Date of Award
Chemistry and Biochemistry
Substituents are capable of affecting their molecules via induction, resonance, and field effects. Using the Hammett equation, the effect a range of substituents has on different properties of a molecule can be quantified.1 Hammett’s parameters have been correlated with the substituent effects on 1H NMR chemical shifts in different molecules, including experiments on substituted chalcones (1,3-diarylpropenones).2 The effect a substituent has on a molecule can also be observed using 13C NMR. The chemical shifts of carbons throughout the molecule may be affected by the substituent. This was shown to be true by Wilson and Zehr in para-substituted terphenyls.3 In order to determine how an alkyne bridge affected the ability of substituents to affect carbons throughout certain molecules, we observed substituent effects in para-substituted tolans.
In previous work, substituents were found to affect the chemical shifts of every carbon in the molecule, but there was difficulty in distinguishing certain carbons from one another due to a close proximity in chemical shift.4 This phenomenon was observed in the 13C NMR spectra for acetyltolan, cyanotolan, and methyltolan. This uncertainty was attempted to be resolved using isotopic substitution. Deuterating one of the ambiguous carbons would theoretically cause this peak to essentially vanish on a 13C NMR spectrum, allowing one to determine that the identity of the remaining peak was the carbon that was not deuterated. The synthesis of methyltolan was attempted using multiple variations of the Sonogashira coupling procedure to test this hypothesis. Once both methyltolan and d5-methyltolan were synthesized, their resulting 13C NMR spectra were compared to assign identities to every peak in the original 13C NMR spectrum of methyltolan. The deuterated carbon peaks disappeared in the 13C NMR spectrum of d5-methyl tolan, which allowed for the determination of the identity of each peak in the 13C NMR spectrum of methyltolan. This technique was found to be a promising method for determining the identities of ambiguous peaks on a 13C NMR spectrum.
Crumpler, Reece, "13C NMR Substituent Effects on para-Substituted Tolans: Using Deuteration to Assign 13C Signals in Methyltolan" (2020). Honors Theses. 1342.