Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 2 - Acids and Bases: Central to Understanding Organic Chemistry

Problem 42a

Chapter 3, Problem 42a

Indicate whether a protonated amine (RN⁺H₃) with a pKa value of 9 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.
1. pH = 1
2. pH = 3
3. pH = 5
4. pH = 7
5. pH = 10
6. pH = 13

Verified step by step guidance

Determine the relationship between pKa and pH using the Henderson-Hasselbalch equation: \( \text{pH} = \text{pKa} + \log \left( \frac{[\text{Base}]}{[\text{Acid}]} \right) \). Here, the base is the neutral amine (RNH2), and the acid is the protonated amine (RN+H3).

Rearrange the equation to solve for the ratio of base to acid: \( \frac{[\text{Base}]}{[\text{Acid}]} = 10^{(\text{pH} - \text{pKa})} \).

Substitute the given pKa value (9) and the pH values from Problem 41 into the equation to calculate the ratio of neutral (base) to charged (acid) molecules for each pH value.

Interpret the ratio: If the ratio \( \frac{[\text{Base}]}{[\text{Acid}]} \) is greater than 1, there are more neutral molecules (RNH2). If the ratio is less than 1, there are more charged molecules (RN+H3).

For each pH value, compare the pH to the pKa: If \( \text{pH} > \text{pKa} \), the solution favors the base (neutral form). If \( \text{pH} < \text{pKa} \), the solution favors the acid (charged form).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

pKa and pH Relationship

The pKa value of a compound indicates the pH at which half of the species are protonated and half are deprotonated. A lower pKa means a stronger acid, while a higher pKa indicates a weaker acid. In this case, the pKa of 9 suggests that at pH values below 9, the amine will predominantly exist in its protonated form (charged), while at pH values above 9, it will be more neutral.

Protonation of Amines

Amines can exist in both protonated (charged) and deprotonated (neutral) forms. The protonated form (RN+H3) occurs when the amine accepts a proton (H+), making it positively charged. The equilibrium between these forms is influenced by the solution's pH relative to the amine's pKa, determining the predominant species in solution.

Acid-Base Equilibrium

Acid-base equilibrium describes the balance between protonated and deprotonated species in a solution. The Henderson-Hasselbalch equation can be used to predict the ratio of these species based on pH and pKa. Understanding this equilibrium is crucial for determining whether more charged or neutral molecules of the amine will be present at specific pH levels.

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Determining Acid/Base Equilibrium

Related Practice

Textbook Question

For each of the following compounds (here shown in their acidic forms), write the form that predominates in a solution with a pH = 5.5:

i. HON⁺H₃ (pK_a = 6.0)

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Textbook Question

A naturally occurring amino acid such as alanine has a group that is a carboxylic acid and a group that is a protonated amine. The pK_a values of the two groups are shown.

c. Draw the structure of alanine in a solution at physiological pH (pH 7.4).

Textbook Question

As long as the pH is not less than ___________, at least 50% of a protonated amine with a pK_a value of 10.4 will be in its neutral, nonprotonated form.

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Textbook Question

Indicate whether an alcohol (ROH) with a pK_a value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.

3. pH = 7

4. pH = 10

5. pH = 13

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Textbook Question

Indicate whether an alcohol (ROH) with a pK_a value of 15 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.

1. pH = 1

2. pH = 3

3. pH = 5

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Textbook Question

For each of the following compounds (here shown in their acidic forms), write the form that predominates in a solution with a pH = 5.5:

g. HNO₂ (pK_a= 3.4)

h. HNO₃ (pK_a= −1.3)

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Indicate whether a protonated amine (RN+H3) with a pKa value of 9 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.1. pH = 1 2. pH = 3 3. pH = 5 4. pH = 7 5. pH = 10 6. pH = 13

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Key Concepts

pKa and pH Relationship

Protonation of Amines

Acid-Base Equilibrium

Indicate whether a protonated amine (RN⁺H₃) with a pKa value of 9 has more charged or more neutral molecules in a solution with the pH values given in Problem 41.
1. pH = 1
2. pH = 3
3. pH = 5
4. pH = 7
5. pH = 10
6. pH = 13