Question

From each pair, select the stronger base. For each stronger base, write a structural formula of its conjugate acid. (a) \(\mathrm{OH}^{-}\)or \(\mathrm{CH}_{3} \mathrm{O}^{-}\)(each in \(\mathrm{H}_{2} \mathrm{O}\) ) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\)or \(\mathrm{CH}_{3} \mathrm{C}^{-} \mathrm{C}^{-}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~S}^{-}\)or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\)or \(\mathrm{NH}_{2}^{-}\)

Short answer

(a) Stronger Base: \(CH_3O^-\); Conjugate Acid: \(CH_3OH\) (b) Stronger Base: \(CH_3C^-C^-\); Conjugate Acid: \(CH_3C=CH\) (c) Stronger Base: \(CH_3CH_2S^-\); Conjugate Acid: \(CH_3CH_2SH\) (d) Stronger Base: \(NH_2^-\); Conjugate Acid: \(NH_3\)

Step-by-step solution

Determine the stronger base in each pair

Let's compare the bases given in each pair: (a) \(\mathrm{OH}^{-}\) or \(\mathrm{CH}_{3} \mathrm{O}^{-}\): Here, both bases have oxygen as the central atom. But \(\mathrm{OH}^{-}\) has one less alkyl group compared to \(\mathrm{CH}_{3}\mathrm{O}^{-}\). Alkyl groups are electron-donating groups, so they make the oxygen more willing to donate its electron pair. Therefore, \(\mathrm{CH}_{3}\mathrm{O}^{-}\) is the stronger base. (b) \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\) or \(\mathrm{CH}_{3}\mathrm{C}^{-}\mathrm{C}^{-}\): Here, we have oxygen and carbon as the central atoms. Oxygen is more electronegative than carbon, making it less likely to donate its electron pair. Thus, the base with the carbon atom, \(\mathrm{CH}_{3}\mathrm{C}^{-}\mathrm{C}^{-}\), is stronger. (c) \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{S}^{-}\) or \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\): Here, we have sulfur and oxygen as the central atoms. Oxygen is more electronegative than sulfur, making it less likely to donate electrons. The base with the sulfur atom, \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{S}^{-}\), is stronger. (d) \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{O}^{-}\) or \(\mathrm{NH}_{2}^{-}\): Here, we have oxygen and nitrogen as the central atoms. Oxygen is more electronegative than nitrogen, making it less likely to donate its electron pair. Thus, the base with the nitrogen atom, \(\mathrm{NH}_{2}^{-}\), is stronger.

Write the structural formula for the conjugate acid of each stronger base

Now, let's find the structural formula of the conjugate acid for each stronger base we found above: (a) \(\mathrm{CH}_{3}\mathrm{O}^{-}\): The conjugate acid of this base is formed by the addition of a proton \(\mathrm{H}^{+}\), so its structural formula will be \(\mathrm{CH}_{3}\mathrm{OH}\). (b) \(\mathrm{CH}_{3}\mathrm{C}^{-}\mathrm{C}^{-}\): The conjugate acid of this base is formed by the addition of a proton \(\mathrm{H}^{+}\), so its structural formula will be \(\mathrm{CH}_{3}\mathrm{C}=\mathrm{C}\mathrm{H}\). (c) \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{S}^{-}\): The conjugate acid of this base is formed by the addition of a proton \(\mathrm{H}^{+}\), so its structural formula will be \(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{SH}\). (d) \(\mathrm{NH}_{2}^{-}\): The conjugate acid of this base is formed by the addition of a proton \(\mathrm{H}^{+}\), so its structural formula will be \(\mathrm{NH}_{3}\).