Question

Compound Z, C5H160. Deduce its structure. IR: significant peaks at 2962 and 1718 (intense) cm! 13 Mass spectrum: a few of the significant peaks include 128, W, 57 m/z. H-NMR: s, at's 2 H'S 5,34's th's PPM 13 C-NMR 3 signals 220 200 180 160 140 120 PPM 100 80 60 40 20 0

Answer 1

Firstly we can calculate the double bond equivalence, for a compound C_aH_bN_cO_d type of compound it is

$DBE=a+1-\frac{b-c}{2}$ (if halogen is there count them with number of H)

So, here it is = 1.

• At 1718 cm^-1 IR peak refers to C=O stretching frequency, so, the one double bond is detected. The 2962 cm^-1 is for sp³ C-H stretching, so there are only sp³ carbons in the compound and the carbonyl is ketone.
• The overall molecular mass is 128 and the strong peak appers due to carbonyl. Now , in case of carbonyl both alpha and beta cleavage occurs. So, in the fragmentation peaks we can search for them. This will be a test for whether our predicted structure is correct or not.
• 9 H singlet is a common phenomenon for tertiary butyl system. So, we can think of a t-butyl system there. The chemical shift value suggests it is not in the vicinity of carbonyl.
• 3 H singlet is a common phenomenon for methyl having no neighbouring proton. Also its chemical shift value is quite high. So, we may think there is a CH₃ group adjacent to carbonyl.

Up to this we got fragments,

"CHз Нас. Нас- Нас

Now, we have only 2 C and 4 H left. Also, there are two 2H triplets in the ¹H NMR spectrum. So, there must be a -CH₂-CH₂- linkage with no neighbouring H in the left or in the right. So, we can join these fragments with the linker. So, proposed molecule:

о Н2 С. Нас. "CHз На Hас Hас

Now, compare mass spectrometry data and ¹³C NMR data:

• Due to alpha cleavage on the right side, -CH₃ group gets lost, having mass = 15. So, we'll have a peak at 128-15=113. Due to beta cleavage the left most neopentyl group gets lost, so we'll have a peak ( for -CH₂-CO-CH₃) at 57.
• For ¹³C, the peak above 200 is due to carbonyl carbon. All other peaks are in the range of sp³ C. The alpha carbons with respect to carbonyl have a higher value near 40. Rest of the carbons come nearly at 30.