Question

Which spectral technique/spectrometer is your favorite instrumental technique? Explain its advantages (especially in the structural data it provides) and its disadvantages (especially the structural information it does not provide).

Answer 1

NMR spectroscopy is best analytical technique. Based on the ¹H NMR chemical shift values we can come to an idea how many types of Hydrogen are present in the compound. if the peaks present in the region of 11-12, itthe unknown compound has carboxylic acid group, it the peak at 9-10 ppm its having aldehyde group. if the peaks are present in the region of 6.5-8.5 the compound have aromatic unit, if there is 4.5-7 ppm there is a alkene unit is there. if the peak is present in the region of 2.5 - 4.5 ppm carbon attached to either electronegative atom (ex O, F, Cl, Br). normal alkyl chains will give peaks in the region of 0.7-1.2 ppm depend on the substitutes on the alkyl chain. Based on the splitting pattern we can get some idea of the H attached Carbon Environment

From the 13C, aliphatic alkyl chains give peaks at 10-30 ppm, the carbon attached to either electronegative atom (ex O, F, Cl, Br will give peaks from 30-70 ppm. The alkenes and aromatic Carbons will give peaks at 110-160 ppm. The carboxylic , anhydride and acyl chloride carbon will give peak at 165-180 ppm. aldehyde peak will come at 190-200, ketone will come at 210ppm.

DEPT 135 and DEPT 90 will details about the Carbon environment like how many hydro gens are attached to the carbon ( CH₃ CH₂ , CH and C).

all above detail will give an idea to solve the structure.

disadvantages

It is difficult some time to solve the structure, In case of isomers. NMR along with mass spectroscopy will give better idea to solve structure