Question

Given the spectral data, determine the structure of the following compound. Show all work. Combustion analysis: C: 85.7% H: 6.67% MS: Molecular ion at m/z= 210, base peak at m/z=167 1H-NMR: 7.5-7.0 ppm (m, 10H) 5.10 ppm (s, 1H) 2.22 ppm (s, 3H) 13C-NMR: 206.2 (C) 128.7 (CH) 30.0 (CH3) 138.4 (C) 127.2 (CH) 129.0 (CH) 65.0 (CH) IR: Strong absorbance near 1720 cm -1

Answer 1

Firstly we will find out the molecular formula from the combination of combustion analysis and mass spectroscopy. As here our molecular weight is 210, So with percentage of carbon we can find out the moles of C present in the compound by 85.7 % of 210 is 179.97 dividing by 12g is equal to the 15 which is the no.of moles of C . Same going to the procedure our no.of moles of H is 14 here and the rest of (100-(85.7+6.67)) 7.6% of 120 is approx 16 which is assumed of O atom . Overalll, the molecular formula is C₁₅H₁₄O.

Now seeing 1H-NMR data , because we have ten protons with a signals in the aromatic region(7.5-7.0ppm) , we are probably dealing with two phenyl groups, each with one substituted carbon.Notice that carbon spectrum has only four peaks in the aromatic region! This indicates that two phenyl groups are equivalent.The absorption band at 1720cm-1 in IRspectrum has a characterstic carbonyl absorption band, its confirming the presence of O atom in the formula.

Now we have only 2C and 4H left to account for.. The proton spectrum tells us we have a methyl group (2.22ppm singlet) that is not split by the neighbouring protons, this chemical shift value is in the range of a methyl group adjascent to a carbonyl.

Finally there is a last proton at 5.10ppm singlet which will adjust itself in the puzzle compound we are going to here it together..

So, our final compound will be 1,1-diphenylacetone.

The base peak at m/z= 167 in the mass spectrum comes from loss of the acetyl group.