The correct decreasing order of acidity is (a) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}>\mathrm{HC}=\mathrm{CH}>\mathrm{H}_{2} \mathrm{O}>\mathrm{NH}_{3}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}>\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}>\mathrm{NH}_{3}>\mathrm{HC}=\mathrm{CH}\) (c) \(\mathrm{HC}=\mathrm{CH}>\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}>\mathrm{H}_{2} \mathrm{O}>\mathrm{NH}_{3}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}>\mathrm{H}_{2} \mathrm{O}>\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}>\mathrm{HC}=\mathrm{CH}\)

Understanding the factors that influence acidity in organic molecules is fundamental for students studying organic chemistry. Acidity in organic compounds is mainly influenced by the bond strength between the hydrogen atom and the atom to which it is attached, and the stability of the resulting conjugate base after the hydrogen ion is lost.

When comparing bond strengths, hydrogen atoms connected to more electronegative elements usually create weaker bonds and thus more acidic compounds. Moreover, the size and hybridization of the atom bound to hydrogen play roles; for example, sp-hybridized carbons compared to sp3-hybridized carbons are bonded to more acidic hydrogens due to the higher s-character and thus stronger pull on the hydrogen's electron.

For the stability of the conjugate base, factors include the ability to delocalize the negative charge through resonance, the induction effect from nearby electronegative atoms, and the overall molecular structure, which can affect how readily the compound can donate a proton. All of these factors coalesce to determine the acidity of a molecule.

The stability of a conjugate base is paramount when assessing the acidity of a compound. A stable conjugate base correlates with an acid's strength because a stable species is more willing to accept the negative charge left behind after ionization. This stability often results from resonance stabilization, inductive effects, and the overall electron distribution within the molecule.

Resonance Stabilization

Resonance stabilization occurs when the negative charge can be delocalized over a structure, as in the case with phenolate ion (C6H5O-), the conjugate base of phenol. This dispersal of charge across multiple atoms decreases electron-electron repulsion and stabilizes the molecule.

Inductive Effect

Electronegative atoms adjacent to the site of deprotonation can pull negative charge away through sigma bonds, lending additional stability to the conjugate base. For example, in halogenated acids, the presence of a halogen can increase the acidity of the compound because of the inductive withdrawal of electron density.

The broader implication is that the more stable the conjugate base, the stronger the acid, as the acid more readily loses its proton in the presence of a base.

Resonance stabilization is a crucial concept in determining the acidity of compounds containing conjugated systems. It refers to the distribution of a negative charge across multiple atoms through resonance structures after an acid donates a proton. Compounds with this trait are typically more acidic because the corresponding conjugate base is more stable as a result of charge delocalization.

For instance, phenol (C6H5OH) displays a significant resonance stabilization in its conjugate base, the phenoxide ion (C6H5O-), where the negative charge can be spread over the ortho and para positions of the benzene ring. This delocalization contrasts with simple alcohols like ethanol (C2H5OH), whose conjugate base (ethoxide ion) lacks the aromatic ring to distribute the charge. Consequently, phenol is more acidic than ethanol due to the enhanced stability provided by resonance in its conjugate base.

Comparing the relative acid strength of different molecules is a fundamental exercise in organic chemistry. It provides insight into reaction mechanisms and the behavior of compounds in various chemical contexts. Acid strength comparison often involves examining the acidity of protons in different molecular environments.

To compare acid strength effectively, one should consider a combination of factors that include electronegativity, resonance stabilization, inductive effects, and hybridization of the atoms to which the hydrogen is bonded. A helpful approach to make these comparisons is through the use of pKa values, with lower pKa values indicating stronger acids.

When comparing the acidity of phenol, acetylene (HC≡CH), water (H2O), ethanol (C2H5OH), and ammonia (NH3), it becomes apparent that phenol, with significant resonance stabilization, is the most acidic, followed by acetylene with its sp-hybridized carbon. Water and ethanol, both having less stable conjugate bases than phenol and acetylene, exhibit lower acidity, and ammonia with its weak acid character is the least acidic, lacking both resonance stabilization and electronegative elements to stabilize the conjugate base.