Question

Most organic acids can be represented as $\mathrm{RCOOH}$ where $\mathrm{COOH}$ is the carboxyl group and $\mathrm{R}$ is the rest of the molecule. [For example, $\mathrm{R}$ is ${\mathrm{CH}}_3$ in acetic acid $\left({\mathrm{CH}}_3\mathrm{COOH}\right).]$ (a) Draw a Lewis structure for the carboxyl group. (b) Upon ionization, the carboxyl group is converted to the carboxylate group $\left({\mathrm{COO}}^{-}\right)$. Draw resonance structures for the carboxylate group.

Short answer

(a) Draw C=O and C-O-H. (b) Draw two resonance structures: C=O, C-O⁻ and C-O⁻, C=O.

Step-by-step solution

Understand the Carboxyl Group

The carboxyl group is represented by the chemical formula $\mathrm{COOH}$. In a Lewis structure, each atom shows its bonding and lone pairs. The carbon (C) atom forms a double bond with one oxygen (O), a single bond with another oxygen, and a single bond with the hydrogen (H) atom.

Draw Lewis Structure for Carboxyl Group

Draw the carboxyl group in the following way:1. Connect the carbon (C) atom to an oxygen (O) atom via a double bond: $\mathrm{C}=\mathrm{O}$.2. Connect the same carbon (C) atom to another oxygen (O) atom via a single bond and then to a hydrogen (H) atom: $\mathrm{C}-\mathrm{O}-\mathrm{H}$.3. Ensure each oxygen atom has 2 lone pairs to satisfy the octet rule.

Ionization to Form Carboxylate Group

Upon ionization, the hydrogen atom is removed from the $-\mathrm{OH}$ part, converting $\mathrm{COOH}$ to ${\mathrm{COO}}^{-}$. This results in the charge being delocalized between the two oxygen atoms.

Draw Resonance Structures for Carboxylate Group

The carboxylate ion ${\mathrm{COO}}^{-}$ can be represented by two resonance structures. Draw these as follows:1. In the first structure, draw the carbon double-bonded to one oxygen atom $\mathrm{C}=\mathrm{O}$, while the other oxygen is single-bonded $\mathrm{C}-{\mathrm{O}}^{-}$.2. In the second structure, switch the double and single-bond roles of the oxygen atoms: $\mathrm{C}-{\mathrm{O}}^{-}$ changes to $\mathrm{C}=\mathrm{O}$ and vice versa.3. Both oxygens should have two lone pairs, and the negative charge should be on the negatively charged oxygen in both structures.

Key concepts

Lewis Structure

A Lewis structure is a useful tool for visualizing the layout of atoms in a molecule using lines for bonds and dots for electrons. In a carboxyl group ( COOH ), we focus on the carbon (C) atom bonded to two oxygen atoms (O) and one hydrogen atom (H). The C atom is the central atom. Visualizing this:

• A double bond connects C to one O: C=O .
• A single bond connects C to another O and then to H: C-O-H .
• Oxygen atoms each receive two pairs of electrons (lone pairs) to fulfill the octet rule.

Each element aims to achieve a full outer shell—carbon reaches four bonds by sharing electrons with oxygen and hydrogen.

Resonance Structures

A molecule like a carboxylate ion ( COO^- ) can be represented in different forms that convey the distribution of electrons. These are called resonance structures. In essence, these structures demonstrate how electrons can shift places but the overall position of the atoms remains unchanged.

• In one structure, we depict the carbon with a double bond to one oxygen ( C=O ) and a single bond to another activated with a negative charge ( C-O^- ).
• The alternative moves the double bond to the other oxygen, making the first a single bond with a negative charge ( C-O^- ).
• This shows that neither structure is more correct but a resonance hybrid of both representations is more accurate.

Resonance indicates that electron pairs can flow between atoms, stabilizing the molecule energetically.

Carboxylate Group

Upon ionization, the carboxyl group ( COOH ) loses a hydrogen ion (H^+) and becomes the carboxylate group ( COO^- ). This transition is important for the chemistry of acids. Here's how this transformation occurs:

• The loss of an H^+ from the hydroxyl group ( -OH ) results in the carboxylate ion.
• We note two oxygen atoms now possibly sharing a delocalized negative charge between them.
• This negative charge is reflected in the resonance forms we discussed.

Understanding this group is crucial while analyzing organic acids because it affects both their reactivity and how they dissociate in solutions.

Organic Acids

Organic acids feature prominently in chemistry due to their carboxyl group ( COOH ). They're mainly characterized by their ability to donate protons (H^+ ions).

• For example, acetic acid ( CH_3COOH ) uses the methyl group ( CH_3 ) as its R group.
• Organic acids like acetic acid are weak acids, meaning they do not completely dissociate in water.
• These acids are crucial in biochemistry, participating in reactions like the Krebs cycle.

Their role in both biochemical cycles and industrial applications make them foundational to furthering knowledge in chemistry.

Ionization Process

The ionization process of an organic acid involves losing an H^+ ion, which is effectively a proton. In the carboxyl group, this often refers to the hydroxyl group losing an H^+. Here's what takes place during ionization:

• The OH group donates its hydrogen, leaving a negative charge on the oxygen, creating COO^- .
• This transformation is reversible, which is significant for dynamic equilibria in solutions.
• Ionization alters the pH and influences how the organic acid participating in chemical reactions behaves.

Understanding ionization helps us comprehend processes such as acid-base reactions, determining the strength and behavior of acids in different contexts.