Chapter 27: Problem 39

A chemically modified guanidino group is present in cimetidine (Tagamet), a widely prescribed drug for the control of gastric acidity and peptic ulcers. Cimetidine reduces gastric acid secretion by inhibiting the interaction of histamine with gastric \(\mathrm{H}_{2}\) receptors. In the development of this drug, a cyano group was added to the substituted guanidino group to alter its basicity. Do you expect this modified guanidino group to be more basic or less basic than the guanidino group of arginine? Explain.

Short Answer

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Answer: The modified guanidino group in cimetidine is less basic compared to the guanidino group of Arginine.

Step by step solution

Understanding the structures of guanidino groups

The guanidino group of Arginine consists of a central carbon atom attached to three nitrogen atoms, with two of those nitrogen atoms connected by a double bond. The third nitrogen atom is part of an amino acid side chain. In the case of cimetidine, the guanidino group is chemically modified, by adding a cyano group to the substituted guanidino group.

Evaluating the effect of adding a cyano group

A cyano group is a functional group in which a carbon atom is triply bonded to a nitrogen atom (C≡N). When adding a cyano group to the guanidino group, it may change the electronic properties of the group by withdrawing electron density from the neighboring atoms. This is because the strong electronegativity of the nitrogen in the cyano group attracts the electrons towards itself, reducing the electron density available for other atoms.

Comparing the basicity of guanidino groups

Basicity is the ability of a molecule to accept a proton (H+). In the case of guanidino groups, the nitrogen atoms are the primary centers for accepting protons. As mentioned earlier, adding a cyano group reduces the electron density available for the neighboring atoms, including the nitrogen atoms in the modified guanidino group. Since basicity is directly related to the electron density of the atoms (higher electron density leads to higher basicity), a reduction in electron density caused by the addition of the cyano group will lower the basicity of the modified guanidino group compared to the unmodified guanidino group of Arginine.

Conclusion

The modified guanidino group in cimetidine, which has a cyano group added to it, is expected to be less basic compared to the guanidino group of Arginine. This is due to the electron-withdrawing effect of the cyano group, which reduces the electron density of the nitrogen atoms in the modified guanidino group, and consequently decreases its basicity.

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Short Answer

Step by step solution

Understanding the structures of guanidino groups

Evaluating the effect of adding a cyano group

Comparing the basicity of guanidino groups

Conclusion

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