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Ch. 20 - Carboxylic Acids
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Non è quello che usi tu?Cambia libro di testo
Capitolo 20, Problema 3b

Rank the compounds in each set in order of increasing acid strength.
(b) CH3CH2CH2CHBrCOOH, CH3CH2CHBrCH2COOH, CH3CHBrCH2CH2COOH

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Step 1: Understand the concept of acid strength. Acid strength is influenced by the stability of the conjugate base formed after the acid donates a proton. Electron-withdrawing groups, such as halogens, stabilize the conjugate base by delocalizing the negative charge, increasing acid strength.
Step 2: Analyze the compounds provided. Each compound contains a carboxylic acid group (-COOH) and a bromine atom (-Br) attached to different positions in the carbon chain. The position of the bromine atom relative to the carboxylic acid group affects the acid strength due to the inductive effect.
Step 3: Evaluate the inductive effect. The closer the bromine atom is to the carboxylic acid group, the stronger the electron-withdrawing effect, which stabilizes the conjugate base and increases acid strength.
Step 4: Rank the compounds based on the position of the bromine atom. For CH3CH2CH2CHBrCOOH, the bromine is furthest from the carboxylic acid group, resulting in the weakest acid. For CH3CH2CHBrCH2COOH, the bromine is closer, making it a stronger acid. For CH3CHBrCH2CH2COOH, the bromine is closest to the carboxylic acid group, resulting in the strongest acid.
Step 5: Final ranking in order of increasing acid strength: CH3CH2CH2CHBrCOOH < CH3CH2CHBrCH2COOH < CH3CHBrCH2CH2COOH.

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Acidity and pKa

Acidity refers to the ability of a compound to donate a proton (H+) in solution. The strength of an acid is often quantified by its pKa value; lower pKa values indicate stronger acids. Understanding the relationship between structure and acidity is crucial, as factors such as electronegativity and resonance can significantly influence a compound's ability to stabilize the negative charge of its conjugate base.
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Identifying pKa values

Inductive Effect

The inductive effect describes the electron-withdrawing or electron-donating influence of substituents on a molecule. Electronegative atoms, such as halogens, can stabilize the negative charge of a conjugate base through their electron-withdrawing properties, thereby increasing acidity. In the context of the given compounds, the position and number of halogen substituents will affect their relative acid strengths.
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Understanding the Inductive Effect.

Resonance Stabilization

Resonance stabilization occurs when a molecule can be represented by multiple valid Lewis structures, allowing for the delocalization of electrons. This delocalization can stabilize the conjugate base formed after deprotonation, enhancing acidity. In the compounds provided, the presence of resonance structures can significantly influence their acid strengths, particularly when combined with inductive effects from substituents.
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The radical stability trend.