Acids and Bases

• It may be surprising if some one says that nitric acid can behave as a base. Yes it can under certain conditions. For instance nitric acid behaves as a base with respect to sulphuric acid in the nitration of benzene. At the same time nitric acid is a well known acid and it behaves as such in most reactions.
• The concept of acids and bases therefore is relative. A given compound is an acid with respect to another compound which is a base in a particular reaction.
• Thus water may behave as an acid in some reactions or as a base in others at the same time we consider it as neutral. In short whether a compound is an acid or a base depends on the compound with which it is reacting.
• Here are some features of acids and bases.

ACIDS : turn blue litmus red, react with a base to form salt. BASES: turn red litmus blue, react with acids to form salt.

LOWRY -BRONSTED concept of acids and bases

ACID : proton donor Example: HCl, CH₃COOH,

BASE: proton acceptor Example: NH₃, NaOH,

LEWIS Concept of acids and bases

ACID: Electron pair acceptor(has an atom with a empty orbital which can accomodate a pair of electrons)AlCl₃.

BASE: electron pair donor. NH₃ (has an atom with an orbital which has a pair of electrons)

Strong acid: An acid which dissociates completely in solution (HCl).

H₂O + HCl —> H₃O⁺ + Cl^-

• In the above example HCl donates a proton and water accepts it, therefore HCl is an acid and water is a base, also HCl undergoes complete dissociation so it is a strong acid.
• H⁺ which is a proton is not capable of independent existence in solution, it exists in combination with a water molecule as H₃O⁺ which is called hydronium ion or hydroxonium ion.

Weak acid: An acid which undergoes partial dissociation and exists as an equlibrium mixture of the undissociated acid and the ions produced due to partial dissociation. (CH₃COOH). Most carboxylic acids are weak acids.

CH₃COOH + H₂O —> CH₃COO^- + H₃O⁺

This equilibrium has an equilibrium constant called dissociation constant or acidity constant, K_a which can be determined by experiment.

• The larger the K_a greater is the dissociation of the acid hence stronger is the acid.
• The value of K_a is a fraction which is inconvenient for use in comparison, K_a is transformed into pK_a a by a mathematical transformation K_a = -log pK_a
• Thus for acetic acid K_a = 1.8 x 10^-5 and pK_a = -log [1.8 x 10^-5] = 4.75. It is evident that pK_a is a number which can be easily compared. Since it is related by -log to K_a, smaller value of pK_a means stronger acid.
• Note: stronger acid and not strong acid.
• The relative strength of acids and bases can be compared through their dissociation constant values that is K_a or through pK_a.

Conjugate acid base pairs

It is a pair of acid and base which differ by a proton, each can be formed from the other through the loss or gain of a proton

In the acetic acid equilibrium,

There are two pairs of acids and bases. Acetic acid and acetate ion are a conjugate acid base pair because they differ by a proton and each can be formed from the other through the loss or gain of a proton. Similarly water and hydronium ion are another conjugate acid base pair.

Some features of conjugate acids and bases and the equilibrium

1. The equilibrium is in favor of the weak acid and the weak base.
2. The conjugate acid of a strong base is a weak acid.
3. Strongest base has the weakest conjugate acid.
4. The concept of acids and bases is relative.

Similarly the strength of bases can be expressed through K_b and pK_b values.

1. Larger K_a stronger base, larger pK_b weaker base
2. Larger K_a stronger acid or weaker base.
3. Larger pK_b weaker base or stronger acid.

Acid

pK_a

Base

pK_b

HCOOH

3.75

NH₃

4.75

CH₃COOH

4.76

CH₃NH₂

3.34

ClCH₂COOH

2.86

(CH₃)₂NH

3.27

Cl₂CHCOOH

1.29

(CH₃)₃N

4.19

Cl₃CCOOH

0.70

C₆H₅NH₂

9.37

• Between two acids which one is stronger can be assessed from their K_a or pK_a values.(similarly for bases from K_b and pK_bvalues)
• Organic Chemists however would like to assess the comparative acid or base strength from the structure.

Comparative acid base strength through study of structure

• Between two acids which one is stronger can be assessed from their K_a or pK_a values.(similarly for bases from K_b and pK_bvalues)
• Organic Chemists however would like to assess the comparative acid or base strength from the structure.

• The acid which gives rise to the more stable anion is a stronger acid. (the anion is formed through the loss of a proton)
• The comparative stability of the anion is judged by comparing their structures.
• Electron withdrawing groups stabilise the anion thus making the acid a stronger acid and electron donating groups would therefore lower the acid strength.
• A base which gives rise to a more stable cation is a stronger base (the cation is formed after it accepts a proton).
• Also the base which can donate an elctron pair more easily is a stronger base.
• The comparative stability of the cations is judged by comparing their structures.
• Generally electron donating groups would stabilise the cation and the corresponding base would be a stronger base.