Animated Solution for Chemistry - Organic Compounds: Which of the following is least basic?

When we talk about the basicity of nitrogen-containing organic compounds, we are fundamentally discussing Lewis basicity. A Lewis base is an electron-pair donor. Therefore, the basic strength of an amine or its derivatives is directly proportional to the availability of the lone pair of electrons on the nitrogen atom. If the lone pair is localized and ready to grab a proton, the compound is a strong base. Conversely, if the lone pair is busy elsewhere, the compound becomes a weak base.

Delocalized Lone Pairs

Let's evaluate the aliphatic amines first: triethylamine, $({\text{C}}_2{\text{H}}_5{)}_3\text{N}$, and diethylamine, $({\text{C}}_2{\text{H}}_5{)}_2\text{NH}$. In these molecules, the nitrogen atom is bonded only to alkyl groups (ethyl groups) and hydrogen. Alkyl groups are electron-donating through the $+I$ (inductive) effect. They push electron density towards the nitrogen atom, making the localized lone pair even more electron-rich and highly available for protonation. Thus, these are relatively strong bases.

Now, consider N-ethylacetamide, $({\text{CH}}_3\text{CO}){\text{NHC}}_2{\text{H}}_5$. This is an amide. The nitrogen atom is directly attached to a carbonyl group ($\text{C}=\text{O}$). The highly electronegative oxygen atom pulls electron density towards itself, creating a partial positive charge on the carbonyl carbon. To stabilize this, the lone pair on the adjacent nitrogen atom delocalizes into the carbonyl $\pi$ system through resonance. Because the lone pair is spending a significant amount of time forming a partial double bond with the carbonyl carbon, it is much less available to accept a proton. This makes amides significantly less basic than aliphatic amines.

Finally, we arrive at diacetamide, $({\text{CH}}_3\text{CO}{)}_2\text{NH}$. This molecule is an imide, characterized by a nitrogen atom flanked by two carbonyl groups.

Here, the lone pair on the nitrogen atom is subjected to a tug-of-war. It can delocalize into the $\pi$ system of the left carbonyl group, and it can also delocalize into the $\pi$ system of the right carbonyl group. This phenomenon is known as cross-conjugation. Because the lone pair is extensively delocalized over two strong electron-withdrawing groups, the electron density on the nitrogen atom is drastically reduced.

Conclusion: The lone pair in diacetamide is the least available for donation among all the given options. Consequently, $({\text{CH}}_3\text{CO}{)}_2\text{NH}$ is the weakest base.