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Thursday, 10 September 2015


Preparation of 4-(4'-methoxyphenyl)-3-buten-2-one (Product A)

A p-anisaldehyde (1.2mL, 10mmol) is prepared in acetone (15mL) in a 100mL round bottom flask. A magnetic flea is then added and the flask is clamped over a magnetic stirrer. A potassium hydroxide (1.0 g) solution in water (20mL) is prepared in a separate beaker and added gradually to the mixture in the flask with continuous stirring for 20 minutes.
Now, around 40mL of water is added to the reaction mixture to ascertain the precipitation of all products. The resulting solid is filtered by vacuum filtration, followed by water washing. The solid is then dried and recrystalized from ethanol (Figure 1).
Figure 1. Recrystallized 4-(4’-methoxyphenyl) but-3-en-2-one (Product A).
Figure 2 presents the 1H NMR spectrum of p-anisaldehyde, clearly showing the resonances for the aldehyde and oxymethyl proton environments. Inductive effects of the substituents are considered to assign the two remaining resonances that belong to the 1,4-disubstituted benzene ring.
Figure 2. 1H NMR spectrum of p-anisaldehyde in CDCI3.
Figure 3. 1H NMR spectrum of 4-(4’-methoxyphenyl)-3-buten-2-one (Product A) in CDCI3.
The 1H NMR spectrum of recrystallized 4-(4'-methoxyphenyl)-3-buten-2-one is presented in Figure 3. The diagnostic methyl resonances at 3.83ppm and 2.35ppm belong to the oxymethyl and terminal methyl at position 1on the aromatic ring, respectively. The remaining resonances between 6.37- 7.69ppm represent the lingering six protons under four chemical conditions. Assigning the chemical shifts and multiplicities of these resonances is a complicated process due to their overlapping. Detailed observation of this region is carried out in the 2D J-resolved and COSY experiments (Figures 4 and 5).
Figure 4. 2D J-resolved spectrum of 4-(4’-methoxyphenyl)-3-buten-2-one (Product A) in CDCI3.
The 2D J-resolved experiment involves mapping of J-coupling constants against the proton chemical shift, thereby allowing accurate measurement of the chemical shifts of proton resonances through f2 dimension examination and their coupling constants in the fl dimension. Spin-coupling partners, typically over 2-4 bonds, are identified in the COSY experiment. Figure 5 presents the COSY of Product A, showing the correlation among the same proton pairs determined in the 2D J-resolved experiment by interpreting cross-peaks in the diagonal of the 2D spectrum. The proximity among these resonances is further validated by these correlations.
Figure 5. COSY spectrum of 4-(4’-methoxyphenyl)-3-buten-2-one (Product A) in CDCI3.

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