The simplest amino acid is glycine, \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CO}_{2} \mathrm{H}\). The common feature of amino acids is that they contain the functional groups: an amine group, - \(\mathrm{NH}_{2}\), and a carboxylic acid group, \(-\mathrm{CO}_{2} \mathrm{H}\). An amino acid can function as either an acid or a base. For glycine, the acid strength of the carboxyl group is about the same as that of acetic acid, \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H},\) and the base strength of the amino group is slightly greater than that of ammonia, \(\mathrm{NH}_{3}\). (a) Write the Lewis structures of the ions that form when glycine is dissolved in \(1 M \mathrm{HCl}\) and in \(1 \mathrm{M} \mathrm{KOH}\). (b) Write the Lewis structure of glycine when this amino acid is dissolved in water. (Hint: Consider the relative base strengths of the \(-\mathrm{NH}_{2}\) and \(-\mathrm{CO}_{2}^{-}\) groups.)

Step by step solution

01

Structure of Glycine

First, recognize that the structure of glycine involves an amino group (-NH2) and a carboxylic acid group (-CO2H). These groups are capable of reacting with acids and bases, respectively.

02

Lewis Structure in HCl

When glycine is dissolved in a strong acid like 1M HCl, it donates a proton to the amino group, which results in the formation of a positively charged ammonium ion (-NH3+), while the carboxyl group stays the same.

03

Lewis Structure in KOH

When glycine is dissolved in a strong base like 1M KOH, the base deprotonates the carboxylic acid group, resulting in the formation of the negatively charged carboxylate ion (-CO2-), while the amino group stays the same.

04

Glycine in Water

In water, the amino group can accept a proton, forming an ammonium ion, and the carboxylic acid group can donate a proton, forming a carboxylate ion. Based on relative base strengths, the amino group of glycine, being slightly stronger than ammonia, is protonated while the carboxyl group deprotonates, which leads to the formation of a zwitterion with both positive and negative charges.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Lewis structures

Understanding the Lewis structures is fundamental when exploring chemical bonds within molecules. Lewis structures depict how electrons are arranged around atoms and indicate the type of bonds between them. For glycine, each dot represents a valence electron, while lines signify chemical bonds. To construct a Lewis structure, count the total number of valence electrons from each atom, then allocate them to form bonds or as lone pairs to satisfy the octet rule.

When creating the Lewis structure for glycine (H_{2}NCH_{2}CO_{2}H), we align the amino group (NH_{2}) and carboxylic acid group (CO_{2}H) around the central carbon atom. In acidic or basic solutions, these groups react differently, altering the Lewis structure by either gaining or losing a proton – key for visualizing the acid-base reactions of glycine.

Acid-base chemistry

Acid-base chemistry is pervasive in the study of amino acids. An acid is a substance that donates protons (H+ ions), while a base accepts them. Glycine, like other amino acids, has the unique ability to function as both an acid and a base due to the presence of both amine and carboxyl groups. This dual functionality is critical for its participation in biochemical reactions.

For instance, when glycine is added to an acidic solution like 1M HCl, its amino group, with a lone pair of electrons, grabs a proton from the acid, transforming into an ammonium group (NH_{3}^{+}). Conversely, in a basic solution such as 1M KOH, the acidic hydrogen from the carboxylic acid group is stripped away by the base, leaving a negatively charged carboxylate group (CO_{2}^{-}). These reversible reactions illustrate how glycine responds in different pH environments.

Amino group

The amino group (NH_{2}) is an integral part of amino acids, including glycine. It consists of a nitrogen atom bonded to two hydrogen atoms and the carbon skeleton of the amino acid. This group acts as a base due to its lone pair of electrons on nitrogen that can readily accept a proton. It is this property that lends the amino group its basicity.

In an acidic environment, the amino group acts as a nucleophile, attracting a proton to form an ammonium ion (NH_{3}^{+}). The amino group's base strength is slightly higher than that of ammonia (NH_{3}), a common base, which allows it to effectively participate in the body's physiological processes where it can accept protons to maintain homeostasis or act as a neurotransmitter precursor.

Carboxylic acid group

The carboxylic acid group (CO_{2}H) is another functional group central to the structure of amino acids. It comprises a carbon atom double-bonded to an oxygen, and a hydroxyl group (OH) attached to the same carbon, characterizing the acidic property of this group. It tends to donate a proton (acidic behavior) due to the electronegative oxygen atoms that stabilize the resultant anion.

In the presence of a base such as KOH, the hydrogen from the hydroxyl group in the carboxylic acid is removed, forming a carboxylate anion (CO_{2}^{-}). This anion's stability is due to resonance structures where the negative charge is delocalized between the two oxygen atoms, emphasizing the importance of the carboxylic acid group in the transfer of protons and the overall acid-base balance in biological systems.

Zwitterion

A zwitterion is a molecule that carries both positive and negative charges, but overall is electrically neutral. In the context of amino acids, the zwitterionic form is particularly relevant. For glycine dissolved in water, it gains a proton on the amino group and loses one from the carboxylic acid group due to the differing strengths of its acidic and basic sites.

The resulting structure has an ammonium ion (NH_{3}^{+}) and a carboxylate ion (CO_{2}^{-}) simultaneously present. The ability of glycine to form a zwitterion in neutral environments like water results from its internal acid-base reactions, and is essential for its solubility, reactivity, and overall function in biological systems, such as protein structure and function. This highlights the significance of zwitterions for the structural integrity and function of amino acids within living organisms.