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Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry
Bruice - Organic Chemistry 8th Edition
Bruice8th EditionOrganic ChemistryISBN: 9780135213711Non è quello che usi tu?Cambia libro di testo
Capitolo 2, Problema 59

Rank the following compounds from strongest to weakest acid:
CH3CH2OH; CH3CH2NH2; CH3CH2SH; CH3CH2CH3

Guida verificata passo dopo passo
1
Step 1: Understand the concept of acidity. Acidity is determined by the ability of a compound to donate a proton (H⁺). The more stable the conjugate base after losing a proton, the stronger the acid.
Step 2: Analyze the functional groups in each compound. CH3CH2OH contains an alcohol (-OH) group, CH3CH2NH2 contains an amine (-NH2) group, CH3CH2SH contains a thiol (-SH) group, and CH3CH2CH3 is an alkane with no acidic functional group.
Step 3: Compare the electronegativity of the atoms directly bonded to hydrogen in each functional group. Oxygen in CH3CH2OH is more electronegative than sulfur in CH3CH2SH, and sulfur is more electronegative than nitrogen in CH3CH2NH2. Electronegativity affects the stability of the conjugate base.
Step 4: Consider the polarizability of the atoms. Sulfur in CH3CH2SH is larger and more polarizable than oxygen in CH3CH2OH, which can stabilize the conjugate base better, making thiols generally more acidic than alcohols.
Step 5: Evaluate CH3CH2CH3. Alkanes like CH3CH2CH3 lack acidic functional groups and are extremely weak acids. Rank the compounds based on the above analysis: CH3CH2SH > CH3CH2OH > CH3CH2NH2 > CH3CH2CH3.

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Acidity and pKa

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

Functional Groups and Their Effects

Different functional groups have varying effects on acidity. For example, alcohols (like CH3CH2OH) can donate protons due to the hydroxyl (-OH) group, while amines (like CH3CH2NH2) are generally basic and less acidic. Thiols (like CH3CH2SH) are more acidic than alcohols due to the lower electronegativity of sulfur compared to oxygen, which stabilizes the conjugate base.
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Identifying Functional Groups

Inductive Effect and Hybridization

The inductive effect refers to the electron-withdrawing or electron-donating effects of substituents on acidity. Electronegative atoms can stabilize the negative charge of a conjugate base, enhancing acidity. Additionally, the hybridization of the atom bearing the acidic proton affects acidity; sp-hybridized carbons are more acidic than sp2 or sp3 due to increased s-character, which holds the electrons closer to the nucleus.
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Understanding the Inductive Effect.