Question

The correct decreasing order of acid strength of the following compounds is \(\begin{array}{cc}\mathrm{CH}_{3} \mathrm{COOH} & \mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{COOH} & \mathrm{a}^{\mathrm{COOH}} & \mathrm{COO}_{3}-\mathrm{m}\end{array}\) \(\mathrm{CF}_{3} \mathrm{COOH}\) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCOOH}\) (a) III > I > II > IV (b) III \(>\mathrm{II}>\mathrm{I}>\mathrm{IV}\) (c) III > I \(\geq\) IV \(>\) II (d) II > III > I > IV

Short answer

Question: Arrange the following compounds in decreasing order of acid strength: Acetic acid (I), Methyl ester of acetic acid (II), Trifluoroacetic acid (III), 2-Methylpropanoic acid (IV). Answer: (c) III > I ≥ IV > II

Step-by-step solution

Identify the compounds

Firstly we need to identify the given compounds: 1. \(\mathrm{CH}_{3} \mathrm{COOH}\): Acetic acid 2. \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{COOH}\): Methyl ester of acetic acid 3. \(\mathrm{CF}_{3} \mathrm{COOH}\): Trifluoroacetic acid 4. \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCOOH}\): 2-Methylpropanoic acid Now let's compare the acidity of these compounds.

Compare acid strength

We will consider the factors that can stabilize the anions left after donating a proton. 1. Acetic acid (\(\mathrm{CH}_{3} \mathrm{COOH}\)) donates a proton to form acetate ion (\(\mathrm{CH}_{3} \mathrm{COO}^{-}\)). This ion is resonance stabilized. 2. Methyl ester of acetic acid (\(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{COOH}\)) forms a species with low resonance stabilization after donating a proton. 3. Trifluoroacetic acid (\(\mathrm{CF}_{3} \mathrm{COOH}\)) donates a proton to form trifluoroacetate ion (\(\mathrm{CF}_{3} \mathrm{COO}^{-}\)). This ion has a greater resonance stabilization as well as a strong inductive effect due to the presence of three highly electronegative fluorine atoms. 4. 2-Methylpropanoic acid (\(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCOOH}\)) forms an anion with less resonance stabilization after donating a proton. The inductive effect is weaker compared to that in trifluoroacetic acid. Now, let's arrange these compounds in decreasing order of acid strength: Trifluoroacetic acid \(>\) Acetic acid \(\geq\) 2-Methylpropanoic acid \(>\) Methyl ester of acetic acid

Find the correct option

Compare this order with the given options: (a) III > I > II > IV (b) III \(>\mathrm{II}>\mathrm{I}>\mathrm{IV}\) (c) III > I \(\geq\) IV \(>\) II (d) II > III > I > IV The correct option is (c) III > I \(\geq\) IV \(>\) II.

Key concepts

Trifluoroacetic Acid

Trifluoroacetic acid (TFA) is a fascinating molecule in organic chemistry, known for its exceptional acid strength compared to its close relative, acetic acid. Why is this so? The answer lies in the unique presence of three fluorine atoms bonded to the carbon adjacent to the carboxylic acid group. Fluorine, being extremely electronegative, exerts a strong inductive effect, pulling electron density away from the carboxyl group. This effect enhances the acid's ability to donate a proton, making the trifluoroacetate ion more stable than the acetate ion.

Moreover, the highly electronegative fluorine atoms help disperse the negative charge after deprotonation, which contributes to the increased acid strength of TFA. This makes TFA a superacid compared to many other carboxylic acids and an incredibly useful reagent in organic syntheses and protein chemistry due to its strong acidity.

Acetic Acid

Acetic acid, often recognized as the sour component in vinegar, is a much milder acid compared to trifluoroacetic acid. Structurally simpler, acetic acid (\(CH_3COOH\)) lacks the highly electronegative elements that could strongly influence the electron distribution. However, it's not without its own chemical charm. Upon donating a proton, acetic acid forms an acetate ion (\(CH_3COO^-\)), which is resonance stabilized.

Resonance stabilization allows the charge to be shared across different atoms through a process known as resonance. The acetate ion has two equivalent resonance structures where the negative charge is delocalized between the two oxygen atoms. This charge delocalization contributes to the stability of the acetate ion and to the overall acidity of acetic acid, albeit less effectively than the inductive effect found in TFA.

Resonance Stabilization in Organic Chemistry

Resonance stabilization is a powerful concept in organic chemistry, instilling stability in many molecules with conjugated systems. It refers to the delocalization of electrons across adjacent atoms by the overlapping of p-orbitals, creating a set of structures known as resonance structures or contributors.

This phenomenon plays a pivotal role when comparing acid strengths. For instance, when acetic acid loses a proton, the resulting acetate ion benefits from resonance stabilization. In more complex systems, the more extensive the resonance stabilization, the more stable the anion, and often, the stronger the acid. However, resonance is just one factor in acid strength. When both resonance and electronegative substitutions like in trifluoroacetic acid come into play, the acidity is further increased, demonstrating how different factors in organic chemistry work in concert to dictate the properties of a compound.

Inductive Effect in Organic Chemistry

The inductive effect is another key player in determining the acidity of organic compounds. Simply put, it's the electronegative pull exerted by certain atoms or groups within a molecule, impacting the electron density distribution. The more electronegative the substituent, the stronger the inductive effect it will have, drawing electron density toward itself.

In the context of carboxylic acids, substituents that have a strong inductive effect can enhance acid strength by stabilizing the negative charge that forms on the oxygen after the acid donates a proton. For example, the presence of fluorine atoms in trifluoroacetic acid exerts a strong inductive effect, which accounts for its higher acidity compared to acetic acid, which does not have such electronegative substituents. This concept demonstrates why even subtle changes in molecular structure can lead to significant differences in chemical behavior.