The ammonium ion (NH4^+, p Ka=9.25) has a lower p Ka than the methylammonium ion (CH3 NH3^+, p K

step 1 So in this problem, we're given that the ammonium ion has a PCA of 9 .25, which is lower than th

The ammonium ion (NH4^+, p Ka=9.25) has a lower p Ka than...

The ammonium ion $\left(\mathrm{NH}_{4}^{+}, \mathrm{p} K_{\mathrm{a}}=9.25\right)$ has a lower $\mathrm{p} K_{\mathrm{a}}$ than the methylammonium ion $\left(\mathrm{CH}_{3} \mathrm{NH}_{3}^{+}, \mathrm{p} K_{\mathrm{a}}=10.66\right) .$ Which is the stronger base, ammonia $\left(\mathrm{NH}_{3}\right)$ or methylamine $\left(\mathrm{CH}_{3} \mathrm{NH}_{2}\right)$ ? Explain.

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Key Concepts

Conjugate Acid-Base Relationship

In acid-base chemistry, every base has a conjugate acid and the strength of the base can be inferred from the acidity (pKa) of its conjugate acid. A higher pKa value indicates a weaker conjugate acid and consequently a stronger base, while a lower pKa reflects a stronger acid and a weaker base. This relationship is critical for comparing the basicity of molecules such as ammonia and methylamine through the pKa values of their conjugate acids.

pKa Values and Base Strength

The pKa value of a conjugate acid serves as a quantitative measure of its acidity. When comparing two bases, the one whose conjugate acid has the higher pKa is typically the stronger base. This concept explains why methylamine, whose conjugate acid has a higher pKa than that of ammonia, is considered the stronger base.

Inductive Effects

Inductive effects describe how electron-donating or electron-withdrawing groups attached to a molecule influence its electron density. In the context of nitrogen bases, the presence of an electron-donating group, such as a methyl group in methylamine, increases the electron density on the nitrogen atom, thereby enhancing its ability to accept a proton. This effect is why methylamine is a stronger base than ammonia.

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