Patricia Yang; James R. O’Haro

Department of Chemistry,

University of Missouri

205 Schlundt Hall

600 S. College Avenue

Columbia, MO 65211

Telephone: (636) 579-0747

Email: py896@mizzou.edu

Professor Rainer Glaser, Associate Editor

The Journal of Organic Chemistry

321 Chemistry Building

601 S. College Avenue

Columbia, MO 65211

April 26, 2010

RE: jo-2010-0689a – Revised

Meta-positioned Methyl and Nitro Substituents on Acidity of Benzoic Acids

By Patricia Yang and James R. O’Haro

Dear Dr. Glaser:

Thank you very much for your letter regarding our manuscript, Meta-positioned Methyl

and Nitro Substituents on Acidity of Benzoic Acids. We are very pleased with the

thorough review of our work and are glad for its favorable reception. After consideration

of our reviewers’ comments, we have effected a revision and the changes are as follows.

Response to Reviewer 1.

[1.1] Give “more weight” on the abstract section by adding pKa values, etc: the values of

the pKa of both disubstituted benzoic acids and of benzoic acid itself were added to the

abstract.

[1.2] Define “acidity” and why it is important: while we respect the request to define

acidity, we decided not to as we believe this is an elementary enough principle that

readers of such prestigious journals will already understand its definition. A further

description of why acidity is important was added to the introduction.

[1.3] Is the effect of the methyl substituent significant enough to mention: The difference

it made on the pKa of the molecule was small enough to be within the range of error,

therefore it was considered an insignificant change, as explained in the Conclusion.

[1.4] Reformat figures in Appendix: reformatted as requested.

[1.5] “Upmost” was suggested as a replacement for “utmost”: we looked up the

dictionary definition of both terms and deduced that utmost was a better fit for what we

were trying to say.

Response to Reviewer 2.

[2.1] Rename schemes: renamed as requested.

[2.2] Reformat spectroscopy: reformatted as requested.

Response to Reviewer 3.

[3.1] “a” was added before highly on line 5 of the Introduction.

[3.2] In line 8 of the Introduction, “especially” was removed to prevent redundancy.

[3.3] Move citation 3 to the end of the sentence with citation 4: this sentence was left as-

is to keep clarity, as citation 3 and citation 4 both address different topics.

[3.4] Rename schemes: renamed as requested.

[3.5] Fix the text of schemes in Materials and Methods to match the rest of the document:

fixed as requested.

[3.6] On page 5, use full name of DMSO for first reference: fixed as requested.

[3.7] Remove endnote separator: removed as requested.

[3.8] Explain why the solution changes color in the synthesis of 3,5-dinitrobenzoic acid

in the appendix section: explained as requested (the yellow precipitate again dissolves in

the solution).

[3.9] Reformat figures in appendix: reformatted as requested.

[3.10] Don’t capitalize “Water”: fixed as requested.

[3.11] Clarify the difference between predicted spectra and other spectra: We were

unable to locate any true spectra for 3-methyl-5-nitrobenzoic acid for this project and

therefore had to use predicted spectra. The spectra for 3,5-dimethylbenzoic acid were

included to show that we weren’t just slacking off, even though it was not required to be

in the appendix.

We believe that these changes will address all of the reviewer comments and put our

manuscript in good stead for publication.

Regards,

James and Patricia

Meta-positioned Methyl and Nitro Substituent Effects

on Acidity of Benzoic Acids

Patricia Yang* and James R. O’Haro

Department of Chemistry, College of Arts & Sciences, University of Missouri, Columbia,

Missouri, 65201. PY896@mail.missouri.edu

Abstract

Two disubstituted benzoic acids, 3,5-dinitrobenzoic acid and 3-methyl-5-

nitrobenzoic acid, were synthesized via a novel procedure. The pKa of these molecules

were then assayed by potentiometric titration. The results were analyzed to determine the

effect of the meta substituents on the acidity of the benzoic acid, and compared with the

acidities of related benzoic acids. The electronegativity of the substituent group was

found to correspond to the change in the pKa of the substituted benzoic acid. The higher

electronegativity of the nitro group caused the pKa to decrease, while the lower

electronegativity of the methyl group had relatively little effect on the pKa of the

substituted benzoic acid. The pKa of 3,5-dinitrobenoic acid in DMSO was determined to

be 7.67, and the pKa of 3-methyl-5-nitrobenzoic acid was found to be 9.36. These values

were compared to the pKa of benzoic acid, having a value of 11.00.

