Part 8: SPECTROSCOPY

The following data is available from the question. 

Note : Remember to cross reference things to confirm.... e.g. IR may show C=C, use NMR to confirm that etc.

MS: M+ seen at 136 g/mol but also an m+2 at 138. While we can't see the ions here because they are too weak, the information suggests either Cl or Br is present. Looking at other peaks in the MS, 3:1 ratios in the ions at 121 & 123 or 72 & 74 implies a Cl isotope patterns.

IR:  There is a strong absorption at about 1750 cm-1 which is probably C=O. There are also peaks near 1100 and 1200 cm-1which suggest C-O. There are no other absorptions of note.... no OH, NH, C=C, C≡C or C≡N

13C NMR: The proton decoupled spectrum shows a total of 4 peaks indicating 4 types of C. By analysis of the chemical shifts, we have a peak at 167ppm (which must relate to the C=O as a carboxylic acid or derivative), 70 ppm C-O and then 2 peaks between 0-50 ppm that are most likely from an sp3 hydrocarbon portion. 

1H NMR: The proton spectrum shows a total of 3 peaks indicating 3 types of H. 

d/ppm
multiplicity
integration
Inference
4.86
7 lines
1
CH coupled to 6H, deshielded
4.4
s
2
CH2 not coupled, deshielded
1.38
d
6
2 x CH3 coupled to 1H

(d = doublet, s = singlet)

The most significant structural information from this are:

Summary....
The MS indicated MW = 136, Cl.
 The IR showed the presence of C=O and C-O
13C NMR shows 4 C types
H NMR gives (CH3)2CH- and a -CH2-

This information suggests a molecular formula = C5H9ClO2 which matches the MW = 136 and has an IHD = 1 consistent with just the C=O.

Altogether...

The fragments we have are:

(CH3)2CH- , -CH2-, Cl, C=O, O

The IR and C-NMR suggest that an ester, O=C-O- is present.

The H-NMR data indicates that the -CH2- is at 4.4 ppm which it can not be attached to the -O- and either the C=O or the Cl, therefore it must be connected to the C=O and the Cl. This means the -O- is connected to the (CH3)2CH- to give an isopropyl ester.

isopropyl chloroethanoate

The final step should always be to check what you have drawn. The easiest thing to check is usually the coupling patterns you would expect to see, and the chemical shifts of each unit.  You should be asking yourself : "Does my answer give me what the H-nmr shows ?"