Question

problem 3 R Spectrum 2984 (iquid flm) 1741 1243 1600 1200 800 0.0 3000 100 Mass Spectrum0.5 60 아る29 20 UV spectrum solvent: ethanol 154 mg/10 mls palh length: 1.00 cm M 88 C4H802 1.5 250 300 350 40 80 120 160 200 240 280 λ(nm) m/e 13C NMR Spectrum (50.0 MHz, CDCl3 solution) proton decoupled 200 160 120 80 40 0 8 (ppm) H NMR Spectrum (200 MHz. CDC solulion) TMS 10 5 4 δ (ppm)

Can you please dumb this down and do it step by step closely with full explanation? I have the simple walkthrough already but it's not helping much!

Thanks so much in advance!

Answer 1

The Chemical formula and molar mass of the compound is got from Mass Spectroscopy C₄H₈O₂ and 88 respectively.

First we have to calculate the Double Bond Equivalent(DBE) to know how many double bonds or rings may be present.

$DBE= C- \frac{H}{2}+ \frac{N}{2}+ 1$

Where H= No of hydrogen atoms = 8, C= No of carbon atoms = 4 , N= No of nitrogen atoms = 0

DBE = 1, That means 1 double bond or ring is present .Ring is not the case here as here the carbon number is only 4.

Next we can find the functional groups present from the IR spectra.

There are a strong peak at 1741 cm^-1 which is the characteristic peak of ester C=O stretching .Again a peak at 2984 cm-1 which is for alkyl C-H stretching .A peak at 1243 cm^-1 is for C-O stretching.So we can conclude a ester group is present in the molecule.

Look the picture for the characteristic peaks.

Ester C-O stretching IR Spectrum2984 1741 1243 č-o stretching Alkyl C-H stretching 2000 1600 1200 800 4000 3000 V (cmi')

Now 2 structures are possible.

3 3

Now we have to look at ¹H NMR spectra for choosing between two.

We know O-C-H chemical shift comes in 3.5-5 ppm .There is a peak near 4 and it is quatret.It means the carbon beside it have 3 hydrogen.So the structure is second(2) one .Again the structure is confirmed from other peaks. There is a singlet peak around 2 which is due to the keto methyl hydrogen.And there is a triplet around 1.5 which is due to methyl hydrogen.

See the following picture for clearer view

Again the structure is in accordance with UV-Vis spectra.Esters give a absorption peak around 200 nm.

H NMR Spectrum 200 MHZ CDsoluin) Methyl(CH3) Triplet(1.29) Keto Methyl (C-OCH3) Singlet(2.21) Methylene(CH2) Quatret (4.13) TMS 10 8 7 6 5 4 3 2 8 (ppm)

The mass spectra is also explained by this structure.

me 88 m/e 43

For esters, the most easily recognisable peak is generally due to [RCO]⁺ .
In ethyl acetate, the molecular ion peak is at m/e = 88. The base peak at m/e = 43 is obtained after the fragmentation(by alpha cleavage)
of ethoxyl radical [OCH₂CH₃]^. . The smaller peak at m/e = 29 is due to ethyl radical fragment.

The structure also explains the 13C NMR data

13C NMR Spectrum (50.0 MHz. Cocl, solution) Methyl Carbon Carbonyl Carbon (170) Methylene Carbon (61) Keto Methyl (14) Carbon(20) proton decoupled solvent 200 160 120 80 40 o 8 (ppm)

So the structure of the compound is

ethyl acetate