Problem 12 - Chemistry: Principles and Reactions

Question

Glycine, an essential amino acid, has the formula \(\mathrm{NH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\). Its skeleton structure has \(\mathrm{C}-\mathrm{C}\) and \(\mathrm{C}-\mathrm{N}\) bonds but \(\mathrm{no}\) N-O bonds. Write its Lewis structure.

Step-by-Step Solution

Answer

Answer: The Lewis structure of glycine is drawn by counting the total number of valence electrons (23), arranging the atoms with the lowest electronegative atom (carbon) at the center, connecting atoms with single bonds, and distributing the remaining electrons to satisfy the octet rule. The final structure is: H O | || H-N-C-C-O-H | H

Step 1: Count the valence electrons

To do this, we find the valence electrons of each atom in the molecule and sum them up. - N has 5 valence electrons - H has 1 valence electron (there are 4 H atoms, total 4 valence electrons) - C has 4 valence electrons (there are 2 C atoms, total 8 valence electrons) - O has 6 valence electrons Summing up, we have a total of 23 valence electrons in this molecule.

Step 2: Arrange the atoms

As carbon has a lower electronegativity in comparison to nitrogen and oxygen, we start with a carbon atom at the center. Place the second C and N adjacent to central the C, and the O adjacent to the second C. Connect the hydrogen atoms to the first carbon atom and the oxygen atom. The preliminary arrangement can be written as \(\mathrm{H-N-C-C-O-H}\).

Step 3: Connect the atoms with single bonds

Connect the atoms using single bonds to construct the molecule's backbone. Now, we have \(\mathrm{H-N-C-C-O-H}\). Each single bond contains 2 electrons, and in this arrangement, we've used 10 of the total 23 valence electrons.

Step 4: Distribute the remaining electrons

With 13 valence electrons remaining, we distribute them to satisfy the octet rule for each atom, except for hydrogen which only needs 2 electrons in total. Assign the remaining electrons as lone pairs, starting with the most electronegative atoms (O and N). After doing this, we find that there are still 3 electrons remaining. We assign these to nitrogen, thus forming a lone electron or free radical on N, which is commonly seen in amino acid structures.

Step 5: Verify the formal charges

Lastly, we verify the formal charges for each atom to make sure the structure is correct. In our case, the formal charges are: - N: -1 - C: 0 (for both carbon atoms) - O: 0 - H: 0 (for all hydrogen atoms) Since the sum of the formal charges is zero, which equals the net charge of the molecule, we can conclude that the Lewis structure is drawn correctly. The final Lewis structure of glycine is: H O | || H-N-C-C-O-H | H

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom. These electrons are crucial because they determine how an atom interacts and forms chemical bonds with other atoms. For glycine, an amino acid, it involves understanding the number of valence electrons for each type of atom in the molecule. Here's how they are evaluated for each atom in glycine:

• Nitrogen (N) has 5 valence electrons.
• Carbon (C), with 2 atoms present, contributes 8 valence electrons (4 each).
• Oxygen (O) holds 6 valence electrons.
• Hydrogen (H), appearing 4 times, adds up to 4 valence electrons (1 each).

Summed up, these valence electrons total 23 for the entire molecule of glycine. Understanding these totals is crucial because it sets the stage for drawing a correct Lewis structure and predicting molecular interactions.

Octet Rule

The octet rule is a general guideline that atoms tend to follow to achieve stability. Atoms aim to have eight electrons in their valence shell, similar to the electron configuration of noble gases. This rule applies to glycine as we construct its Lewis structure, focusing on ensuring that carbon, nitrogen, and oxygen satisfy their octet requirements.

• Carbon and nitrogen aim for eight electrons in their outermost shell.
• Oxygen also follows the octet rule, carrying its requirement for eight electrons.
• Hydrogen is an exception, needing only two electrons for a full shell.

By distributing the remaining electrons after single-bond construction, we ensure that each atom achieves this stable electronic arrangement. The octet rule is pivotal in defining the structure and stability of amino acids like glycine.

Formal Charge Calculation

Calculating formal charges is key in validating the accuracy of a Lewis structure. It helps ensure that the molecule is balanced electronically. To calculate the formal charge, use the formula: \( \text{Formal Charge} = \text{Valence electrons} - (\text{Non-bonding electrons} + \frac{1}{2}\text{Bonding electrons}) \). Let's see how this applies to glycine:

• Nitrogen: Formal charge is \(-1\)
• Each Carbon: Both have a formal charge of \(0\)
• Oxygen: Formal charge is also \(0\)
• Each Hydrogen: Formal charge of \(0\)

Adding these charges shows they sum to zero, reflecting a stable structure. This mathematical check confirms that the drawn Lewis structure is indeed accurate for glycine, with electrons optimally arranged. Understanding how to calculate formal charges aids in drawing correct molecular structures in chemistry.

Amino Acids Chemistry

Amino acids are building blocks of proteins, essential for numerous biological functions. Glycine, with the formula \(\text{NH}_2\text{CH}_2\text{COOH}\), is the simplest amino acid but plays vital roles in the body, such as forming proteins and acting in neurotransmitter pathways. Learning about its chemistry involves understanding how atoms are arranged and bonded in the structure.

• The amino group (\(\text{-NH}_2\)) shows how nitrogen participates in peptide bonds.
• A carboxylic acid group (\(\text{-COOH}\)) showcases typical acidic behavior.
• The carbon backbone connects these functional groups, determining molecule shape and reactivity.

Understanding glycine's structure and featuring parts supports the larger exploration of how proteins are made and how they function in the body. This fundamental knowledge in amino acids chemistry forms the basis for studying biological processes and drug design.