Laboratory Report on Properties of Carboxylic acids

• Ramona Mae S. Rajaratnam

Abstract:

This report presents the different properties of carboxylic acids including solubility, acidity of some carboxylic acids, difference in strength of carboxylic acids compared to phenols, action of oxidizing agent on the carboxylic group and the neutralization equivalent of carboxylic acids. Carboxylic acids like acetic acid, butyric acid, oleic acid, succinic acid, stearic acid and benzoic acid were each mixed with water to test their solubility. The same acids were each mixed with 10% sodium bicarbonate to test their acid strength. The typical pKa values of carboxylic acids, phenols, HCO₃^– and CO₃^2- were used to compare the acid strength of carboxylic acids with phenols and to judge whether both Na₂CO₃ and NaHCO₃ can be used to successfully separate phenols from carboxylic acids. Carboxylic acids like acetic acid, formic acid, lactic acid, succinic acid and oxalic acid were each mixed with 0.5% KMnO₄ to look at the action of KMnO₄, an oxidizing agent, on the carboxylic acid group. This report also focuses on the finding the neutralization equivalent to determine the unknown molar mass of a carboxylic acid. An accurately weighed sample of an unknown carboxylic acid was dissolved, heated and titrated with a previously standardized NaOH solution to find the neutralization equivalent and ultimately, the molar mass of the unknown carboxylic acid.

Introduction:

This experiment focuses on the different properties of carboxylic acids. The experiment aims to compare the solubility of acetic acid and stearic acid in water and to describe the relationship between molecular weight and solubility of carboxylic acids in water. The experiment also intends to infer the relative acidities of carboxylic acids and phenols based on the relative differences of their reaction with NaHCO₃ and explain how NaHCO₃ can be used to separate a mixture containing a water-insoluble carboxylic acid and a water insoluble phenol. The experiment also aims to identify reducing acids and the functional groups responsible for their reduction potential. The experiment also intends to describe a physical property such as physical state, color, odor or solubility that can differentiate succinic acid and oxalic acid, acetic acid and lactic acid, acetic acid and formic acid, benzoic acid and stearic acid and acetic acid and butyric acid. And lastly, the experiment looks into the determination of the neutralization equivalent and molar mass of an unknown mono- and dicarboxylic acid.

Experimental Details:

The following apparatus were used in the experiment:

• Vials
• Vial rack
• Micro spatula
• Dropper
• Test Tubes
• Test Tube Rack
• Weighing boat
• 50 mL Buret
• Iron stand
• Buret clamp
• Erlen Meyer flasks
• Funnel
• Corks
• Graduated Cylinder
• Bunsen burner
• Wire gauze
• Test tube brush
• Vial brush
• Safety goggles

The following materials were used in the experiment:

• Distilled water
• Acetic acid
• Butyric acid
• Oleic acid
• Stearic acid
• Succinic acid
• Benzoic acid
• Formic acid
• Lactic acid
• Oxalic acid
• 10% NaHCO₃
• 0.5% KMnO₄
• 0.09413 M NaOH
• Bromthymol blue indicator
• unknown carboxylic acid (at least 0.2 g)

The following procedures were carried out in the experiment:

a. Solubility in Water. The solubility of carboxylic acids in water was tested by mixing water with the following acids: acetic, butyric, oleic, stearic, succinic and benzoic. Three drops of the liquid or one micro spatula of the solid acid were added to 2 mL of water. The qualitative results obtained with the solubilities listed for the compounds were checked in a chemical handbook. The data were tabulated.

