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Magazine Chapter 2: Problem 46
Which of the following bases are strong enough to deprotonate CH\(_3\)CH\(_2\)CH\(_2\)C =CH (p\(K_a\) = 25), so that equilibrium favors the products: (a) H\(_2\)O; (b) NaOH; (c) NaNH\(_2\); (d) NH\(_3\); (e) NaH; (f) CH\(_3\)Li?
Short Answer
Expert verified
NaNH鈧�, NaH, and CH鈧僉i can deprotonate the molecule.
Step by step solution
01
Understand the Problem
We need to determine which bases can deprotonate CH\(_3\)CH\(_2\)CH\(_2\)C=CH, which has a p\(K_a\) of 25. A strong enough base must have a conjugate acid with a p\(K_a\) greater than 25 to drive the equilibrium towards the products.
02
Review Base Strength
Review the p\(K_a\) values of the conjugate acids of the given bases. A base can successfully deprotonate a molecule if its conjugate acid is weaker (higher p\(K_a\)) than the molecule being deprotonated.
03
Evaluate H鈧侽 as a Base
The conjugate acid of H\(_2\)O is H\(_3\)O\(^+\), with a p\(K_a\) of approximately -1.7. This is much stronger (lower p\(K_a\)) than 25, so H\(_2\)O is not strong enough.
04
Evaluate NaOH as a Base
The conjugate acid of NaOH is H\(_2\)O, with a p\(K_a\) of 15.7. It is also not strong enough to deprotonate the molecule.
05
Evaluate NaNH鈧� as a Base
The conjugate acid, NH\(_3\), has a p\(K_a\) of about 36. Since 36 is greater than 25, NaNH\(_2\) can deprotonate the molecule.
06
Evaluate NH鈧� as a Base
The conjugate acid of NH\(_3\) is NH\(_4^+\), with a p\(K_a\) of about 9.25, which is much lower than 25, so NH\(_3\) is not strong enough.
07
Evaluate NaH as a Base
The conjugate acid of NaH is H\(_2\), with a p\(K_a\) of around 35. This is greater than 25, so NaH can deprotonate the molecule.
08
Evaluate CH鈧僉i as a Base
The conjugate acid of CH\(_3\)Li is CH\(_4\), with a p\(K_a\) of around 50. Since this is much greater than 25, CH\(_3\)Li can deprotonate the molecule.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Deprotonation
Deprotonation is a fundamental concept in acid-base chemistry and involves the removal of a proton (a hydrogen ion, H+) from a molecule. This process is critical in many organic reactions, particularly when forming anions from neutral molecules.
In organic chemistry, deprotonation often plays a vital role in generating reactive intermediates, such as carbanions, which can participate in further chemical reactions. The ability of a base to deprotonate a molecule depends on the base's strength and the acidity of the proton to be removed.
In organic chemistry, deprotonation often plays a vital role in generating reactive intermediates, such as carbanions, which can participate in further chemical reactions. The ability of a base to deprotonate a molecule depends on the base's strength and the acidity of the proton to be removed.
- For deprotonation to occur, the base must be strong enough to abstract the hydrogen ion.
- The relative acidity of the hydrogen being removed is often represented by its p\(K_a\) value; a lower p\(K_a\) indicates stronger acidity.
pKa values
The p\(K_a\) value of a compound is a measure of its acidity, specifically the tendency of an acid to donate a proton. The smaller the p\(K_a\) value, the stronger the acid. For base-driven reactions, the p\(K_a\) values help in predicting whether a particular base can deprotonate a given acid.
- In general, deprotonation is feasible if the conjugate acid of the base has a higher p\(K_a\) than the acid being deprotonated.
- This indicates that the base is strong enough to steal the proton from the acid, shifting equilibrium towards the products.
Conjugate acid-base pairs
Conjugate acid-base pairs are pairs of compounds that transform into each other by the gain or loss of a proton. In any acid-base reaction, an acid and a base form their respective conjugate base and acid.
In the reaction where a base deprotonates an acid, the base gains a proton to become its conjugate acid. Likewise, the acid becomes its conjugate base.
In the reaction where a base deprotonates an acid, the base gains a proton to become its conjugate acid. Likewise, the acid becomes its conjugate base.
- The strength of a base is inversely related to the strength of its conjugate acid.
- A stronger base will have a weaker conjugate acid with a higher p\(K_a\).
Base strength
Base strength is a key determinant of a base's ability to deprotonate other molecules. The strength of a base is evaluated in terms of its willingness to accept a proton. Factors contributing to base strength include:
- The electronic environment around the base.
- Stability of the conjugate acid formed after protonation.
Equilibrium in acid-base reactions
Acid-base reactions achieve a balance, or equilibrium, between the acid-base pairs in solution. Whether equilibrium favors the reactants or products depends on the relative strengths of the acids and bases involved.
- When equilibrium favors the products, it indicates that the base involved is strong enough to effectively deprotonate the acid.
- In our original problem, bases such as NaNH\(_2\), NaH, and CH\(_3\)Li push the equilibrium towards products due to their strong base nature and their conjugate acids having higher p\(K_a\)'s than 25.
