Question

The melting points and solubility in water of amino acids are generally higher than that of the corresponding halo acids. Explain.

A. Due to the presence of amino groups
B. Due to the presence of carboxyl groups
C. Due to the aromatic nature of amino acids
D. Due to the absence of hydrogen bonding in halo acids

Answer 1

Amino acids have higher melting points and solubility than haloacids due to their ability to form hydrogen bonds and their zwitterionic nature, with both negatively and positively charged groups. B. Due to the presence of carboxyl groups

Step-by-step explanation:

The melting points and solubility in water of amino acids are generally higher than that of the corresponding haloacids due to the presence of both the amino and carboxyl groups which allow for hydrogen bonding and zwitterionic formation. Amino acids, being zwitterions, have both a negatively charged carboxylate anion and a positively charged ammonium group in neutral solutions, leading to solid structures more akin to inorganic salts with high melting points. They also display increased solubility due to their ability to form hydrogen bonds with water, as hydrophilic amino acids like lysine (Lys) show high solubility over a wide pH range because of the ionized ɛ-amine group.

In contrast, haloacids, which are derivatives of alkanoic acids by replacing a hydrogen atom with a halogen, do not typically form hydrogen bonds with water, leading to their generally lower solubility. The presence of hydrogen bonding in amino acids substantially increases their interaction with water, making them more soluble than many haloacids which lack this feature. Additionally, the solubility of amino acids is modulated by the nature of their side chains, with hydrophilic side chains contributing to higher solubility.

Answer 2

Amino acids have higher melting points and solubility in water than halo acids primarily due to the presence of amino groups (A). Amino acids form hydrogen bonds and can exist as zwitterions, which contribute to their higher solubility and melting points similar to inorganic salts. So, the correct option is A. Due to the presence of amino groups.

Step-by-step explanation:

The melting points and solubility in water of amino acids are generally higher than halo acids. This is because amino acids have amino groups that contribute to higher melting points and solubility due to the ability to form hydrogen bonds and exist in zwitterionic form at certain pH levels. A zwitterion contains both a positively charged group (ammonium) and a negatively charged group (carboxylate). This dual charge contributes to their solubility in water and high melting points similar to inorganic salts.

Moreover, the solubility of amino acids is affected by the polarity of their side chains. Hydrophilic amino acids with uncharged but polar side groups or those with ionizable side groups are generally more soluble compared to amino acids with nonpolar side groups. Halo acids, on the other hand, show limited solubility in water as they do not form hydrogen bonds as effectively as amino acids do.

When considering hydrophobic or hydrophilic characteristics, the solubility in water of amino acids is largely a function of the polar or nonpolar nature of the side chain. For example, increasing the hydrocarbon content in the side chain leads to a decrease in water solubility, which illustrates how the nature of the side chain impacts the solubility properties of amino acids. In conclusion, the presence of amino groups (A) is the main reason for the higher melting points and solubility of amino acids compared to halo acids. The zwitterionic nature of amino acids along with their ability to form hydrogen bonds results in these increased properties.