Question

A compound of formula C₁₁H₁₆N₂gives the IR, ¹HNMR, and ¹³CNMR spectra shown. The proton NMR peak atδ 2.0disappears on shaking with D₂O. Propose a structure for this compound, and show how your structure accounts for the observed absorptions.

Short answer

The required structure is as follow.

Step-by-step solution

Structure determination information from IR spectrum:

Molecular formula C₁₁H₁₆N₂ has five elements of unsaturation, enough for a benzene ring and no oxygens precludes nitro group and amide. If cyanide group is present then there are not enough elements of unsaturation left for a benzene ring, so benzene and cyanide group are mutually exclusive.

From IR spectrum, we can deduce that, one spike is present around 3300 cm^-1 which suggests a secondary amine. No cyanide group is present. -CH and C=C regions suggest an aromatic ring.

Structure determination from proton and carbon NMR spectrum:

From proton NMR spectrum, -5H multiple at chemical shift value 7.3 indicates a monosubstituted benzene ring (the fact that all the peaks are huddled around 7.3 precludes nitrogen being bonded to the ring). 1H singlet at chemical shift value of 2.0 is exchangeable which indicates NH of secondary amine. 2H singlet at chemical shift value 3.5 is methylene; the fact that it is so strongly DE shielded and unsplit suggests that it is between a nitrogen and benzene ring.

So far, you have fragments as follows.

From carbon NMR spectrum, there are four signals around chemical shift values 125-138 which indicates aromatic carbons. The signal at chemical shift value 65 is the methylene bonded to the benzene. The other four carbons come as two signals at chemical shift value 46 and 55, each is a methylene carbon. So, there are two sets of two equivalent methylene groups each bonded to nitrogen atom to shift it downfield.

So far, you have fragments as follows.

There is no evidence for an alkene in any of the spectra, so the remaining element of unsaturation must be a ring. The simplicity of NMR indicates a fairly symmetric compound.

Structure determination from the given data.