Question

1. How can you use 1H NMR to distinguish between isopropyl ethanoate and methyl 2-methylpropanoate (also called ethyl isobutyate)?

Isopropyl ethanoate

20180528_inova_500_isopropyl_acetate-PROTON_01 -2.02 - BC 66 LOS -727 5.02 EO'S 96 LS497498 Chemical Shift (ppm)

2. This spectrum contains two peaks that are shifted ~3 or more ppm downfield of the other three peaks. Why are these peaks shifted?

1-pentene

Answer 1

Ans-1: Yes, we can distinguish between isopropyl ethanoate and ethyl isobutyrate. Refer below the structures.

H3C. OCH CH3
                       
H, сосна

           

                        Ethyl Isobutyrate    Iso-propyl ethanoate

By looking at the structure, we see that the ethyl iso-butyrate contains one ethyl group attach to the oxygen. It will give distinct splitting pattern of triplet and quartet from the coupling of the protons of the –CH2- and –CH3 groups, following the n+1 rule. In the provided NMR it is not seen. Whereas in the iso-propyl ethanoate, we can expect a septet and doublet from the iso-propyl group. The –CH-(CH3)2, again following the n+1 rule.

In the provided NMR it is present.

CH3 ot che isopropyl ethanoate

The 1H-NMR assignments of the iso-propyl ethanoate is represented above, which is fully concordant with the provided NMR. The protons marked as A, B & C corresponds to the peak markings given in the attached NMR.

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Ans-2: Please refer the below structure of 1-pentene with the labelling on protons, i.e., A, B, C, D & E corresponding to signal labels in the provided NMR.

① h 1-pentene

The down field shifting of these two peaks appearing in the NMR is due to the hydrogens attached to the double bond having π electrons. The magnetic field created by these hydrogens is perpendicular to the double bond axis and forces the electrons of the π-bond to enter into a circular motion. The circular motion creates a local field that opposes the external magnetic field in the center of the double bond, thereby the hydrogens of the double bond experience de-shielding and appears about 3 ppm downfield compared to the other protons.