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b. Reaction with 10% Sodium Bicarbonate. The solubility test of the same acids was repeated with 10% sodium bicarbonate solution. Three drops of the liquid or one micro spatula of the solid acid were added with 2 mL of 10% sodium bicarbonate solution. The evidence for reaction when water soluble acetic acid and succinic acid when added to reagent was noted. The typical pKa values of carboxylic acids, phenols, HCO₃^– and CO₃^2- were compared.

c. Action of an Oxidizing Agent on the Carboxylic Acid Group. Five drops of acetic acid were added to three to five drops of 0.5 KMnO₄ in a vial. The test was repeated with the following acids: formic, lactic, oxalic and succinic.

d. Neutralization Equivalent of Carboxylic Acids. A 0.2 g sample of unknown carboxylic acid was weighed accurately to four significant figures. The acid was dissolved in 50 mL water or ethanol. The mixture was heated to dissolve completely the compound. The solution was titrated with a previously standardized NaOH solution. A bromthymol blue indicator was used. The neutralization equivalent and molar mass of the unknown carboxylic acid were calculated.

Results and Discussion:

Carboxylic acids are organic compounds containing a carboxy group (COOH). The carbon atom of a carboxy group is surrounded by three groups, making it sp₂ hybridized and trigonal planar, with bond angles of approximately 120^â-¦.

Figure 1: Carboxylic Acid structure

Carboxylic acids exhibit dipole-dipole interactions because of their polar C-O bond and O-H bond. They also exhibit intermolecular hydrogen bonding because they possess a hydrogen atom bonded to an electronegative oxygen atom. Carboxylic acids are one of the most polar organic compounds. Most carboxylic acids exist as cyclic dimmers, held together by two hydrogen bonds.

Figure 2: Carboxylic acid dimer

Acetic acid is soluble in water. Carboxylic acids with less than 5 carbons in their alkyl group are soluble in water. The carbon skeleton is not too large for the OH group to solubilize by hydrogen bonding. The hydrophilic nature of the carboxylic group dominates than the hydrophobic nature. This is the reason why acetic acid and butyric acid are soluble in water.

Figure 3: Acetic acid and butyric acid

On the other hand, oleic acid and stearic acid are insoluble in water. Both have long, bulky carbon chains exceeding the five carbon limit. The OH group cannot solubilize the carbon skeleton via hydrogen bonding. Its hydrophobic character dominates than its hydrophilic nature.

Figure 4: Oleic acid

Figure 5: Stearic acid

A good solvent for stearic acid would be organic solvents like ether, chloroform and carbon tetrachloride.

Figure 6: Solvents for stearic acid

Benzoic acid is insoluble in water because the benzene ring is too bulky and large, and because of its stability, the OH group cannot solubilize it using hydrogen bonding.

Figure 7: Benzoic acid

Succinic acid contains two COOH groups because it is a dicarboxylic acid. This tells us that there is an increase in the hydrogen bonding capacity which makes it slightly soluble only because the carbon chain exceeds the five carbon chain limit and its hydrophobic character also shows.

Figure 8: Succinic acid

Carboxylic acids readily react with Bronsted Lowry bases to form carboxylate ions which are done through deprotonation.

Figure 9: Carboxylic acids react with sodium carbonate

In the experiment, sodium bicarbonate was used to deprotonate the carboxylic acid. This was a simple neutralization reaction forming a carboxylate salt, carbon dioxide and water. Acetic acid, butyric acid, succinic acid and benzoic acid react with the sodium bicarbonate. Succinic acid undergoes two deprotonation steps because it contains two COOH groups.

An acid can be deprotonated by a base that has a conjugate acid with a higher pKa. The pKa values of acetic acid, butyric acid, benzoic acid and succinic acid are all ~5, thus bases that have conjugate acids with pKa values higher than 5 are strong enough to deprotonate them. Oleic acid and stearic acid have pKa values of 9.85 and 10.15 respectively. These pKa values are higher than the conjugate acid of the base (NaOH) which is H₂CO₃. This tells us that sodium bicarbonate is not strong enough to deprotonate both carboxylic acids. Stronger bases are needed to deprotonate them such as NaOH which has a conjugate acid with a pKa of 15.7.

Figure 10: Dissociation and pKa values of carboxylic acids

When comparing the pKa values of carboxylic acids and phenols, phenols always have a higher pKa value which tells us that phenols are weaker acids than carboxylic acids.

