CHEM 202 California State University San Marcos Acidity of Benzoic Acids Worksheet

When benzoic acid is substituted with a chloro- group, the ortho isomer is the most acidic and the para isomer is the least acidic; when benzoic acid is substituted with nitro group, the ortho isomer is the most acidic and the meta isomer is the least acidic; and when benzoic acid is substituted with an amino group the meta isomer is the most acidic and the ortho isomer is the least acidic.

Use the provided data to explain these trends:

Cl: ortho > meta > para NO2: ortho > para > meta NH2: meta > para > ortho

Write an essay (between 400–500 words in length) that thoroughly explains the difference in the pKa’s of these benzoic acids. Your response should consider the

Guiding Questions

but should be written in essay form. Be extremely clear in your written arguments. Avoid using ambiguous nouns such as “it” – instead, refer to species by their proper names (e.g., name of the acid, name of conjugate base, the amino group, nitro group, the negative charge, etc.).

Guiding Questions

1. 1) What do the conjugate bases of these benzoic acid look like and how they influence their acidity?
2. 2) What is the effect of halogens on benzoic acid acidity?
3. 3) What is the effect of nitro- or amino group on benzoic acid acidity?
4. 4) Draw out the conjugate base of each of these 9 acids and their resonance forms.
5. 5) Is the nitro-, amino- or chloro- group involved in the resonance? If so, how?
6. 6) Is the resonance affecting each of the ortho- meta- and para positions? If not, which position is notaffected and why?
7. 7) How is the induction affecting the benzoic acid acidity?
8. 8) In conclusion, what effect does resonance and induction have on the relative acidity of benzoic acids andhow they affect each position in benzoic acid molecule?

Student Essay:
Comparing Acidity of Benzoic Acids (15 pts)
The values of pKa’s for several carboxylic acids are provided below:
When benzoic acid is substituted with a chloro- group, the ortho isomer is the most acidic and the para isomer is
the least acidic; when benzoic acid is substituted with nitro group, the ortho isomer is the most acidic and the meta
isomer is the least acidic; and when benzoic acid is substituted with an amino group the meta isomer is the most
acidic and the ortho isomer is the least acidic.
Use the provided data to explain these trends:
Cl: ortho > meta > para
NO2: ortho > para > meta
NH2: meta > para > ortho
Write an essay (between 400–500 words in length) that thoroughly explains the difference in the pKa’s of these
benzoic acids. Your response should consider the Guiding Questions but should be written in essay form. Be
extremely clear in your written arguments. Avoid using ambiguous nouns such as “it” – instead, refer to species
by their proper names (e.g., name of the acid, name of conjugate base, the amino group, nitro group, the negative
charge, etc.).
Guiding Questions
1)
2)
3)
4)
5)
6)
What do the conjugate bases of these benzoic acid look like and how they influence their acidity?
What is the effect of halogens on benzoic acid acidity?
What is the effect of nitro- or amino group on benzoic acid acidity?
Draw out the conjugate base of each of these 9 acids and their resonance forms.
Is the nitro-, amino- or chloro- group involved in the resonance? If so, how?
Is the resonance affecting each of the ortho- meta- and para positions? If not, which position is not
affected and why?
7) How is the induction affecting the benzoic acid acidity?
8) In conclusion, what effect does resonance and induction have on the relative acidity of benzoic acids and
how they affect each position in benzoic acid molecule?
Pay attention to the essay format and spelling!
If is hand written make sure I could understand what you meant to say and all the drawings are
carefully done! Make reference to the drawings while explaining concepts.
The way you present the answer is as important as the answer!
This assignment was designed to help student improve their grades by evaluating them with an
instrument other than an exam, to stimulate student to apply classroom information in explaining
concepts of organic chemistry, to help students in making connections between chemistry information,
to have students practice their writing skills.