Synthesis of Trans-Benzalacetophenone
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Chem 44.1 exer 5
Organic compounds with a carbonyl functional group, C=0, include aldehydes and ketones. Hydrogen or alkyl or aryl substituents can occupy the two left bonds on the carbon atom of this group. An aldehyde compound has at least one hydrogen substituents while a ketone has no hydrogen. (Reusch, 2012)
Similarly, aldehydes and ketones double bond functional group can be compared with that of the alkenes. Relative with the dipole moments of alkenes, aldehydes and ketones have larger molecular dipole moments due to the higher electronegativity of oxygen making the carbonyl group polar. This characteristic also indicates the higher boiling point of aldehydes and ketones than that of alkenes. Moreover, aldehydes and ketones are said to be good hydrogen-bond acceptors due to the presence of oxygen containing non-bonding electron pairs. This affects the chemical reactivity of aldehydes and ketones with water as well. (reush, 2012)
As said, only most of carbonyl compound can form enolates. This is based on the alpha-substitution reaction of carbonyl cmpds. Apparently, this substitution occurs at the alpha-position which is a position next to the carbonyl group. Illustration:
As mentioned, the carbonyl group causes the acidity of the hydrogens of the alpha-C hence making it reactive with strong bases to produce enolate ions. A carbonyl compound can be considered enolizable if it has an alpha-hydrogen. Furthermore, the enolate is easily formed in this case due to the nucleophilicity of the a-C due to the acidity of its a-hydrogens.
In this experiment, trans-benzalacetophenohne will be synthesized using mixed-aldol condensation or ClaBenzaldehyde and acetophenone will be the aldehyde and ketone to be used, respectively. In here, acetophenone is the only enolizable carbonyl compound since benzaldehyde has no alpha-hydrogen hence, it is not a good acceptor of nucleophile.
Over all rxn:
The reaction involves the formation of enolate then the nucleophilic attack of the enolate ion to the carbonyl group of benzaldehyde. This is followed by the protonation and dehydration reactions. This is shown as follows:
There are two isomeric forms of benzalacetophenone, the cis and trans isomers. Obviously, the main goal here is to synthesize the trans isomer. This is because the formation of cisisomer isl ess favorable due to the hindrance. Illustration:
During the synthesis, there is a possibility that the reaction would not be spontaneous. Some aldol reactions tend to stop at the B-hydroxyl carbonyl stage and do not proceed with the dehydration. This is because of the stability of the product formed cause by its conjugation. To prevent this, there are driving forces for the reaction to proceed to dehydration. These include the presence of heat and