Alpha-Carbons and Tautomerization

Concept: Concept: Unusual Acidity of the Alpha Carbon

Video Transcript

In this video, I’m going to introduce two very important new concepts that we need to know for this section. That’s the concept of carbons and tautomerization.
Previously to this chapter, we've discussed how carbonyl carbons are very reactive because they have a partial positive on them. So far, we’ve always been talking about reactions at the carbonyl carbon. It turns out that carbonyls have another reactive component other than the carbonyl carbon. This is evidenced by pKas. I know it's been a long time since your acid and base chapter in Orgo 1. But does anyone happen to remember what is the pKa of an sp3-hybridized CH bond? Basically an alkane. Do you guys remember what the pKas of just a typical alkane? I heard someone say it. Great job. It's been forever since we’ve mentioned that. It’s 50. If you said 45, anything above. It’s 50. It’s something really high, something crazy high. The acidity of a normal alkane is 50. But alpha-carbons are uniquely acidic. Alpha-carbons don’t have a pKa of 50. Guess what their pKa is. 20. What is responsible for this crazy difference? Just so you guys know, this is on a log scale. That means that it’s 10 to the thirty times more acidic than an alkane. That’s an indescribable number. That's a huge, huge number. What could possibly be responsible for this difference? The answer is tautomerization. That's what I’m going to show you guys in the next video. 

Problem: Draw the enol tautomer for cyclopentanone


Alpha-Carbons and Tautomerization Additional Practice Problems

The following reaction process is an example of

A. Resonance

B. Conjugation

C. Oxidation

D. Acetalization

E. Tautomerism

Watch Solution

Consider each of the equilibria shown below. Does the equilibrium lie to the left or to the right?

Watch Solution

Identify the keto form of the following enol.

a 1-penten-3-one

b (E)-3-penten-2-one

c 2-pentanone

d (E)-3-pentenal

e (Z)-3-penten-2-one

Watch Solution

Which of the following is not correct?

a Tautomerization is catalyzed by both acids and bases.

b Tautomers are constitutional isomers.

c Tautomers rapidly interconvert.

d The enol form is generally more stable.

e All of the above are correct with respect to tautomers.

Watch Solution

How many α-hydrogens does 2-hexanone have?

a 1

b 2

c 3

d 4

e 5


Watch Solution

Refer to the reaction drawn below to answer the following questions.


In the presence of acid or a base a ketone can be converted into an ___________. This process is called


The ketone predominates because _______________________________________________________.


Watch Solution

Estimate Keq for the following reaction and explain if the equilibrium favors the reactants or the products.

pKa α-H cyclohexanone 20; pKa HCN 9; pKa β-H cyclohexanone 40 

Watch Solution

Write out the mechanism for the following transformation.

Watch Solution

Write out the mechanism for the following transformation.

Watch Solution

The final step in the hydration of an alkyne under both acidic and basic conditions is the tautomerization of an enol intermediate to give the corresponding carbonyl. Provide an arrow-pushing mechanism for the tautomerization under acidic conditions. 

Watch Solution

How many tautomers can you draw? 

Watch Solution

How many acidic protons (pKa ≤ 20) are in the following compound?

Watch Solution

Draw the enol tautomer for the following ketone:

Watch Solution

Consider the structural formula of acetaldehyde below.

(a) Give a chemical equation for the reaction between hydroxide ion (HO -) and acetaldehyde in which HO- acts as a Lewis base and acetaldehyde acts as a Lewis acid. IMPORTANT: The Lewis base that you give cannot be a Bronsted-Lowry base; the Lewis acid that you give cannot be a Bronsted-Lowry acid. Note: Use Lewis structures for each reactant/product and use curved arrows appropriately. 






(b) Using Lewis structures for each reactant and each product and using curved arrows appropriately to show the flow of electrons in the reactants, give a chemical equation for the reaction between hydroxide ion (HO -) and acetaldehyde in which HO- acts as a Bronsted-Lowry base and acetaldehyde acts as a Bronsted-Lowry acid. Do not give the same answer that you gave in the first part of this question. 






Watch Solution