Page 10 - Asam Basa

The Effect of subsituens on
the strength of acid and base

The acidity of this acid is due to the
presence one acidic hydrogen bonds to

oxygen atom. In aqueous solution this
acidic hydrogen may be released as

+
hydrogen ion (H ).
Three hydrogen atoms bond to
carbon atom are non-acidic

hydrogens. They can not be released as
hydrogen ions. These hydrogen atoms

can be replaced with electron
withdrawing substituents such as
halogen (F, Cl, Br, or I), Halogen behaves

as electron withdrawing substituent
because it is more electronegative than

hydrogen. Halogen substituent will
decreases electron cloud or electron density of O-H bond. This

may weaken and lengthen the O-H bond. Because of this the
acidic hydrogen atom is easier to release and the streng of acid

increases, Replacement one non-acidic hydrogenat with chlorine
atom gives chloroacetic acid as shown in figure 1.6.

Further replacement of non-acidic hydrogen atoms w chlorine

increases the strength of acid formed. For e the strength of
CCI COOH>CHCl COOH>CH CICOOH >CH COOH. Replacement of
3
2
2
3
non-acidic hydrogen atom with fluorine atom also gives acid
stronger than the acetic acid. Since electronegativity of fluorine is
higher than that of chlorine fluorine substituted acetic acid is

stronger than chlori substituted acetic acid. For example
trifluoroacetic acid (CF COOH) is stronger than trichloroacetic acid
3
(CCI COOH).
3
+