Question

Consider the following acids: (1) \(\mathrm{O}-\mathrm{HOC}_{6} \mathrm{H}_{4} \mathrm{COOH}\) (2) \(\mathrm{O}-\mathrm{CH}_{3} \mathrm{OC}_{6} \mathrm{H}_{4} \mathrm{COOH}\) (3) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{COOH}^{\circ}\) Arrange these acids in the decreasing order of their acidities. (a) \(1>3>2\) (b) \(3>2>1\) (c) \(1>2>3\) (d) \(2>3>1\)

Short answer

Option (a) 1>3>2 is incorrect. The correct order is (b) 3>1>2.

Step-by-step solution

Analyze the Structures

Start by looking at the structure of each compound and identifying functional groups and possible substituents. The substitution patterns and groups on the benzene ring can affect acidity by influencing the electron density at the carboxylic acid group.

Assess Electron Donating/Withdrawing Effects

The acidity of benzoic acids is affected by groups on the benzene ring. - Compound 1 has a hydroxyl group (OH) which is electron donating via resonance, making the acid less acidic. - Compound 2 has a methoxy group (CH₃O) which can donate electrons via resonance, also decreasing acidity. - Compound 3 is benzoic acid with no substituents, so it maintains its default acidity level.

Compare Acidity Contributions

Considering resonance and electron donation: - Compound 3 C₆H₅COOH has no electron donating group, thus it remains more acidic compared to compound 1 and compound 2. - Compound 1 and compound 2 both have groups (OH, OCH) that donate electrons. The methoxy group in compound 2, being a stronger electron donor compared to hydroxyl in compound 1, reduces acidity more.

Determine Correct Order

The order of acidity is influenced by the extent to which the substituents affect the electron density around the carboxylic acid group. Since having no electron donating group increases acidity (compound 3) and methoxy results in more electron donation, reducing more acidity compared to the hydroxyl (compound 1): Order: Compound 3 > Compound 1 > Compound 2.

Key concepts

Electron Donating and Withdrawing Effects

In organic chemistry, the acidity of a compound like benzoic acid can be influenced significantly by the substituents attached to its benzene ring. To understand this, let's think about electron donating and withdrawing effects. If a group attached to the benzene ring donates electrons, it's known as an electron donating group (EDG). Conversely, if it pulls electrons away, it's termed an electron withdrawing group (EWG).

Here's how it plays out:

• Electron donating groups like a hydroxyl group (\(\mathrm{OH}\)) or a methoxy group (\(\mathrm{OCH}_3\)) add electron density to the benzene ring.
• These groups stabilize the negative charge on the conjugate base (which forms when the hydrogen is lost), thus reducing overall acidity.
• When no substituents are present, the electron density is unaltered, resulting in a more acidic compound compared to ones with donating groups.

Overall, the more an electron donating group influences the molecule, the less acidic the compound remains.

Resonance Effects in Acidity

Resonance is another concept that can dramatically change the acidity of a benzoic acid. Through resonance, electrons are delocalized within the molecule, affecting the stability of its conjugate base.This delocalization can either enhance or reduce acidity depending on the nature of the substituent.

Let's break it down:

• A hydroxyl group (\(\mathrm{OH}\)) on the benzene ring donates electrons via resonance. It causes electron delocalization across the structure.
• Similarly, a methoxy group (\(\mathrm{OCH}_3\)) also participates in resonance. However, it has a stronger electron donating effect than hydroxyl, thus making the acid even less acidic.
• Without any substituents, the resonance doesn't occur, allowing the compound to manifest its natural acidity.

This means that resonance effects can either enhance or dilute the electron density alterations caused by substituents, profoundly impacting acidity.

Structure-Function Relationship in Organic Chemistry

Structure and function in organic chemistry often go hand in hand, especially when evaluating acidity. The position and type of substituents in aromatic compounds like benzoic acids can significantly alter their chemical behavior.

Consider the following:

• The base structure of benzoic acid includes the benzene ring and carboxylic acid group (\(\mathrm{COOH}\)).
• Substituents like hydroxyl (\(\mathrm{OH}\)) or methoxy (\(\mathrm{CH}_3\)) groups alter the acidity based on their ability to donate or withdraw electrons.
• These changes in structure lead to functional differences, such as shifts in acidity.Through understanding these relationships, one can predict and manipulate chemical properties for desired outcomes.

In essence, subtle changes in molecular structure significantly influence the role and characteristics of the compound, showcasing the dynamic interplay in organic chemistry.