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 17b,c

Chapter 3, Problem 17b,c

Ethyne (HC≡CH) has a pK_a value of 25, water has a pK_a value of 15.7, and ammonia (NH₃) has a pK_a value of 36. Draw the equation, showing equilibrium arrows that indicate whether reactants or products are favored, for the acid–base reaction of ethyne with
b. ⁻NH₂.
c. Which would be a better base to use if you wanted to remove a proton from ethyne, HO⁻ or ^-NH₂?

Verified step by step guidance

Step 1: Understand the acid-base reaction. Ethyne (HC≡CH) acts as an acid, and the base in question is −NH2 (amide ion). The reaction involves the transfer of a proton (H⁺) from ethyne to −NH2, forming the conjugate base of ethyne (HC≡C⁻) and the conjugate acid of −NH2 (NH3).

Step 2: Write the chemical equation for the reaction. The equation is:

HC≡CH + NH_2^- 
ightleftharpoons HC≡C^- + NH_3

. Use equilibrium arrows to indicate the direction of the reaction.

Step 3: Compare the pKa values to determine the direction of equilibrium. The pKa of ethyne is 25, and the pKa of ammonia (NH3) is 36. Since the pKa of ammonia is higher, it is a weaker acid compared to ethyne. The equilibrium will favor the side with the weaker acid and weaker base, which in this case is the products (HC≡C⁻ and NH3).

Step 4: Address part c of the problem. To determine whether HO⁻ or −NH2 is a better base for deprotonating ethyne, compare their conjugate acids' pKa values. The conjugate acid of HO⁻ is water (pKa = 15.7), and the conjugate acid of −NH2 is ammonia (pKa = 36). Since a stronger base has a weaker conjugate acid (higher pKa), −NH2 is a stronger base than HO⁻.

Step 5: Conclude that −NH2 is the better base to use for removing a proton from ethyne because it is stronger than HO⁻, as indicated by the higher pKa of its conjugate acid (NH3).

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above.

Video duration:

Was this helpful?

Key Concepts

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

pKa and Acid-Base Strength

The pKa value is a quantitative measure of the strength of an acid in solution; lower pKa values indicate stronger acids. In acid-base reactions, the equilibrium favors the formation of the weaker acid and base. Understanding pKa values helps predict the direction of the reaction and which species will act as the acid or base.

Equilibrium in Acid-Base Reactions

Acid-base reactions can be represented by equilibrium arrows, indicating the relative stability of reactants and products. The position of equilibrium is influenced by the strengths of the acids and bases involved. In this context, the reaction between ethyne and -NH2 will favor the formation of the weaker acid and base, which can be determined by comparing their pKa values.

Comparing Basicity of HO− and -NH2

To determine which base is better for deprotonating ethyne, one must compare the basicity of HO− (hydroxide ion) and -NH2 (amide ion). The stronger base will have a higher tendency to accept protons. Since -NH2 has a higher pKa than HO−, it is a stronger base and more effective at removing a proton from ethyne.

Recommended video:

Guided course

06:21

Understanding the difference between basicity and nucleophilicity.

Related Practice

Textbook Question

Rank the ions (⁻CH₃, ⁻NH₂, HO⁻, and F⁻) from most basic to least basic.

views

Textbook Question

For each of the acid–base reactions in [Section 2.3], compare the pK_a values of the acids on either side of the equilibrium arrows to prove that the equilibrium lies in the direction indicated.

1. <IMAGE>

2. <IMAGE>

views

Textbook Question

For each of the acid–base reactions in [Section 2.3], compare the pK_avalues of the acids on either side of the equilibrium arrows to prove that the equilibrium lies in the direction indicated.

3. <IMAGE>

4. <IMAGE>

views

Textbook Question

Ethyne (HC≡CH) has a pK_a value of 25, water has a pK_a value of 15.7, and ammonia (NH3) has a pKa value of 36. Draw the equation, showing equilibrium arrows that indicate whether reactants or products are favored, for the acid–base reaction of ethyne with

a. HO^-.

views

Textbook Question

Calculate the equilibrium constant for the acid–base reaction between the reactants in each of the following pairs:

c. CH₃NH₂+ H₂O

d. CH₃N⁺H₃ + H2O

views

Textbook Question

Which of the following bases can remove a proton from acetic acid in a reaction that favors products?

HO⁻ CH₃NH₂ HC≡C⁻ CH₃OH H₂O Cl⁻

views