Question

please help me answer the IR, H NMR and 13C NMR questions based on the pictures below.

IR List the important peaks seen in the IR of the product and label with the associated stretches and bends (eg. C-H stretch, etc). 13C NMR List all the peaks in the 13C NMR and give them each a number. Write the structure of the product and label the carbons with the associated number for the peak. (The peaks from 21-25 can be given the same number.)

Answer 1

Answer: The structure of amyl acetate is shown below.

The IR spectrum shows the sp³ -C-H stretching below 3000 cm^-1 . The peak at nearly 1740 cm^-1 is due to C=O stretching of ester carbonyl. Normally ester carbonyl appeared between 1750-1735 cm^-1. Another stronger peak observed at 1230 cm^-1 is due to  C-O stretching. Normally the C-O stretching is observed in the range 1300- 1000cm^-1 .

The ¹H NMR shows 5 signals this indicates that the molecule contains 5 non-equivalent sets of protons which are labelled in below structure.

E B D A E C

The proton labelled as B in structure of isoamyl acetate shows an triplet of 2 hydrogens at 4.10 ppm.This is an etheric proton (-O-CH₂) normal range is 3.8- 4.2 ppm. It shows an triplet because it has two neighbour of C type protons according to n + 1 rule (n = number of neighbouring hydrogens) 2 + 1 = 3.

The proton labelled as A shows an singlet of 3 hydrogens at 2.00 ppm. A type protons attached to electron withdrawing carbonyl group. So, this protons are deshielded normally appeared between 2.0- 2.5 ppm.It has no neighbour therefore it appeared as a (0 + 1 = 1) singlet.

The proton labelled as D shows an multiplet at 1.64 ppm of 1 hydrogen because it has 6 neighbour hydrogens of E type protons and 2 neighbour of C types protons. So according to n + 1 rule the multiplicity of D is 8 + 1 = 9.

The proton labelled as C shows an quartet of 2 hydrogens at 1.48 ppm. The multiplicity of C is quartet because it has 2 neighbour of B type protons and 1 neighbour of D .So according to n + 1 rule the multilpicity of C is 3 + 1 = 4.

The proton labelled E shows an doublet of 6 hydrogen at 0.88 ppm. The E types protons are equivalent.Its multiplicity is doublet  because it has 1 neighbour of D type protons (1 + 1 = 2).

The ¹³C NMR shows 6 signals this indicates that the molecule contains 6 non-equivalent sets of protons which are labelled in below structure.

F B A С E F. D

The carbon labelled as B is ester carbonyl carbon appeared at 171.43 ppm. The normal range of the ester carbonyl carbon 155-185 ppm.

The carbon labelled as C is appeared at 63.38 ppm. This carbon is etheric -O-C normally appeared between range 40- 80 ppm.

The carbon labelled as D is appeared at 37.56 ppm. This carbon normally appeared between range of 15- 55 ppm.

The carbon labelled as E is appeared at 25.30 ppm. This carbon normally appeared between range of 20- 60 ppm.

The carbon labelled as F is appeared at 22.69 ppm. This carbon normally appeared between range of 8- 30 ppm.

The carbon labelled as A is appeared at 21.26 ppm. This carbon normally appeared between range of 8- 30 ppm.

The molecular weight of isoamyl acetate is 130. Due to the alpha cleavage of isoamyl acetate in Mass- spectrum the peak at m/z = 87 is to the loss of (CH₃C=O⁺ ) ion. The peak at m / z = 43 is due to (CH₃C=O⁺ ) ion.The another peak at m/z = 59 is due to (CH₃ COO⁺ ) ion.