Q 60P - Organic Chemistry (Mcmurry)

Question

Compound A, a hydrocarbon with $M^{+}$= 96 in its mass spectrum, has the ${}^{13}C$ spectral data given below. On reaction with $B{H}_3$ , followed by treatment with basic $H_2{O}_2$, A is converted into B, whose ${}^{13}C$ spectral data are also given below. Propose structures for A and B. 

 Compound A Broadband-decoupled ${}^{13}C$ NMR: 26.8, 28.7, 35.7, 106.9, 149.7 d DEPT-90: no peaks DEPT-135: no positive peaks; negative peaks at 26.8, 28.7, 35.7, 106.9 d 

 Compound B Broadband-decoupled ${}^{13}C$ NMR: 26.1, 26.9, 29.9, 40.5, 68.2 d DEPT-90: 40.5 d DEPT-135: positive peak at 40.5 d; negative peaks at 26.1, 26.9, 29.9, 68.2 d

Step-by-Step Solution

Answer

Step-by-Step Solution Step 1: Mass spectra

It is a spectroscopy used to determine the molecular mass of the compound and the structure by plotting the ion signal as a function of mass to charge ratio.

Step 2: Identify compound from mass spectra

The peak of the mass spectra at the mass to charge ratio is 96 which means the two unsaturation bonds are present by ${}^{13}C$ NMR method we can see that compound A has 5 peaks and the molecule is symmetrical and cyclic.

Step 3: Structure of compounds A and B

The DEBT-135 shows that the compound A has three different methylene groups, one $=C{H}_2$ and one $- C =$ and the last two carbons are $s{p}^2$ hybridise as per the chemical shift

In compound b the absorption due to the double bond carbon replace by CH bond and the methylene bond to an electronegative atom as shown:

                                                  Chemical Shift