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Chapter 22: Problem 90

When pure crystalline amino acids are heated, decomposition generally occurs before the solid melts. Account for this observation. (Hint: Crystalline amino acids exist as \(\mathrm{H}_{3} \mathrm{NCRHCOO}^{-},\) called zwitterions.)

Short Answer

Expert verified
In conclusion, the decomposition of crystalline amino acids upon heating occurs primarily due to the strong ionic interactions within the amino acid's zwitterion structure (\(\mathrm{H}_{3} \mathrm{NCRHCOO}^{-}\)). These interactions, when disrupted by heat energy, cause the amino acids to decompose rather than melt. The strong ionic interactions form a stable crystal lattice that requires significant energy to disrupt, so the decomposition reaction consumes heat energy and prevents the solid from melting.

Step by step solution

01

Understand zwitterions and their properties

Amino acids are the building blocks of proteins and have both an amino group (-NH2) and a carboxyl group (-COOH). In a solid-state, amino acids exist as zwitterions. A zwitterion is a molecule with both positive and negative charges. In the case of amino acids, the positive charge comes from the protonated amino group (NH3+), and the negative charge from the deprotonated carboxyl group (COO-). The overall formula for zwitterion is \(\mathrm{H}_{3} \mathrm{NCRHCOO}^{-}\).
02

Discuss the decomposition of amino acids

When amino acids are in their crystalline state, the zwitterions are held together by strong ionic interactions between the positively charged amino groups and the negatively charged carboxyl groups. These interactions form a stable crystal lattice.
03

Understand the effect of heating on amino acids

When crystalline amino acids are heated, the energy provided by the heat weakens the bonds between the positively and negatively charged groups of the zwitterions. As the temperature increases, these bonds break before the amino acid molecules have a chance to reach the melting point. As a result, the amino acids decompose into other constituents rather than melting, producing gases, vapors, and other breakdown products. The decomposition reaction is endothermic, consuming heat energy, and further preventing the solid from melting.
04

Conclusion

In conclusion, the decomposition of crystalline amino acids upon heating occurs primarily due to the strong ionic interactions within the amino acid's zwitterion structure. These interactions, when disrupted by heat energy, cause the amino acids to decompose rather than melt.

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