Aldehydes and ketones experiment discussion

ALDOL CONDENSATION WITH UNKNOWN KETONES AND ALDEHYDES OBJECTIVES: to perform aldol condensation utilizing as starting material unknown aldehydes and ketones. To determine the structure of the starting materials and product using NMR, and melting point.

TIME ALLOCATED: ONE LAB PERIOD

Introduction

The aldol condensation is a synthetic reaction resulting in the formation of a new carbon-carbon bond. The

carbon adjacent to the carbonyl group (C=O) is the - carbon and the hydrogens on this carbon are known as

-hydrogens. -Hydrogens of ketones, aldehydes and esters are more acidic than those in alkanes and alkenes. There are two main reasons for this: 1) electron-withdrawing inductive effect of the carbonyl group, 2) the negative charge present in a deprotonated ketone, aldehyde or ester is stabilized through resonance delocalization. When an aldehyde

or a ketone possessing -hydrogens is treated with a basic catalyst, a rapid and reversible reaction occurs. Because this step is reversible, a small amount aldol (b-hydroxy aldehyde) is formed at equilibrium.

However, the β-hydroxy aldehyde is usually not observed, since it readily dehydrates to yield a product in which a carbon-carbon double bond is in conjugation with the carbonyl group. The equilibrium constant for this step is much higher, therefore is possible to obtain

large quantities of , -unsaturated aldehyde or ketone. As you know, conjugation lowers the energy of the molecule, and thus elimination of water is a favorable process.

The mechanism for the reaction Is depicted below

Note that the double bond can be either cis or trans. A way to identify which isomer is formed is to measure the coupling constant (J value) of the alkene hydrogens: cis-alkene H's are typically 6-12Hz, and trans-alkene H's are typically 12-18Hz.

While this reaction is useful to form new carbon-carbon bonds, it does have a drawback. This is the lack of selectivity when two different aldehydes or ketones are used. In this case a crossed or mixed aldol condensation results in which the four possible aldol products are formed. This problem is minimized when one of the aldehydes or ketones does not have an α- hydrogen, since two of the normally expected products do not form. Also, with symmetrical ketones, there will be two additions of the aldehyde to give a symmetrical product.

MATERIAL AND METHODS

Chemicals

• Unknown aldehyde "A"

• Unknown aldehyde "B"

• Unknown aldehyde "C"

• Unknown aldehyde "D"

• Unknown ketone "A"

• Unknown ketone "B"

• Unknown ketone "C"

• Unknown ketone "D"

• Sodium Hydroxide

• Ethanol

Equipment

10 mL round bottom flask, 10 mL graduated cylinder, cork, steam bath, 3-pronged clamp, watch glass, pipettes, balance, melting point capillary tubes, Mel- Temp.

EXPERIMENTAL

During this experiment you will react an unknown aldehyde with an unknown ketone to form an aldol. You will then determine the identity of the two starting materials and the product, using NMR and melting point.

General procedure

• In a 10 mL round bottom flask, add the ketone (0.1 g), aldehyde (0.4 mL), 95% ethanol (2

2

mL), and 2N aqueous sodium hydroxide (1.5 mL).

• Stir for 30 minutes. If no precipitate forms after 15 min, heat in a small beaker on the hot plate for 5 min and then allow it to cool to room temperature.

• Cool your round bottom flask in ice for 5 minutes. A precipitate should form.

• Collect the product by Buchner funnel filtration and wash with ice-cold 95% ethanol (4 mL), 4% acetic acid in 95% ethanol (4 mL), and 95% ethanol (4 mL).

After performing the synthesis, you will characterize your aldol condensation product by measuring the melting point, and recording NMR. Using this information, you should be able to determine the structure of your product and, therefore, of your unknown starting materials. The table of data for comparison is provided below. Table 1. melting points of possible aldol products

Ketones

aldehyde A B C D

a 113 189 118 98-99

b 175 235 170 135-136

c 129 212 159 141-142

d 144 225 180 163-164

POST LAB ASSIGNMENT

Write your lab report including the following sections:

• Abstract (include which unknown you used and which is their name)

• Introduction: include at least one reference that is not the handout. Explain why the condensation is important and list one application (industry, medicine, etc)

• Discussion: explain the rationale behind your unknowns assignment. Comment the NMR spectra and m.p. Describe the procedure and your thought process. Include a structure of your final compound AND the two starting materials. Write the mechanism of the reaction.

• Experimental: concise procedure (use passive form). Include: yield, m.p. NMR assignment using the scientific format. DO NOT include personal comments.

• Conclusions. Brief statement about the outcome of the experiment.

List of possible unknowns listed In the table below

Ketones Aldehydes

Acetone benzaldehyde

Cyclopentanone 2-pentenal

Cyclohexanone 4-methyl- benzaldehyde

4-methylcyclohexanone 4-methoxy- benzaldehyde

Benzophenone trans cinnamaldehyde

Cycloheptanone acetaldehyde

2-Pentanone hexanal