Inferring SN2 and E2 Reaction Selectivity Using NMR Spectroscopy

In organic chemistry, the substitution reaction (SN2) and the removal reaction (E2) are different mechanisms, but when actually reacted, not only one of the two occurs, but the two reactions occur simultaneously depending on the conditions. To confirm this difference through the actual product, alkyl bromide and sodium methoxide were reacted and ¹ H NMR analysis was performed.

The difference in the reaction path was clearly revealed in the spectrum obtained after the reaction of 1-bromocyclohexane and 3-bromopentane, respectively. First, a new vinylic proton signal was observed near 5.6 to 6.2 ppm, indicating the formation of an alkene and indicating that the E2 reaction occurred. On the other hand, the peak corresponding to the singlet of about 3.3 ppm and the C–O bond around 4 ppm indicated the presence of the methoxy substitution product, and it was confirmed that the SN2 reaction proceeded together.

Calculated by normalizing the integral values against the CH–Br signal of the reference material, the E2 product was more predominant on both substrates, but the ratio differed with the substrate structure. In particular, in 1-bromocyclohexane with a ring structure, backside attack was difficult due to three-dimensional limitations, and E2 was more advantageous because anti-periplanar conditions were well formed. On the other hand, in 3-bromopentane, the proportion of SN2 products increased relatively.

Through this experiment, it was directly confirmed through NMR data that the reaction mechanism was not a simple rule, but a result of the molecular structure and reaction conditions working together. Seeing invisible molecular selectivity in the form of peaks in the

spectrum, I came to understand that organic reactions can be clearly confirmed not only by theory but also by actual evidence.

글 하단에 캡션 달아 달래요.  ¹H NMR spectrum of products from 1-bromocyclohexane reaction

¹H NMR spectrum of products from 3-bromopentane reaction