Graphical Abstract

Introduction

Substituted benzoic acids are widely used in medicinal and other biological

applications, as well as in the synthesis of more complex molecules. Benzoic acids have a

broad range of uses: from food preservatives – in its salt form, sodium benzoate1 – to

NSAID painkillers such as aspirin, the derivative acetylsalicylic acid. The carboxy

substituent is a highly reactive functional group from which a variety of other functional

groups can be formed. Benzoic acid-derived monomers are used to make highly

specialized polymers such as nylon, Kevlar, and Mylar.

In particular, methyl and nitro-substituted benzoic acids are relevant in practical

application. Terephthalic acid is a precursor to the insect repellent DEET, and

furthermore, many billions of kilograms of terephthalic acid are produced each year in

the formation of PET plastics and polyesters.2 Nitrobenzoic acids can be used as highly

versatile catalytic interfaces for NADH electrooxidation3 and also have the additional

property when reacted with lead oxides to be used in electro-explosive device

manufacture.4 With biological applications, acidity is of utmost concern because of its

possible consequences on molecular activity and function. For example, the efficacy of

the preservative sodium benzoate is dependent upon the acidity of its medium.5

Additionally, many drugs require a certain acidity in order to be effective in the human

body. The purpose of this paper is to explore the effects of methyl and nitro substituents

on the acidity of benzoic acid.

Specifically, here we report the results of the synthesis of 3-methyl-5-nitrobenzoic

acid and 3,5-dinitrobenzoic acid by novel methods, and the analysis of these molecules

for their relative acidities as affected by the identity of its meta-substituents (Scheme 1).

We expect that the electron-withdrawing effect of the electronegative nitro group will

additively lower the pKa of the compound with each substitution. Additionally, we

anticipate that the electron donating effect of the methyl groups will additively raise the

pKa of the compound with each substitution, and the addition of both nitro and methyl

groups will offset each other.

3,5-dimethylbenzoic acid 3-methyl-5-nitrobenzoic acid 3,5-dinitrobenzoic acid

Scheme 1. Disubstituted Benzoic Acids By Increasing Acidity.

Materials and Methods

Synthesis of 3-methyl-5-nitrobenzoic acid was conducted by a nucleophillic acyl

substitution of methyl 3-methyl-5-nitrobenzoate (synthesized by the procedure outlined

in the accompanying appendix). Aqueous NaOH with methanol and THF were added to

the starting ester under heat. Ethyl acetate was added and the carboxylate product was

acidified with hydrochloric acid (Scheme 2).

Scheme 2. Synthesis of 3-methyl-5-nitrobenzoic acid.

The synthesis of 3,5-dinitrobenzoic acid was accomplished via a simple nitration

of benzoic acid. The benzoic acid reagent was subjected to a steam bath of fuming HNO3

and concentrated H2SO4 (Scheme 3). Please refer to appendix for additional synthetic

details and procedures.

Scheme 3. Synthesis of 3,5-dinitrobenzoic acid.

The pKa measure of acidity can be experimentally determined. One commonly

used method is that of acid-base titration: the acid, in this case our disubstituted benzoic

acid, is titrated against a base of known concentration. Changes in the pH during the

titrations can be monitored with a glass pH electrode (probe); this procedure is called

potentiometric titration.6 The electrode generates a potential in proportion to the

hydronium ion concentration in the solution. The pH meter will electronically convert

this potential to pH. Plotting the experimentally determined data points of pH versus

volume of base added generates an s-shaped titration curve, where the equivalence point

represents the pKa value.

