Array transformations in this experiment

Numbers in red are mole fractions for each array and the rest are NMR assignments. Converted from the CRS Array Table. [JCB]
CRSEXP001arrays.jpg

Chemspider links: 4-chlorobenzaldehyde ethanol 4-chlorobenzaldehyde ethanol hemiacetal

Reaction Attempts for ethanol 4-chlorobenzaldehyde hemiacetal

Researcher

Cuepil Choi

Objective

To observe the possible products of mixing together 4-chlorobenzaldehyde and ethanol at room temperature over the course of a few hours using a co-axial NMR tube with CDCl3 in the outer compartment. Experiment was prompted by UsefulChem post which hypothesized the role of electron withdrawing groups on aromatic aldehydes and hemiacetal formation.

Procedure

1.4220g of ethanol was added to 0.1137g of 4-chlorobenzaldehyde crystals in a 1-dram vial then transferred to the inner compartment of an NMR co-axial tube with CDCl3 in the outside compartment. HNMRs were taken starting at 30 minutes after the mixture was prepared.

Results

HNMR 1 30 min, about 10:1 aldehyde/hemiacetal (mole fraction sheet-1)
HNMR 2 60 min, about 10:1 aldehyde/hemiacetal (mole fraction sheet2 )
HNMR 3 90 min, about 10:1 aldehyde/hemiacetal (mole fraction sheet3)

Discussion

The small amounts of 4-chlorobenzaldehyde ethanol hemiacetal that was created through the reaction of 4-chlorobenzaldehyde and ethanol was expected. However, comparing the results of ONSEXP033 that tested 4-chlorobenzaldehyde and methanol showed a 4-chlorobenzaldehyde to 4-chlorobenzaldehyde methanol hemiacetal ratio of 2:1 compared to the 10:1 from this experiment. The drastically different ratios between methanol and ethanol reacting with 4-chlorobenzaldehyde may be the result of steric hindrance.
In these spectra the hydroxyl group of the hemiacetal (near 7.5 ppm) couples as an expected doublet with the benzylic H (near 6.1 ppm). This can arise when the lack of trace acid or basic impurities slow down H bonding exchange to longer than the NMR timescale. However the same is not observed for the ethanolic OH, which appears as a broad peak instead of a triplet. [JCB]

Conclusion

The reaction of 4-chlorobenzaldehyde with excess ethanol reached equilibrium within 30 minutes and was stable for at least 90 minutes. The aldehyde to hemiacetal ratio was about 10:1.

Log

2013-07-08

11:38 weight of empty 1-dram vial 1-E was 4.7488g
11:38 0.1137g of 4-chlorobenzaldehyde was put into 1-E
11:39 1.4220g of ethanol was added to 1-E
11:40 mixture was retriturated several times, all the 4-chlorobenzaldehyde crystals went in.
12:08 HNMR spectrum 1 was taken
12:37 HNMR spectrum 2 was taken
13:37 HNMR spectrum 3 was taken