The Aldol Condensation

 

 

Prior reading: Bruice, Chapter 19, 19.11 19.13

http://www-class.unl.edu/chem254/Lab9/

For a complete description of the lab go to http://courses.chem.psu.edu/chem36/, Experiment #10

 

 

Introduction

The aldol condensation relies on the reactivity of a carbonyl group to build a new carbon-carbon bond.  The aldol reaction is one of the most powerful methods available for forming a carbon-carbon bond.  In this reaction the conjugate base of an aldehyde or ketone adds to the carbonyl group of another aldehyde or ketone to give a b-hydroxy aldehyde or ketone product.  The simplest example of this reaction, outlined below, is the aldol condensation of acetaldehyde (ethanal) catalyzed by hydroxide ion. 

 

In the first step of this reaction, a base (sodium hydroxide is commonly used) removes a proton from the a-carbon (i.e., the carbon adjacent to the carbonyl group) of an aldehyde or ketone to form an anion.  Although simple carbanions are unstable, this anion gains significant stability by being a resonance hybrid of two contributors, a carbanion and an enolate ion in which the negative charge is on the more electronegative oxygen atom.


This enolate anion is a nucleophile that can add to the carbonyl group of another aldehyde or ketone.  In principle, either the negatively charged carbon of the carbanion or the negatively charged oxygen of the enolate could act as a nucleophile; a stronger bond forms in this reaction if carbon serves as the nucleophile.  In this example, a second molecule of acetaldehyde provides the carbonyl group that undergoes nucleophilic attack.

 

This gives an alkoxide ion that abstracts a proton from water to form the neutral product.  The reaction is called an aldol reaction because of the two functional groups that the product contains an aldehyde and an alcohol.

 


 

The initial aldol product sometimes spontaneously dehydrates to give an α-β-unsaturated system.  The ease of the dehydration of the aldol product increases in compounds that can lose water to form an extended conjugated system.

 

 
 

In this experiment, dibenzalacetone, or 1,5-diphenyl-1, 4-pentadien-3-one, will be prepared by the crossed aldol condensation of acetone with two moles of benzaldehyde.  The base is sodium hydroxide and the solvent is a mixture of ethanol and water.

 

                Benzaldehyde             Acetone                                             Dibenzalacetone

 [1,5-diphenyl-1,4-pentadien-3-one]

 

Not shown is the fact that both alkene bonds have trans stereochemistry.

 

 

Procedure

Dispense 200 μL (210 mg, 2.0 mmol) of benzaldehyde directly into a clean, dry test tube (13 x 100 mm), and then add 1 mL 95% ethanol and a magnetic spin vane.  Clamp the test tube to a ring stand, centered above a hot plate/stirrer and start stirring (the hot plate will not be used for heating!).  Add 75 μL (59 mg, 1.0 mmol) acetone, 1 mL 3 M NaOH, and stopper the test tube.  Stir the reaction mixture at room temperature (~20C) for 30 minutes.  Note the appearance of product as a yellow precipitate.  Vacuum filter this precipitate and wash it thoroughly with 3 x 1 mL of cold water.  Determine the weight and melting point of the crude sample, and then recrystallize it from ethyl acetate.  Once your sample has air-dried (allow 15 minutes after recrystallization) obtain the weight and melting point of purified product and calculate the percent yield.  Take an IR spectrum and a UV-Vis spectrum of the recrystallized sample. 

 

 

Safety Summary: NaOH in aqueous ethanol is corrosive and particularly dangerous to the eyes.  If contacted, remove with plenty of water. Acetone is highly flammable.  Benzaldehyde is listed as moderately toxic (but contributes to the flavor of almonds!)

 

 

Questions

1. What can you conclude about the nature (i.e. purity) of the compound that you made?

2. Check this website for stability of the isomers of dibenzalacetone:  http://www.hyper.com/science/tutorials/dibenzal.html

Stability of Dibenzalacetone Isomers:

In the synthesis of dibenzalacetone by aldol condensation of acetone with benzaldehyde the primary product melts at 110-112 C and has a intense UV absorption at 330 nm.  Two other isomers have much lower melting points.  Predict structures for all three.  

dibenzalacetone

 

 

Experimental data on the three isomers are summarized in the table below.

Isomer #

melting point, C

UV spectrum,  (nm)

UV spectrum, ε

1

110-111

330

34,300

2

60

295

20,000

3

<20

287

11,000

 

3. Why is the C=O stretching frequency at lower wavenumber than a standard C=O frequency?