Results and Discussion

The equilibrium constant, Ka, of a reaction is the ratio of products to reactants.

With an acid, such as the substituted benzoic acids we studied here, the reaction is an

equilibrium with the acid solvated with dimethyl sulfoxide (DMSO) present in its acid

and conjugate base forms. A larger Ka value for an acid indicates its greater tendency to

drive towards the products (its conjugate base form) by donating a proton; thus a greater

Ka designates a stronger acid. Customarily, however, the pKa value (equal to the

negative logarithm of Ka) is used. The smaller the pKa value is, the stronger the acid.7

The values of the experimentally determined pKa’s of the molecules we

synthesized can be seen in Table 1. The lowest pKa was that of 3,5-dinitrobenzoic acid,

having a value of 7.67. The 3-methy-5-nitrobenzoic acid had a higher pKa of 9.36,

which was closer to the value of the pKa of benzoic acid, being 11.00.

When a single nitro group is added in the 3-position, the pKa is dropped by a

value of about 1.6 to become 9.26. When a second nitro group is added to the 5-position,

the pKa is dropped again by a nearly identical amount (to a pKa value of 7.67). When a

methyl group is added in the 3-position to a benzoic acid, the pKa stays roughly the same,

at 11.21. A further addition of a methyl group to the 5-position also results in only small-

scale changes in the pKa, rising to 11.29. Similarly, the addition of a methyl group to a

nitrobenzoic acid causes almost no change in that pKa as well.

The drop in pKa with the addition of a nitro group can be attributed to the

substituent’s relatively high electronegativity, having a value of 8.61 eV, which pulls the

electron density away from the acidic hydrogen.8 This then pulls electron density away

from the O-H bond and allows the hydrogen atom to more easily dissociate from the

substituted benzene molecule. The lack of influence by the methyl group on the value of

pKa of the various methyl-substituted benzoic acids is caused by the relatively low

electronegativity of the methyl group, with a value of 4.41 eV.8 This low

electronegativity does not noticeably change the electron density near the acidic

hydrogen and therefore does not cause the pKa to change significantly in value.

Table 1. Experimental pKa for the various substituted benzoic acids.

No. Compound Name Structure Solvent Experimental

pKa

Reference

No.

1 3-methyl-5-nitrobenzoic

acid

DMSO 9.36 This Work

2 3,5-dinitrobenzoic acid

DMSO 7.67 This Work

3 3,5-dimethylbenzoic acid

DMSO 11.29 This Work

4 3-methylbenzoic acid

DMSO 11.21 9

5 3-nitrobenzoic acid

DMSO 9.26 9

6 benzoic acid

DMSO 11.00 10

Conclusion

After determining the results of our experiments, we found that our original

hypothesis was both correct and incorrect. We were correct in assuming that the nitro

groups would lower the pKa of the substituted benzoic acids cumulatively—lowering it

each time by a value of around 1.6. However, we were incorrect in thinking the methyl

group would raise the pKa. Though the experiments did point to an average increase in

the value of the pKa of about 0.2, this was not statistically significant enough to exceed

the possibility of error. Our belief that the effects of the nitro and methyl groups would

offset was also wrong, since the methyl groups do not raise the pKa high enough to

impede the decrease caused by the presence of the nitro groups.

What this means is that methyl groups can be added to a benzoic acid group

without affecting the equilibrium of the solution, whereas nitro groups cannot. This

knowledge can be further applied in the event that one needs to increase the acidity of a

benzoic acid in order to react it with a base and produce a salt containing the benzene

ring.

Supplemental Material

The appendix contains detailed explanations for the syntheses of the two new

compounds (compound 1 and compound 2) as well as spectroscopic characterization and

identification of synthesized compounds. The appendix can be obtained from the

corresponding author at: PY896@missouri.edu.

References

Maki, T.; Takeda, K. Benzoic Acid and Derivatives. Verlag GmbH & Co. KGaA. 2005,

1, 1-14.

