Question

Guanidine and the guanidino group present in arginine are two of the strongest organic bases known. Account for their basicity.

Short answer

Question: Explain why guanidine and the guanidino group present in arginine are considered strong bases. Answer: Guanidine and the guanidino group present in arginine are considered strong bases due to the presence of multiple electron-donating nitrogen atoms and resonance stabilization in their structures. The resonance delocalizes electrons across the molecules, increasing the electron density on the nitrogen atoms and resulting in stronger proton acceptance. Furthermore, the stability of their conjugate acids makes these molecules more basic.

Step-by-step solution

Identify the guanidine and guanidino group structures

Guanidine is a molecule with the chemical formula (NH2)2C=NH, while the guanidino group in arginine is -NH-C(NH2)2. Both of these molecules share the same central carbon atom double bonded to a nitrogen atom with two amine groups (-NH2) attached to it. The double bond and amine groups are the key features we need to focus on when determining their basicity.

Recognize the electron-donating groups

The main electron-donating groups in both guanidine and the guanidino group are the three nitrogen atoms attached to the central carbon atom. Each of these nitrogen atoms has a lone pair of electrons, which are vital for the basicity of the molecules as they can donate these electrons to form bonds with protons (H+ ions).

Explain the importance of resonance in basicity

Resonance stabilization plays a significant role in the basicity of guanidine and the guanidino group. Due to the presence of the double bond and the electron-donating groups in these molecules, they can undergo resonance, where the electrons can be delocalized across the molecule. This resonance stabilization increases the electron density of the nitrogen atoms and thus makes them stronger proton acceptors.

Describe the stability of the conjugate acids

When guanidine and the guanidino group accept a proton, they form their respective conjugate acids. The stability of these conjugate acids is crucial for determining the strength of a base. In the case of both guanidine and the guanidino group, their conjugate acids are stabilized through resonance, which disperses the positive charge across the multiple nitrogen atoms, making the conjugate acids more stable. As a result, the starting molecules (guanidine and the guanidino group) are more likely to accept protons and act as strong bases.

Conclusion

Guanidine and the guanidino group present in arginine are two of the strongest organic bases known due to the presence of multiple electron-donating nitrogen atoms and resonance stabilization in their structures. The resonance delocalizes electrons across the molecules, thereby increasing the electron density on the nitrogen atoms and resulting in stronger proton acceptance. Furthermore, the stability of their conjugate acids makes these molecules more basic.