Acids and Bases - Organic Chemistry Concepts: An EFL Approach - page 101

E In CF 3 CH 2 CH 2 OH, the CF 3 group is too far from the OH to change its acidity. The
resulting acidity is about the same as CH 3 CH 2 CH 2 OH. However, in CH 3 CF 2 OH, the
electronegative fluorine substituents are next to the OH. Therefore, the acidity is greatly
increased.
We can use this same method to compare carboxylic acids. Program 17 shows that
carboxylic acids all have the same resonance forms, but inductive effects cause their
pK a values to be different.
Try to explain the order of acidity shown in the following series of acids.
F As stated earlier, all of the acids have the same possible resonance forms.
The direction and size of any inductive effect of R changes the polarization of the
acidic O-H bond and the electron density of the carboxylate conjugate base.
The least acidic is ethanoic acid (R ] CH 3 ). It has a +I effect from the CH 3 - group
compared to the hydrogen atom of formic acid (R ] H). All the other examples have
increasing −I effects at the α -carbon. The halogenated examples show the effect of the
number, type, and placement of the halogen substituents.
The remaining examples show the similar −I effect of methylammonium and acetyl groups.
Q 6.8. Arrange the following compounds in order of increasing acidity.
CH 3 CO 2 H; CH 3 CH 3 ; Cl 3 CH 2 CH 2 CO 2 H; ClCH 2 CO 2 H; CH 3 CH 2 OH;
CH 3 CHO