2 Ogata, Y.; Tsuhida, M.; Muramoto, A. The Preparation of Terephthalic Acid from

Phthalic or Benzoic Acid. J. Am. Chem.. Soc., 1957, 79, 6005-6008.

Santhiago, M.; Lima, P.; Santos, W. In situ activated 3,5-dinitrobenzoic acid covalent

attached to nanostructured platform for NADH electrooxidation. Electrochimica Acta.

2009, 54, 6609–6616.

4 Orbovic, N.; Luco, A. Production of Exploding Materials for Electro Explosive

Devices. Propellants, Explosives, Pyrotechnics. 2008, 33, 271-278.

5 Goshorn, R. H.; Degering, Ed F.; Tetrault, P. A. Antiseptic and Bactericidal Action of

Benzoic Acid and Inorganic Salts Effect of PH. Ind. Eng. Chem. 1938, 30, 646-648.

Pytela, O.; Kulhánek, J.; and Ludwig, M. Chemometrical Analysis of Substituent

Effects. IV. Additivity of Substituent Effects in Dissociation of 3,5-Disubstituted Benzoic

Acids in Organic Solvents. Collect. Czech. Chem. Commun. 1994, 59, 1637-1644.

Jover, J.; Bosque, R.; Sales, J. QSPR Prediction of pKa for Benzoic Acids in Different

Solvents. QSAR Comb. Sci. 2008, 27, 563-581.

Proft, F. De; Langenaeker, W.; Geerlings, P. Ab Initio Determination of Substituent

Constants in a Density Functional Theory Formalism: Calculation of Intrinsic Group

Electronegativity, Hardness, and Softness. J. Phys. Chem. 1993, 97, 1826-1831.

Pytela, O.; Kulhánek, J.; and Ludwig, M. Chemometrical Analysis of Substituent

Effects. XIII. Compariston of Substituent Effects on Dissociation and Chemical Shift in

13C NMR Spectra of Mono- and Disubstituted Benzoic Acids. Collect. Czech. Chem.

Commun. 2000, 65, 106-116.

Bordwell, Frederick G. Equilibrium Acidities in Dimethyl Sulfoxide Solution. Acc.

Chem. Res. 1988, 21, 456-463.

Supporting Information

Meta Substituent Effects on Benzoic Acid Acidity

Patricia Yang* and James R. O’Haro

Department of Chemistry, College of Arts & Sciences, University of Missouri, Columbia,

Missouri, 65201. PY896@mail.missouri.edu

Table of Contents

3,5-Dinitrobenzoic Acid ................................................................................................................. 3

Synthesis of 3,5-Dinitrobenzoic Acid .......................................................................................... 3

Spectroscopy of 3,5-Dinitrobenzoic Acid ................................................................................... 3

3-Methyl-5-Nitrobenzoic Acid....................................................................................................... 6

Synthesis of 3-Methyl-5-Nitrobenzoic Acid ................................................................................ 6

Spectroscopy of 3-Methyl-5-Nitrobenzoic Acid ......................................................................... 6

3,5-Dimethylbenzoic Acid .............................................................................................................. 8

Spectroscopy of 3,5-Dimethylbenzoic Acid ................................................................................ 8

3,5-Dinitrobenzoic Acid

Synthesis

In a large round-bottom flask, 61 g (0.5 mol) of benzoic acid was added to 300

mL of conc. sulfuric acid. Under the hood, 100 mL of fuming nitric acid was then added

dropwise to the reaction mixture while maintaining at constant 70-90˚C by cold water

bath. The flask was covered with a watch glass and allowed to stand 1 h. Following, the

flask was heated on a steam bath for 4 h, during which large amounts of brown gas is

evolved. The mixture was cooled to room temperature, whereupon a yellow precipitate

settles from the solution. An additional portion of 75 mL fuming nitric acid was added to

the solution and again heated in the steam bath for 3 h, and in an oil bath (135-145˚C) for

3 h. Brown fumes should continue to be evolved during heating, and the solution should

become a light to reddish yellow as the precipitate dissolves in the solution. The flask

was allowed to cool and poured onto 800 g of ice and 800 mL of water for 30 min to

complete cooling. The product was isolated by vacuum filtration and washed with water,

and recrystallized using hot 50% ethanol. Experimental yield turned out to be 58%.

