Identify what arginine is

Arginine is an amino acid. Like other amino acids, it exists predominantly as a zwitterion in the solid state and in many conditions in solution: $\ce{^{+}H3N-CHR-COO^-}$ It also has a basic side chain (guanidino group), making it strongly polar.

Evaluate each statement

Option A: It has a fairly high melting point.

Amino acids are ionic/zwitterionic solids, so they generally have high melting points compared with ordinary covalent organic compounds.

Hence, A is true.

Option B: It is associated with more than one $\mathrm{pK_a}$ values.

Arginine has multiple ionizable groups:

• the carboxyl group
• the amino group
• the basic side chain (guanidino group)

Therefore, it has more than one $\mathrm{pK_a}$ value.

Hence, B is true.

Option C: It has high solubility in benzene.

Benzene is a non-polar solvent. Arginine is highly polar/ionic due to its zwitterionic nature and ionizable groups. Such compounds are generally insoluble or very poorly soluble in benzene.

Therefore, the statement that arginine has high solubility in benzene is not true.

Hence, C is false.

Option D: It is a crystalline solid.

Amino acids are generally colorless crystalline solids because of strong ionic interactions in the crystal lattice.

Hence, D is true.

Conclusion

The statement that is not true for arginine is: $\boxed{\text{C}}$

Comparison with stored correct answer

Stored correct answer: C

My derived answer is also C, so they agree.