Aldol reaction Essay
In this preparative lab, an aldol (trans-p-anisalacetophenone) was produced from the reaction between p-anisaldehyde and acetophenone with the presence sodium hydroxide. The reaction also showed the importance of an enolate and the role it played in the mechanism. Sodium hydroxide acts as a catalyst in this experiment and is chosen because of its basic conditions and pH. The acetophenone carries an alpha hydrogen that has a pKa between 18 and 20. This alpha hydrogen is acidic because of its location near the carbonyl on acetophenone. When the sodium hydroxide is added, it deprotonates the hydrogen and creates an enolate ion. This deprotonation creates a nucleophilic carbon that can attack an electrophilic carbon (like a parent carbon of a carbonyl). This enolate ion is a resonance structure and the oxygen atom and the corresponding pi bond it can form can stabilize the negative charge. When the nucleophilic pi bond attacks the carbonyl carbon (the electrophile) it undergoes nucleophilic addition.
This is often known as crossed-aldol condensation and creates a new carbon-carbon bond. Water can donate a proton and form a alpha-beta-hydroxyaldehyde. When performing mixed aldol reactions, there are two potential enolates that can form and two potential carbonyls that can serve as the electrophile. Without taking precautions, you will end up with many different similar products. In order to control mixed aldol reactions so they produce your desired product, you have to worry about the two enolates and two carbonyls.
First off you do not mix them together but instead you separate them. The two carbonyls react and form the aldol. Our product was trans-p-anisalacetophenone and it was produced from the ketone reacting with the aldehyde. The ketone does not react with itself because the aldehyde is sterically favored and the carbon on it is more likely to be attacked by the nucleophile than the carbon on the ketone. If the ketone were to attack itself, much more energy would be required.
Specifically, like stated earlier, our mixed aldol reaction consisted of reacting p-anisaldehyde (aldehyde) and acetophenone (ketone). The initial product is the beta-hydroxyketone, which rapidly undergoes dehydration and creates the final product, trans-p-anisalacetophenone. Technically, both the carbonyls cannot be mixed together with sodium hydroxide to get one product. We will get a dominant product of trans-p-anisalacetophenone. This reaction is an exception and we get away with it. P-anisaldehyde and acetophenone together only make one enolate.
This helps our exception, but there are still two carbonyls. With our weak base, we should be worried about acetophenone reacting with itself but we are not. This is due to steric hindrance, like I stated earlier. Aldehydes are better electrophilic carbons and therefore the ketone will react with the aldehyde faster than reacting with itself. It will quickly form the product trans-p-anisalacetophenone because it is the favored product. We do not have to use expensive LDA, we can use the weaker base and get away with it.
The reaction took place in a conical vial and .2mL of each of the reactant samples were added to it along with some 95% ethanol. Two drops of NaOH were added shortly after and stirred at room temperature for fifteen minutes. The vial was cooled in and ice bath and crystallized. Vacuum filtration was performed to filter the crude product. The crude product was recrystallized using methanol and filtered again. We made one change to the procedure and instead of using .7mL of ethanol we used 1mL. This is because 1mL is easier to measure out and will make sure that the reactant dissolves. Product analysis consists of percent yield, melting point, and IR. This reaction works very well, and the percent yield should be around 70%. I had a percent yield of about 69.22%.
This is a fairly good percent yield with the fact that the experiment was performed on a microscale level. Some product may have been left in the vial or in the vacuum filter. A little loss of product due to technique makes a great difference on the percent yield since it is microscale. I had a melting point of 70 degrees Celsius to 73 degrees Celsius. This is a little low compared to the expected melting point of trans-p-anisalacetophenone at 73 to 76 degrees Celsius. The lower melting point may be due to any present impurities or contaminates. The IR spectrum furthermore proved that the final product was in fact trans-p-anisalacetophenone. My IR matched that of the actual IR for trans-p-anisalacetophenone pretty well. You can see that there is a carbonyl peak present around 1656cm-1. There is also a CH aromatic stretch around 2981cm-1. The fingerprint region has many peaks, which match the fingerprint region of the actual IR.