O (Water)
Brønsted–Lowry acids and bases
The Danish chemist Johannes Brønsted and the English chemist Thomas
Lowry defined an acid as a proton (H
þ ) donor, and a base as a proton (H
þ
)
acceptor.
HNO 2 (Acid)þH 2 O (Base) ÀÀÀ* )ÀÀÀNO
À
2 (Conjugate base)
þ H 3 O
þ (Conjugate acidÞ
6
CH1 INTRODUCTION