Question

The correct order of acidic strength is/are (A) \(\mathrm{H}_{2} \mathrm{O}>\mathrm{HC} \equiv \mathrm{CH}\) (B) \(\mathrm{HCOOH}>\mathrm{Ph}-\mathrm{COOH}\) (C) \(\mathrm{Ph}-\mathrm{OH}<\mathrm{Ph}-\mathrm{SH}\) (D) \(\mathrm{HCN}>\mathrm{H}_{2} \mathrm{O}\)

Short answer

The correct order of acidic strength is given in options (B) and (D): (B) \(\mathrm{HCOOH}<\mathrm{Ph}-\mathrm{COOH\) (D) \(\mathrm{HCN}>\mathrm{H}_{2} \mathrm{O}\)

Step-by-step solution

Identify the conjugate bases

We need to identify the conjugate bases for each compound after losing a proton (H+). The conjugate bases for each compound are: (A) \(\mathrm{H}_{2} \mathrm{O}>\mathrm{HC} \equiv \mathrm{CH}\): Conjugate base of \(\mathrm{H}_{2} \mathrm{O}\) = \(\mathrm{OH}^{-}\) Conjugate base of \(\mathrm{HC} \equiv \mathrm{CH}\) = \(\mathrm{C} \equiv \mathrm{CH}^{-}\) (B) \(\mathrm{HCOOH}>\mathrm{Ph}-\mathrm{COOH}\): Conjugate base of \(\mathrm{HCOOH}\) = \(\mathrm{HCOO}^{-}\) Conjugate base of \(\mathrm{Ph}-\mathrm{COOH}\) = \(\mathrm{Ph}-\mathrm{COO}^{-}\) (C) \(\mathrm{Ph}-\mathrm{OH}<\mathrm{Ph}-\mathrm{SH}\): Conjugate base of \(\mathrm{Ph}-\mathrm{OH}\) = \(\mathrm{Ph}-\mathrm{O}^{-}\) Conjugate base of \(\mathrm{Ph}-\mathrm{SH}\) = \(\mathrm{Ph}-\mathrm{S}^{-}\) (D) \(\mathrm{HCN}>\mathrm{H}_{2} \mathrm{O}\): Conjugate base of \(\mathrm{HCN}\) = \(\mathrm{CN}^{-}\) Conjugate base of \(\mathrm{H}_{2} \mathrm{O}\) = \(\mathrm{OH}^{-}\) Now let's assess the stability of these conjugate bases.

Assess the stability of conjugate bases

A more stable conjugate base implies greater acidic strength. Factors like electron negativity, resonance, and inductive effect can affect the stability of the conjugate base. (A) Stability of \(\mathrm{OH}^{-}\) < Stability of \(\mathrm{C} \equiv \mathrm{CH}^{-}\). The triple bond in \(\mathrm{C} \equiv \mathrm{CH}^{-}\) distributes the negative charge, making it more stable. So, \(\mathrm{H}_{2} \mathrm{O}<\mathrm{HC} \equiv \mathrm{CH}\) (B) Stability of \(\mathrm{HCOO}^{-}\) < Stability of \(\mathrm{Ph}-\mathrm{COO}^{-}\). The negative charge on \(\mathrm{Ph}-\mathrm{COO}^{-}\) is more stabilized due to resonance with the phenyl ring. Therefore, \(\mathrm{HCOOH}<\mathrm{Ph}-\mathrm{COOH}\) (C) Stability of \(\mathrm{Ph}-\mathrm{O}^{-}\) > Stability of \(\mathrm{Ph}-\mathrm{S}^{-}\). Oxygen is more electronegative than sulfur, which means that the negative charge on oxygen is more stable. Thus, \(\mathrm{Ph}-\mathrm{OH}>\mathrm{Ph}-\mathrm{SH}\) (D) Stability of \(\mathrm{CN}^{-}\) > Stability of \(\mathrm{OH}^{-}\). The greater electronegativity of nitrogen combined with the triple bond in \(\mathrm{CN}^{-}\) helps spread the negative charge, making it more stable. Therefore, \(\mathrm{HCN}>\mathrm{H}_{2} \mathrm{O}\)

Compare the given order with the evaluated order

Using the stability of conjugate bases, we obtain the following correct order of acidic strength: (A) \(\mathrm{H}_{2} \mathrm{O}<\mathrm{HC} \equiv \mathrm{CH}\) (not as given) (B) \(\mathrm{HCOOH}<\mathrm{Ph}-\mathrm{COOH}\) (given correctly) (C) \(\mathrm{Ph}-\mathrm{OH}>\mathrm{Ph}-\mathrm{SH}\) (not as given) (D) \(\mathrm{HCN}>\mathrm{H}_{2} \mathrm{O}\) (given correctly) So, only options (B) and (D) display the correct order of acidic strength.

Key concepts

Conjugate Bases

Understanding the concept of conjugate bases is essential when discussing the acidic strength of organic compounds. A conjugate base is formed when an acid donates a proton (H+). This process is part of the Bronsted-Lowry acid-base theory. The stability of the resulting conjugate base directly affects the acidic strength of the original acid. If the conjugate base is more stable, it means that it holds the negative charge more effectively after the acid has given up a proton, rendering the acid stronger.

To put it simply, consider the example where water (H_2O) loses a proton and becomes the hydroxide ion (OH^-), whereas acetylene (HC≡CH) forms a conjugate base (C≡CH^-). The stability and distribution of the negative charge are crucial in determining which of two acids is stronger. Generally, the acid that forms the more stable negative ion, in this case, the acetylene with its triple bond assisting in charge distribution, will be the stronger acid.

The process of acid dissociation and the stability of the resulting conjugate base are foundational concepts for predicting the direction of acid-base reactions and understanding pH in solutions.

Resonance Effect

Diving deeper into understanding acidic strength, the resonance effect comes into play significantly. It refers to the delocalization of electrons in a molecule through p-orbitals, which can lead to stabilization of the molecule. In the context of conjugate bases, a base that can exhibit resonance will generally be more stable than one that cannot because the resonance allows for the distribution of the negative charge over multiple atoms.

For instance, the carboxylate ion (Ph-COO^-) in the benzoic acid conjugate base has the ability to resonate with the aromatic ring (phenyl group), effectively delocalizing the negative charge across the structure. This dispersal of charge lowers the energy and, therefore, stabilizes the ion, making benzoic acid (Ph-COOH) more acidic than formic acid (HCOOH), which lacks this extensive resonance stabilization. The resonance effect is a vital concept that helps us to predict the relative strengths of acids and understand the reactivity of various organic molecules.

Inductive Effect

The inductive effect is another crucial factor influencing the acidic strength in organic compounds. It describes the transmission of charge through a chain of atoms in a molecule, which can either increase or decrease the stability of a conjugate base. This effect is caused by the difference in electronegativity between atoms. Electronegative atoms draw electron density towards themselves, thus polarizing the bonds.

In the context of acidic strength, molecules that have electronegative atoms or groups attached to them can stabilize the negative charge through the inductive effect. For example, in hydrocyanic acid (HCN), the electronegative nitrogen atom exerts a strong inductive effect, pulling electron density towards itself and away from the negatively charged conjugate base (CN^-). This increases the stability of the cyanide ion relative to hydroxide (OH^-), thereby making HCN a stronger acid than water. The inductive effect is a significant aspect in the study of organic compounds, as it helps to explain variations in not just acidic strength but also reactivity patterns in chemical reactions.