Question

Using the provided MS and IR spectra, predict the structure and identity of the unknown. Show all workings and reasoning. Identify important peaks or fragments that led to your conclusion. You are free to use Rule 13 and/or index of hydrogen deficiency when necessary.

2 /3 65% 2. Unknown B 100 MS-Nu-0323 80 60 40 20- 0 ptoinittm 25 50 75 100 125 m/z to three different elemental oxygen present. Note that there be may up L00 1000 st0 aVENUERId Relative Intensity

Answer 1

Here you go:

The answer is salicylic acid. Now, I will tell you why salicylic acid, Before that, the structure is given below:

Он но

First we have to know the molecular formula. This can be obtained from the Mass spectra. The mass spectra says that the molecular peak, [M] i.e. the cationic peak for the full molecular weight at m/z ratio of 138 amu, and the [M+1] peak at 139 amu. Now, the relative intensities of these peaks are, for m/z 138 is 56% and form m/z 139 it is 4% (from the MS spectrum). This will help us to know about the number of carbon atoms in the molecule. Since, we know that the relative abundance of ¹²C isotope in nature is 100% while the relative abundance of ¹³C isotope in nature is 1.1%, THUS, here the relative percentage of the 13C isotope peak i.e. [M+1] peak compared to [M] peak is = 4/56 = 0.071 = 7.1%. Now, since 1 Carbon give relative percent of 1.1%, then 7.1% means around 7 Carbon atoms. Thus, It has 3 oxygen atoms (PROVIDED in the question), 7 carbon atoms, so we can calculate the number of the hydrogen atoms as = 138 - (7*12+3*16) = 6 (since atomic weight of carbon is 12 amu and that of oxygen is 16 amu and hydrogen is 1 amu. Hence, the molecular formula is C₇H₆O₃.

The degree of unsaturation can be calculated using the fomula,

Degree of unsaturation = (C+1) - H/2 = 7+1 - 6/2 = 8 - 3 = 5

That means the molecule has total of 5 double bonds + rings. Thus, most probably it's a benzene ring with an extra double bond since benzene ring has degree of unsaturation of 4 and a double bond is 1.

Now, we will look into the IR spectra to identify the functional groups. As the peak around 3300 cm-1 appears as sharp one, it indicated the presence of alcohol which is intramolecularly bound that is the single is sharp because it doesn't bind with other molecule, rather it intramolecular hydrogen bonding. The peaks close to 3000 cm-1 are indication of presence of C-H stretching and the small peaks from 1700-2000 cm-1 are due to C=C stretching. Sharp peak at 1680 cm-1 indicates the presence of an acidic C=O stretching and thus its a carboxylic acid also along with an alcohol present in the molecule where the groups are adjacent to each other so that there must be an intramolecular hydrogen bond. That is the reason why the -OH and - COOH groups are ortho to each other and the molecule is Salicylic acid.

I hope it helps!!!