Question

How does NMR allow us to understand the connection between bond rotation and the kinetic energy within the sample being analyzed?

Background info: in this lab DEET or N,N-diethyl-m-toluamide is synthesized using the Schotten-Baumann synthetic method using m-Toluoyl chloride abd diethylamine solution

Answer 1

NMR is probably the most widely used technique for the structure elucidation.in the given reaction the product will be characterised by proton NMR and IR. Variable temperature ¹H NmR will be used to determine the effect of amide resonance on the rate of C-N bond rotation. However the rotation about the C-N bond of an amide is much less "free" . This is due to amide resonance which makes the C-N bond have appreciable double bond character. In other words, rotating the C-N bond destrots the pi overlap bertween the carbonyl carbon and the nitrogen resulting in a much higher energy barrier (E_a) for bond rotation.

The rate of a chemical reaction is related to the activation energy(Ea) according to the Arrhenius equation :

                   k = A x e^-Ea/RT

A is the Arrhenius factor & R is the gas constant.

NMR occurs on a fairly slow time scale.Therefore, fast reactions are not discernable by NMR and the observed spectrum is the average spectrum of the interconverting molecules.In general, an averaged spectrum is observed when the rate of the process is 2p times the frequency difference (2p x Dn) between the peaks being observed.At lower temperatures the process is slower and seperate peaks are observed. at higher temperatures, one sharp peak at the average chemical shift value is obtained.This is what can do for the C-N bond rotation in DEET.

we find out the temp at which the two ethyl groups of DEET start to form one peak. Remember at this temp , the rate of bond rotation is approximately 2p x Dn.

hence we will know k, A, R & T so we can calculate Ea by using above equation .

The value of Ea obtained from the nmr exp will be compared to the predicted value from semi-empirical molecular orbital calculation using hyper-chem.