Figure 11: pKa values of phenol and carboxylic acid

Carboxylic acids and phenols are both acidic. Looking into the Arrhenius definition of an acid, both when dissolved in water, increases the H⁺ concentration. Also looking at the Bronsted-Lowry definition of an acid, acids are proton donors.

Figure 12: Bronsted Lowry definition of an acid

Aside from these two famous definitions of an acid, we must also look into the stability of the conjugate base. A rule states that anything that stabilizes a conjugate base makes the starting reagent acidic. When we talk about phenols, its conjugate base which is the phenoxide is resonance stabilized. It has five resonance structures which disperse the negative charge to three carbons and one oxygen atom. This makes phenols more acidic than alcohols which cannot stabilize its conjugate base via resonance.

When we compare phenols with carboxylic acids, carboxylic acids are stronger compared to phenols. For carboxylic acids, their conjugate base which is the carboxylate ion is a lot more stable because they contain two oxygen atoms that delocalize the negative charge. As an effect, carboxylic acids are stronger acids than phenols which is evident in their pKa values.

Looking at the pKa values of phenols and carboxylic acids, we could conclude that NaHCO₃ can be used to separate a water insoluble carboxylic acid and a water insoluble phenol considering that this insoluble carboxylic acid does not exceed the pKa value of HCO₃^– (when protonated H₂CO₃ which is the conjugate acid) which is 6.4. Sodium bicarbonate can successfully separate a water insoluble phenol and a water insoluble carboxylic acid because typical pKa values for phenol which is 10 exceeds 6.4. The NaHCO₃, therefore, is not strong enough to deprotonate the phenol but is strong enough to deprotonate the carboxylic acid. It will most likely form two layers: an organic layer with the phenol and an aqueous layer with the water and carboxylate ion which are products of the reaction of the carboxylic acid with the base.

Sodium carbonate is not effective in separating a mixture containing a water insoluble carboxylic acid and a water insoluble phenol. The pKa of CO₃^2- (when protonated becomes HCO₃^–) is close to 10. This tells us that Na₂CO₃ reacts with some of the phenol and ofcourse with the carboxylic acid. Thus, no complete separation between the two occurs.

Figure 13: Sodium bicarbonate and sodium carbonate

Some carboxylic acids undergo oxidation. These are called reducing acids. In the experiment, lactic acid, formic acid and oxalic acid are all oxidized to carbon dioxide and water with the presence of a brown precipitate which is the reduced KMnO₄. Acetic acid and Succinic acid are both non-reducing acids because they do not oxidize in the presence of a strong oxidizing agent, KMnO₄.

Lactic acid is oxidized into pyruvic acid because it contains an oxidizable group which is OH.

Figure 14: Oxidation of lactic acid

Formic acid is oxidized to carbon dioxide and water.

Figure 15: Oxidation of formic acid

Oxalic acid also oxidizes into carbon dioxide and water.

Figure 16: Oxidation of oxalic acid

The neutralization equivalent of an acid is mathematically defined as:

Neutralization equivalent (NE) =

To determine the molar mass:

Molar mass = (X) x neutralization equivalent

*Where X is the number of COOH groups

The molar mass of an unknown carboxylic sample could be determined by computing its neutralization equivalent. Finding the neutralization equivalent requires titrating the solution of unknown carboxylic acid with a previously standardized solution of NaOH. The exact molarity of the NaOH was found to be 0.09413 M. Two trials were carried out in this section of the experiment. The solid form of the unknown carboxylic acid was water soluble.

Weighing of sample:

Titration:

Computations:

Trial 1:

Volume of NaOH used = Final buret reading – Initial buret reading

= 33.80 mL – 0.50 mL

= 33.30 mL

Neutralization equivalent (NE) =

=

= 66.36 g/mol

Molar mass = 2 x (66.36 g/mol)

= 132.72 g/mol

Trial 2:

Volume of NaOH used = Final buret reading – Initial buret reading

= 32.50 mL -0.30 mL

= 32.20 mL

Neutralization equivalent (NE) =

=

= 66.35 mL

Molar mass = 2 x (66.35 g/mol)

= 132.70 g/mol

Average molar mass =

= 132.71 g/mol

This molar mass was determined to be 95% near the true molar mass of the unknown carboxylic acid.

