Question

\(\mathrm{HC} \equiv \mathrm{N}\) is more acidic than \(\mathrm{HC} \equiv \mathrm{CH}\) because (a) HCN is an acid whereas, \(\mathrm{HC} \equiv \mathrm{CH}\) is a hydrocarbon. (b) HCN has a pK of \(9.2\). (c) In addition to greater s character of the hybrid orbitals, the presence of more electronegative nitrogen further stabilizes the anion - CN. (d) of the triple bond nature of the \({ }^{-} \mathrm{C} \equiv \mathrm{N}\) bond.

Short answer

Answer: In addition to greater s character of the hybrid orbitals, the presence of more electronegative nitrogen further stabilizes the anion - CN.

Step-by-step solution

Evaluate each option

For each statement provided, evaluate if it is related to acidity and if it supports the claim that HCN is more acidic than \(\mathrm{HC} \equiv \mathrm{CH}\). (a) HCN is an acid whereas, \(\mathrm{HC} \equiv \mathrm{CH}\) is a hydrocarbon. - This statement tells us that HCN is an acid, while \(\mathrm{HC} \equiv \mathrm{CH}\) is a hydrocarbon. It does not, however, provide sufficient information to determine why HCN is more acidic. (b) HCN has a pK of \(9.2\). - Although it tells us the pK value of HCN, this statement does not provide any comparative information about the acidity of \(\mathrm{HC} \equiv \mathrm{CH}\), so we cannot determine why HCN is more acidic. (c) In addition to greater s character of the hybrid orbitals, the presence of more electronegative nitrogen further stabilizes the anion - CN. - This statement describes how the nature of the hybrid orbitals and the electronegativity of nitrogen contribute to the stability of the anion. The more stable the anion, the more likely the parent molecule is to release a proton and, therefore, the more acidic the molecule. This statement provides a convincing argument for why HCN is more acidic than \(\mathrm{HC} \equiv \mathrm{CH}\). (d) of the triple bond nature of the \({ }^{-} \mathrm{C} \equiv \mathrm{N}\) bond. - This statement only discusses the nature of the triple bond between carbon and nitrogen but does not provide any information regarding its effect on acidity.

Choose the correct statement

Based on our evaluation, it's clear that option (c) is the correct explanation for why HCN is more acidic than \(\mathrm{HC} \equiv \mathrm{CH}\). It addresses the hybrid orbital's s character, the presence of more electronegative nitrogen, and the stabilizing effect on the anion, all of which contribute to the higher acidity of HCN. So, the correct answer is: (c) In addition to greater s character of the hybrid orbitals, the presence of more electronegative nitrogen further stabilizes the anion - CN.

Key concepts

Acid Strength Comparison

Understanding the relative acid strength of different organic compounds is essential for predicting their reactivity and behavior in chemical reactions.
Acid strength in organic compounds is commonly compared using the pKa value, which measures the propensity of the compound to donate a proton (H+). The lower the pKa value, the stronger the acid. For instance, in the textbook exercise, hydrocyanic acid (HCN) has a pKa of 9.2, which is lower than most hydrocarbons, meaning HCN is a stronger acid.
Several factors influence acid strength; one of the most crucial is the stability of the conjugate base formed after the acid donates a proton. In the case of HCN, the resulting anion is stabilized by the factors discussed in the 'hybrid orbitals and acidity' and 'electronegativity and acidity' sections below, which contribute to HCN's stronger acidity compared to a hydrocarbon like ethyne (HC≡CH).

Hybrid Orbitals and Acidity

The role of hybrid orbitals in determining the acidity of a compound is rooted in electronic structure.
The 's' character of a hybrid orbital affects acidity—the greater the 's' character, the closer the electrons are held to the nucleus, which increases the acidity. In the example provided in the exercise, HCN has a hybrid orbital with a higher s character due to the sp hybridization when compared to the sp2 or sp3 hybridization in other organic molecules. This significant s character results in a highly electronegative triple bond (C≡N).

Stabilizing the Conjugate Base

Furthermore, the high s character contributes to the stability of the conjugate base, in this case, the cyanide ion (CN-). A stable conjugate base implies that the proton release is energetically favorable, making the acid stronger. The electron pair left behind on the carbon atom after deprotonation resides in an orbital with high s character, meaning the negative charge is held tightly, adding to the anion's stability.

Electronegativity and Acidity

Electronegativity is a fundamental concept when discussing acidity. It refers to the ability of an atom to attract bonding electrons towards itself. In acids, a highly electronegative atom attached to the acidic hydrogen can stabilize the negative charge of the conjugate base after deprotonation.
In the exercise, the electronegativity of nitrogen in HCN plays a pivotal role. Nitrogen is more electronegative than carbon, which means it can better stabilize the additional electron density that occurs when HCN releases a hydrogen ion to form the cyanide ion (CN-).

Effects on Conjugate Base Stability

The high electronegativity of nitrogen in the CN- ion not only stabilizes the charge but also makes the loss of a proton from HCN more favorable compared to the less electronegative carbon in ethyne (HC≡CH). This stabilization due to electronegativity is a key factor in making HCN a stronger acid than hydrocarbons containing less electronegative elements.