Spectroscopy

Instrumental analysis performed with an internal TMS standard. Predicted spectra

calculated from ACD/Labs software at 25 °C at a working frequency of 400 MHz. Other

spectra referenced were taken with a JEOL AL-400 (399.65 MHz) and a NEVA NV-14

(15.087 MHz) for proton and carbon NMRs, respectively.

Figure 1. Proton NMR Spectrum of 3,5-Dinitrobenzoic Acid in DMSO.

Figure 2. Carbon-13 NMR Spectrum of 3,5-Dinitrobenzoic Acid in DMSO.

Figure 3. IR Spectrum of 3,5-Dinitrobenzoic Acid by KBr disc.

Figure 4. Mass Spectroscopy of 3,5-Dinitrobenzoic Acid at 75eV.

3-Methyl-5-Nitrobenzoic Acid

Synthesis

900 mg (4.0 mmol) 3-(methoxycarbonyl)-5-nitrophenylboronic acid was placed in

a round bottom flask charged with 27 mg (0.12 mmol) palladium acetate. 16 mL

tetrahydrofuran, 112 mg (0.27 mmol) tri-1-napthylphosphine, 1.70 g (800 mmol)

potassium phosphate, and 0.370 mL (5.9 mmol) methyl iodide were added under constant

flow of nitrogen. 0.14 mL (7.8 mmol) water was added to the solution and stirred at

room temperature overnight. The reaction was diluted with water and extracted three

times with ethyl acetate. The product was then washed with water and brine, dried, and

concentrated in a vacuum. This was then purified via silica gel chromatography to yield

a white solid, methyl 3-methyl-5-nitrobenzoate.

A solution of 0.36 g methyl 3-methyl-5-nitrobenzoate and 3 mL aqueous sodium

hydroxide in 1.5 mL methanol and 3 mL tetrahydrofuran was heated at 80˚C for 1 h and

at rt for 2 h. An additional 1 mL sodium hydroxide solution was added and the mixture

was heated at 80˚C for 5 min.

The reaction mixture was concentrated by vacuum filtration, diluted with ethyl

acetate and acidified with hydrochloric acid. The aqueous layer was extracted twice with

ethyl acetate. The combined organic layers were washed with brine, dried with MgSO4,

and concentrated in vacuo to give 3-methyl-5-nitrobenzoic acid as a white solid. The

percent yield was 94%.

Spectroscopy

Instrumental analysis performed with an internal TMS standard. Predicted spectra

calculated from ACD/Labs software at 25 °C at a working frequency of 400 MHz. Other

spectra referenced were taken with a JEOL AL-400 (399.65 MHz) and a NEVA NV-14

(15.087 MHz) for proton and carbon NMRs, respectively.

Figure 4. Proton NMR Predicted Spectrum 3-Methyl-5-Nitrobenzoic Acid.

Figure 5. Carbon-13 NMR Predicted Spectrum 3-Methyl-5-Nitrobenzoic Acid.

3,5-Dimethylbenzoic Acid

Spectroscopy

Figure 1. Proton NMR of 3,5-Dimethylbenzoic Acid.

Figure 2. Carbon-13 NMR of 3,5-Dimethylbenzoic Acid.

Figure 3. Infrared Spectrum of 3,5-Dimethylbenzoic Acid.

Additional Citations.

1. Pytela, O.; Kulhánek, J.; and Ludwig, M. Chemometrical Analysis of Substituent

Effects. IV. Additivity of Substituent Effects in Dissociation of 3,5-Disubstituted

Benzoic Acids in Organic Solvents. Collect. Czech. Chem. Commun. 1994, 59,

1637-1644.

2. Brewster, R. Q.; Williams, B.; and Phillips, R. 3,5-Dinitrobenzoic Acid. Organic

Syntheses Collective. 1978, 3, 337-338.