Textbook Question
Identify the most acidic proton in each pair. Tell which structural features you analyzed and why you weighted them as you did in picking one answer. [Always start by drawing the conjugate base.]
(a)
3. Acids and Bases
Ranking Acidity
Problem 50
Textbook Question
If reacted with a strong base, which of the labeled protons would you expect to be removed first?
Verified step by step guidance1
Step 1: Identify the labeled protons (Ha, Hb, Hc) in the molecule and their chemical environments. Ha is attached to a sp2-hybridized carbon in the cyclohexene ring, Hb is attached to a sp-hybridized carbon in the alkyne group, and Hc is attached to a sp3-hybridized carbon in the cyclohexane ring.
Step 2: Recall that the acidity of a proton is influenced by the hybridization of the carbon it is attached to. Protons attached to sp-hybridized carbons are generally more acidic than those attached to sp2 or sp3 carbons due to the higher electronegativity of sp-hybridized carbons.
Step 3: Consider the role of the strong base (:NH2−). A strong base will preferentially remove the most acidic proton, which corresponds to the proton with the lowest pKa value.
Step 4: Compare the acidity of the labeled protons. Hb, attached to the sp-hybridized carbon, is the most acidic due to the high s-character of the sp orbital. Ha, attached to the sp2-hybridized carbon, is less acidic, and Hc, attached to the sp3-hybridized carbon, is the least acidic.
Step 5: Conclude that the strong base (:NH2−) will first remove Hb, the proton attached to the sp-hybridized carbon in the alkyne group, because it is the most acidic.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acidity and Basicity
Acidity refers to the tendency of a compound to donate protons (H+ ions), while basicity is the ability to accept protons. In organic chemistry, the strength of an acid is often determined by the stability of its conjugate base. Stronger acids have more stable conjugate bases, making them more likely to lose protons when reacted with a strong base.
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Proton Removal and pKa
The pKa value is a quantitative measure of the acidity of a proton in a molecule. Lower pKa values indicate stronger acids, meaning the corresponding protons are more easily removed. When considering which proton will be removed first by a strong base, one should look for the proton with the lowest pKa, as it is the most acidic and thus the most reactive towards the base.
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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 a conjugate base formed after proton removal. Protons attached to atoms that can participate in resonance are typically more acidic, as the resulting negative charge can be spread over multiple atoms, enhancing stability.
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