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Chapter 10: Problem 161

Select the stronger arid from each pair, aud explain your reasoning. For each stronger acid, write a structural formula for its conjugate hase. (a) \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{H}_{2} \mathrm{CO}_{\mathrm{a}}\) (b) \(\mathrm{CH}_{3} \mathrm{OH}\) or \(\mathrm{CH}, \mathrm{COOH}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{C}=\mathrm{CH}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH}\) \(10.27\) From each pair, select the stronger base. For each stronger base, write a structural formula of its conjugate anid. (a) \(\mathrm{OH}^{-}\)or \(\mathrm{CH}_{3} \mathrm{O}^{-}\)(each in \(\mathrm{H}_{-} \mathrm{O}\) ) (b) \(\mathrm{CH}_{K} \mathrm{CHO}^{-}\)or \(\mathrm{CH}_{4} \mathrm{C}=\mathrm{C}^{-}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2}\) s \(^{-}\)or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\)or \(\mathrm{NH}_{2}^{-}\)

Short Answer

Expert verified
Question: Determine which is the stronger acid or base in each pair and write the structural formula for the conjugate base or acid. (a) Compare the acidity of \(H_2O\) and \(H_2CO_3\) Answer: \(H_2CO_3\) is the stronger acid, and its conjugate base is \(\mathrm{HCO}_{3}^{-}\). (b) Compare the acidity of \(CH_3OH\) and \(CH_3COOH\) Answer: \(CH_3COOH\) is the stronger acid, and its conjugate base is \(\mathrm{CH}_{3}\mathrm{COO}^{-}\). (c) Compare the acidity of \(CH_3CH_2OH\) and \(CH_3C=CH\) Answer: \(CH_3CH_2OH\) is the stronger acid, and its conjugate base is \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\). (d) Compare the acidity of \(CH_3CH_2OH\) and \(CH_3CH_2SH\) Answer: \(CH_3CH_2SH\) is the stronger acid, and its conjugate base is \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{S}^{-}\). (e) Compare the basicity of \(OH^-\) and \(CH_3O^-\) Answer: \(CH_3O^-\) is the stronger base, and its conjugate acid is \(CH_3OH\). (f) Compare the basicity of \(CH_3CHO^-\) and \(CH_3C=C^-\) Answer: \(CH_3CHO^-\) is the stronger base, and its conjugate acid is \(CH_3CHOH\). (g) Compare the basicity of \(CH_3CH_2S^-\) and \(CH_3CH_2O^-\) Answer: \(CH_3CH_2O^-\) is the stronger base, and its conjugate acid is \(CH_3CH_2OH\). (h) Compare the basicity of \(CH_3CH_2O^-\) and \(NH_2^-\) Answer: \(NH_2^-\) is the stronger base, and its conjugate acid is \(NH_3\).

Step by step solution

01

(a) Compare the acidity of \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{H}_{2} \mathrm{CO}_{3}\)

Acid strength is determined by the stability of the conjugate base. Here, the conjugate base of \(\mathrm{H}_{2}\mathrm{O}\) is \(\mathrm{OH}^{-}\) and the conjugate base of \(\mathrm{H}_{2}\mathrm{CO}_{3}\) is \(\mathrm{HCO}_{3}^{-}\). Since \(\mathrm{HCO}_{3}^{-}\) has resonance stabilization, it is more stable than \(\mathrm{OH}^{-}\). So, \(\mathrm{H}_{2}\mathrm{CO}_{3}\) is the stronger acid. Its conjugate base is \(\mathrm{HCO}_{3}^{-}\).
02

(b) Compare the acidity of \(\mathrm{CH}_{3}\mathrm{OH}\) and \(\mathrm{CH}_{3}\mathrm{COOH}\)

The conjugate base of \(\mathrm{CH}_{3}\mathrm{OH}\) is \(\mathrm{CH}_{3}\mathrm{O}^{-}\) and the conjugate base of \(\mathrm{CH}_{3}\mathrm{COOH}\) is \(\mathrm{CH}_{3}\mathrm{COO}^{-}\). \(\mathrm{CH}_{3}\mathrm{COO}^{-}\) is more stable than \(\mathrm{CH}_{3}\mathrm{O}^{-}\) due to resonance stabilization. So, \(\mathrm{CH}_{3}\mathrm{COOH}\) is the stronger acid. Its conjugate base is \(\mathrm{CH}_{3}\mathrm{COO}^{-}\).
03

(c) Compare the acidity of \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\) and \(\mathrm{CH}_{3}\mathrm{C}=\mathrm{CH}\)

Neither of these compounds releases a proton easily. \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\) has an \(\mathrm{-OH}\) group which can release a proton, while \(\mathrm{CH}_{3}\mathrm{C}=\mathrm{CH}\) has acidic protons in the double bond. Since the proton in \(\mathrm{-OH}\) group is easier to remove, \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\) is the stronger acid. Its conjugate base is \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\).
04

(d) Compare the acidity of \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\) and \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{SH}\)

The sulfur atom in \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{SH}\) is larger and less electronegative than the oxygen atom in \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\). The conjugate base of \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\) is \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\) while the conjugate base of \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{SH}\) is \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{S}^{-}\). The negative charge on sulfur in \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{S}^{-}\) is less concentrated and more stable, so \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{SH}\) is the stronger acid. Its conjugate base is \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{S}^{-}\).
05

(a) Compare the basicity of \(\mathrm{OH}^{-}\) and \(\mathrm{CH}_{3} \mathrm{O}^{-}\)

The hydrogen atom in \(\mathrm{OH}^{-}\) is more acidic than the one in \(\mathrm{CH}_{3} \mathrm{O}^{-}\) because the oxygen atom is more electronegative. Thus, \(\mathrm{CH}_{3} \mathrm{O}^{-}\) is the stronger base. Its conjugate acid is \(\mathrm{CH}_{3}\mathrm{OH}\).
06

(b) Compare the basicity of \(\mathrm{CH}_{3}\mathrm{CHO}^{-}\) and \(\mathrm{CH}_{3} \mathrm{C}=\mathrm{C}^{-}\)

Both these species accept a proton, but \(\mathrm{CH}_{3}\mathrm{C}=\mathrm{C}^{-}\) has more resonance structures, making it more stable. Consequently, \(\mathrm{CH}_{3}\mathrm{CHO}^{-}\) is the stronger base. Its conjugate acid is \(\mathrm{CH}_{3}\mathrm{CH}\mathrm{OH}\).
07

(c) Compare the basicity of \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{S}^{-}\) and \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\)

Oxygen is more electronegative than sulfur, making the negative charge more delocalized and stable on \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{S}^{-}\). Thus, \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\) is the stronger base. Its conjugate acid is \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\).
08

(d) Compare the basicity of \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\) and \(\mathrm{NH}_{2}^{-}\)

Comparing the electronegativity of nitrogen and oxygen, nitrogen is less electronegative than oxygen, meaning nitrogen is a better electron-pair donor and \(\mathrm{NH}_{2}^{-}\) is the stronger base. Its conjugate acid is \(\mathrm{NH}_{3}\).

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