My Problems

Published

List of user's published problems.

Problem # 747
 

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

Remember that all enolates come from carbonyls.

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 # 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.

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 # 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 # 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 # 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 # 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 # 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 # 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 # 735
 

Show how each ketone below can be prepared from sodium cyanide and either ethylene or propene.

You may also use methyl Grignard and ethylene oxide.

Problem # 730

N,N-dimethylformamide (DMF) is shown below. Based on its structure, you might expect to see only one -CH3 signal in the 1H NMR spectrum. But instead DMF shows two different -CH3 signals. Explain.

Problem # 729

The ester below was dissolved in a solution of water, a small amount of which was isotopically labeled with O-18, denoted with an asterisk.

After a few hours, some isotopically labeled oxygen was found in the ester. Where was it found in the ester? Can you explain why?

Problem # 728

The acyl group is a protecting group for amines. Amines can be acylated using acetic anhydride, and deacylated with base.

Propose a mechanism for each reaction.

Problem # 727

Show how the ester below can be prepared from propene.