A Guidebook to Mechanism in Organic Chemistry

The Strengths of Acids and Bases

+ HsO®

H—C H—C^0

o

+ H.O

H—C

Oe

H—C

O

O—H O

themselves in solution by collecting around themselves a solvation

envelope of water molecules.

Water has the advantage of being able to function as an acid or a

base with equal facility, which further increases its usefulness and

versatility as an ionising solvent. It does however have the disadvan­

tage as an ionising solvent for organic compounds that some of them

are insufficiently soluble in the unionised form to dissolve in it in the

first place.

(iii) The origin of acidity in organic compounds

Acidity in an organic compound, YH^nay be influenced by

(a) The strength of the Y—H bond,

(b) The electronegativity of Y,

(c) Factors stabilizing Ye compared with YH,

but of these (a) is normally foundto be of little significance. The effect

of (b) is reflected in the fact that the pKa of methanol, CHsO—H, is

«16 while that of methane, H 3 C—H, is « 50, oxygen being consider­

ably more electronegative than carbon. By contrast, the pKa of formic

acid,

O.

II

. H-C-O—H
is 3ml3. This is in part due to the electron-withdrawing carbonyl group
enhancing the electron affinity of the oxygen atom to which the
incipient proton is attached but much moretimportant is the stabilisa­
tion possible in the resultant formate anion compared with the
undissociated formic acid molecule: