Organic Chemistry

Section 1.19 An Introduction to Delocalized Electrons and Resonance 49

PROBLEM 36

For each of the following compounds, indicate which is the stronger base:

PROBLEM 37 SOLVED

HCl is a weaker acid than HBr. Why, then, is a stronger acid than

SOLUTION To compare the acidities of HCl and HBr, we need to compare the stabili-

ties of and Because we know that size is more important than electronegativity in

determining stability, we know that is more stable than Therefore, HBr is a

stronger acid than HCl. In comparing the acidities of the two carboxylic acids, we need to

compare the stabilities of and (An O H bond is broken in both com-

pounds.) Therefore, the only factor to be considered is the electronegativities of the atoms

that are pulling electrons away from the oxygen atom in the conjugate bases. Because Cl is

more electronegative than Br, Cl is better at inductive electron withdrawal. Thus, it is bet-

ter at stabilizing the base that is formed when the proton leaves.

PROBLEM 38

a. Which of the halide ions is the strongest base?

b. Which is the weakest base?

PROBLEM 39

a. Which is more electronegative, oxygen or sulfur?

b. Which is a stronger acid, or

c. Which is a stronger acid, or

PROBLEM 40

Using the table of values given in Appendix II, answer the following:

a. Which is the most acidic organic compound in the table?

b. Which is the least acidic organic compound in the table?

c. Which is the most acidic carboxylic acid in the table?

d. Which is more electronegative, an hybridized oxygen or an hybridized oxygen?

(Hint:Pick a compound in Appendix II with a hydrogen attached to an oxygen and

one with a hydrogen attached to an oxygen, and compare their values.)

e. What are the relative electronegativities of and sphybridized nitrogen atoms?

f. What are the relative electronegativities of and sphybridized carbon atoms?

g. Which is more acidic, or Why?

1.19 An Introduction to Delocalized Electrons

and Resonance

We have seen that a carboxylic acid has a of about 5, whereas the of an alco-
hol is about 15. Because a carboxylic acid is a much stronger acid than an alcohol, we
know that a carboxylic acid has a considerably more stable conjugate base.

pKa pKa

HNO 3 HNO 2?

sp^3 , sp^2 ,

sp^3 , sp^2 ,

sp^3 pKa

sp^2

sp^3 sp^2

pKa

CH 3 OH CH 3 SH?

H 2 O H 2 S?

(F-, Cl-, Br-, I-)

RCOO- R¿COO-. ¬

Br- Cl-.

Cl- Br-.

BrCH 2 COOH?

ClCH 2 COOH

CH 3 CHCO– or CH 3 CHCO–

O O

CH 3 CCH 2 CH 2 O– or CH 3 CH 2 CCH 2 O–

O O

Br F

CH 3 CHCH 2 CO– or CH 3 CH 2 CHCO–

O O

Cl Cl

BrCH 2 CO– or CH 3 CH 2 CO–

O O

a. c.

b. d.