Calculations for determining identity of unknown:

= 139. 69 139.69 – 132.71 = 6.98 (error)

For MM1 = 132.71 +6.98 = 139.69

MM2 = 132.71 – 6.98 = 125.73

For the 1^st probable molar mass:

139.69 – 90.02 (2 X molar mass of COOH) = 49.67

CnH2n = 49.67

(12.01)n + (1.00)2n = 49.67

14.01 n = 49.67

n= 3.5/4

For the 2^nd probable molar mass:

125.73 – 90.02 (2 X molar mass of COOH) = 35.71

CnH2n = 35.71

(12.01)n + (1.00)2n = 35.71

14.01n = 35.71

n= 2.5/3

Possible identities for the carboxylic acid include Glutaric acid, Glutaconic acid and Adipic acid.

Conclusion:

Therefore, the solubility of different carboxylic acids can be rationalized from the structure of the carboxylic acid itself. Acetic acid and butyric acid are soluble since their OH groups are able to solubilize their alkyl chain which does not exceed five carbons. Oleic acid and stearic acid are insoluble in water because their alkyl chain exceeds 5 carbons and the OH group cannot solubilize the long, bulky alkyl chain. A good solvent for stearic acid would be organic solvents like ether, chloroform and carbon tetrachloride. Benzoic acid is insoluble in water because the benzene ring, due to its stability, cannot be solubilized by the OH group. Succinic acid on the other hand, is soluble in water due to greater capacity of hydrogen bonding because it has two OH groups. Carboxylic acids also react with sodium carbonate through deprotonation. Only acetic acid, succinic acid, benzoic acid and butyric acid give a reaction because these acids have a lower pKa value than the conjugate acid of the base which is NaHCO₃. Oleic acid and Stearic acid do not react with NaHCO₃ because they have higher pKa values than the conjugate acid of the base. This tells us that sodium bicarbonate is not strong enough to deprotonate both carboxylic acids. The rule here is: an acid can be deprotonated by a base that has a conjugate acid with a higher pKa. By looking at the pKa values, phenols are weaker acids than carboxylic acids. Phenols are resonance stabilized by carboxylic acids is more stable because they have conjugate bases with two oxygen atoms which delocalize the negative charge. NaHCO₃ can be used to separate a mixture containing a water insoluble carboxylic acid and a water insoluble phenol because phenols do not react with this because it has a higher pKa than its conjugate acid. Na₂CO₃ is not effective because both phenols and carboxylic acids react, therefore, no separation occurs. Some carboxylic acids react with KMnO₄ and are oxidized. Examples are lactic acid which is oxidized to pyruvic acid and formic acid and oxalic acids which are oxidized to carbon dioxide and water. Non-reducing acids include acetic acid and succinic acid. And for the last part of the experiment, the molar mass of an unknown carboxylic acid may be determined by identifying how many COOH groups are present and by computing its neutralization equivalent.

Neutralization equivalent (NE) =

Molar mass = (X) x neutralization equivalent

Supporting information:

In determining the molar mass or formula for an unknown carboxylic acid, it may be possible to have an unsaturated compound.

If the molecular formula is given, plug in the numbers into this formula:

DoU=

C= number of carbons

N= number of nitrogens

X= number of halogens (F, Cl, Br, I)

H= number of hydrogens

References:

Organic Chemistry by John McMurry

Organic Chemistry by Janice Smith

http://chemwiki.ucdavis.edu/Organic_Chemistry/Hydrocarbons/Alkenes/Properties_of_Alkenes/Degree_of_Unsaturation

Wikiperdia.org

www.studymode.com