Textbook: Loudon 5th Ed. (2009)

Chapter 22: The Chemistry of Enolate Ions, Enols, and α,β-Unsaturated Carbonyl Compounds

Practice Problems (No matching mendel sets were found.)

Individual Problems

Problem # 738

Carbonyls are in equilibrium with their enol forms. This process is called keto-enol tautomerization.

This equilibrium happens in both acid and base.

Let's go through this equilibrium under acidic conditions. Draw a mechanism using curved arrows for each reaction below.

Remember that under acidic conditions, most species are either neutral or positively charged, and rarely negatively charged. So your structures will contain either ROH or ROH2+, but not RO-.

 

a) Carbonyl to Enol (acidic)

 

b) Enol to Carbonyl (acidic)

Problem # 739

Carbonyls are in equilibrium with their enol forms. An enolate is the deprotonated form of an enol.

Enolates are formed from carbonyls under basic conditions.

Let's go through this equilibrium under basic conditions. Draw a mechanism using curved arrows for each reaction below.

Remember that under basic conditions, most species are either neutral or negatively charged, and rarely positively charged. So your structures will contain either ROH or RO-, but not ROH2+.

 

a) Carbonyl to Enolate (basic)

 

b) Enolate to Carbonyl (basic)

Problem # 740

Enolates are formed from carbonyls by adding a strong base, such as lithium diisopropyl amide (LDA), to deprotonate the alpha position. The enolate can then act as a nucleophile and attack an electrophile (such as an alkyl halide), to form a new bond at the alpha position. This is called a carbonyl alpha substitution reaction.

 

Let's go through the mechanism of how enolates are formed and how they react with electrophiles.

Draw in the curved arrows to show the formation of the enolate (middle compound), and draw the structure of the carbonyl product (right compound)

 

Problem # 741

Like enolates (Q740), enols can also act as nucleophiles and make new bonds at the alpha position.

Let's go through the mechanism for how this happens, using alpha bromination as our example.

In the reaction below, the carbonyl form is in equilibrium with its enol form (Q738), which can then attack molecular bromine.

 

Complete the structure of the protonated carbonyl, and show the curved arrows that transform it into the final carbonyl product.

Problem # 742

Enamines are similar to enols and enolates in that they also undergo alpha substitution reactions.

The process of performing a carbonyl alpha substitution reaction via an enamine intermediate is called the Stork enamine synthesis. Let's work through this reaction.

 

Draw in the structures for the enamine and show it attacking the alkyl halide to form the  "3º imine." Also draw the structure of the final and carbonyl product.

 

Problem # 749
 

Show how to prepare each compound below from propanal. I've marked the "cuts" for you.

Problem # 306

Rank each group of acids in order of decreasing acidity. (1 = most acidic)

Explain your reasoning. You will have to use more than one rule in your explanation (resonance, electronegativity, atomic radius, etc.). 

 

Problem # 743
 

Enolates are nucleophiles and react with a variety of electrophiles.

Carbonyls are electrophiles. But aldehydes/ketones and  esters/acid chlorides often form different products.

 

Use curved arrows to draw a mechanism for each reaction below. How do the two products differ?

Problem # 744

After a sample of optically pure (S)-2-ethyl-cyclohexanone is dissolved in an aqueous solution for several hours, a significant loss of optical activity is observed. Explain.

Problem # 748

Alpha bromination is usually carried out under acidic conditions via the enol intermediate. 

Alpha bromination is uncontrollable under basic conditions, which goes through the enolate intermediate. Let's explore why.

     

 

a) Rank each carbonyl below in order of decreasing alpha-proton acidity (1= most acidic). Explain.

b) based on a), why does the reaction below lead to polyhalogenation?

Problem # 746
 

Show what combination of aldehyde, ketone, and/or ester can prepare each compound below. Every compound is a Claisen or aldol product.

Problem # 747
 

Show a combination of enolate (nucleophile) and electrophile that can produce each compound below.

Remember that all enolates come from carbonyls.

Problem # 750

Let's work through a Robinson annulation.

Work backwords to determine the starting materials needed to produce each intermediate below, then show a mechanism for the overall reaction.

Problem # 745

The molecule below has five different types of hydrogens (A through E). Rank each in order of decreasing acidity.

(1 = most acidic). Explain your reasoning.